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Magnetocaloric effect in nanoparticles and bulk clathrates

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Title:
Magnetocaloric effect in nanoparticles and bulk clathrates
Physical Description:
Book
Language:
English
Creator:
Rebar, Drew
Publisher:
University of South Florida
Place of Publication:
Tampa, Fla
Publication Date:

Subjects

Subjects / Keywords:
Entropy
Magnetism
Adiabatic demagnetization
Nanomaterials
Cooling
Dissertations, Academic -- Physics -- Masters -- USF
Genre:
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Abstract:
ABSTRACT: The magnetocaloric effect (MCE) has been a recognized phenomenon for over a hundred years and is typically observed as the heating of a magnetic material when a magnetic field is applied and cooling of the material when the field is removed. Besides low temperature physics, MCE has been utilized for cooling at higher temperatures for various applications. High efficiency and no dependence on pressurized gases have pushed forward the research on MCE and magnetic cooling systems. Research into the MCE in nanoparticle systems has mostly been within the last two decades. Besides novel properties inherent to nanoparticle systems, benefits such as ease of tunability and materials processing for thin film applications continue to fuel the pursuit of MCE research in nanoparticle systems. We characterized the MCE in cobalt ferrite nanoparticles, manganese zinc ferrite nanoparticles, cobalt nanoparticles, and CocoreAgshell nanoparticles. In addition, we had the opportunity to^ characterize the MCE in Eu8Ga16Ge30 Type I and Type VIII clathrates. The cobalt ferrite nanoparticles were synthesized in our lab using a decomposition of metalorganic salts technique. The product particles were capped with oleic acid. These particles were characterized with XRD, EDS, and TEM. DC magnetic characterization was performed on all samples. Change in entropy was calculated using a Maxwell's relation and magnetization data as a function of applied field and temperature. MCE studies in cobalt ferrite and manganese zinc ferrite revealed a broad but low magnitude change in entropy over a wide temperature span. MCE studies in cobalt and CocoreAgshell revealed a large MCE correlating with a sharp low temperature transition associated with the blocking of surface spins. MCE studies in both clathrates revealed a large MCE at low temperatures. These studies indicate the possibility of dual functionality for these clathrates since these materials also possess excellent thermo electric properties.
Thesis:
Thesis (M.A.)--University of South Florida, 2006.
Bibliography:
Includes bibliographical references.
System Details:
System requirements: World Wide Web browser and PDF reader.
System Details:
Mode of access: World Wide Web.
Statement of Responsibility:
by Drew Rebar.
General Note:
Title from PDF of title page.
General Note:
Document formatted into pages; contains 80 pages.

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University of South Florida Library
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University of South Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 001797514
oclc - 156995707
usfldc doi - E14-SFE0001630
usfldc handle - e14.1630
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SFS0025948:00001


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Magnetocaloric effect in nanoparticles and bulk clathrates
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ABSTRACT: The magnetocaloric effect (MCE) has been a recognized phenomenon for over a hundred years and is typically observed as the heating of a magnetic material when a magnetic field is applied and cooling of the material when the field is removed. Besides low temperature physics, MCE has been utilized for cooling at higher temperatures for various applications. High efficiency and no dependence on pressurized gases have pushed forward the research on MCE and magnetic cooling systems. Research into the MCE in nanoparticle systems has mostly been within the last two decades. Besides novel properties inherent to nanoparticle systems, benefits such as ease of tunability and materials processing for thin film applications continue to fuel the pursuit of MCE research in nanoparticle systems. We characterized the MCE in cobalt ferrite nanoparticles, manganese zinc ferrite nanoparticles, cobalt nanoparticles, and CocoreAgshell nanoparticles. In addition, we had the opportunity to^ characterize the MCE in Eu8Ga16Ge30 Type I and Type VIII clathrates. The cobalt ferrite nanoparticles were synthesized in our lab using a decomposition of metalorganic salts technique. The product particles were capped with oleic acid. These particles were characterized with XRD, EDS, and TEM. DC magnetic characterization was performed on all samples. Change in entropy was calculated using a Maxwell's relation and magnetization data as a function of applied field and temperature. MCE studies in cobalt ferrite and manganese zinc ferrite revealed a broad but low magnitude change in entropy over a wide temperature span. MCE studies in cobalt and CocoreAgshell revealed a large MCE correlating with a sharp low temperature transition associated with the blocking of surface spins. MCE studies in both clathrates revealed a large MCE at low temperatures. These studies indicate the possibility of dual functionality for these clathrates since these materials also possess excellent thermo electric properties.
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Adiabatic demagnetization.
Nanomaterials.
Cooling.
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M agn et o cal o ri c Ef f ect i n Nan o part i cl es an d Bul k Cl at h rat es by Drew Re b ar A t he s i s s u bm i t t e d i n p a r t i a l f u l f i l l m e nt of th e r e q u i r e m e n ts f or th e d e g r e e of M as t er o f Sci en ce Depa rt m en t o f Ph y s i cs Co l l ege o f A rt s an d Sci en ces Un i v ersi t y o f So ut h F l o ri da M aj o r P ro f es s o r: Hari h aran Sr i kan t h Ph D. Pr i t i s h M ukh erj ee, Ph D. Den n i s K i l l i n ger, Ph D. D a t e o f A p p r o v a l: M ay 22, 2006 Ke y w o r d s : E nt r o p y m a g ne t i s m a d i a ba t i c d e m a g ne t i z a t i o n, na no m a t e r i a l s c o o l i ng Co py ri gh t 2006 Drew J Re b ar

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Dedication I wo ul d l i ke t o dedi cat e t h i s t h es i s t o m y M o m an d Dad wh o h av e al way s f uel ed m y en t h us i as m f o r sci en ce.

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A c k no w l e dg m e nt s T h e r e a r e s e v e r a l p e o p le a n d g r o u p s t h a t mu s t b e a c k n o w le d g e d a n d t h a n k e d fo r t h e ir co n t r i b ut i o n t o war d t h i s re sear ch I wo ul d f i r st l i ke t o t h an k m y Go d an d S av i o r Jesus Ch r i st w h o g a v e m e t h e a b i l i t y a n d s t re n g t h t o c o m p l e t e t h i s p roj e c t It i s m y d e s i re t h a t m y w o rk b o t h in a n d o u t o f t h e la b w o u ld b e t o t h e p r a is e o f H is g lo r y. N e x t I w o u ld lik e t o t h a n k my p a r e n t s f or a l l th e y h a v e d on e f or m e b oth i n l i f e a n d w i th th i s p r oj e c t. I w ou l d l i k e to p a r ti c u l a r l y th a n k m y D a d f or h e l p w i th v a r i ou s a s p e c ts of w or d p r oc e s s i n g I w ou l d l i k e to t h an k m y adv i so r Dr Har i h ar an S r i kan t h t o wh o m I o we a gr eat deb t o f gr at i t ud e. T h e l i st o f t h in g s fo r w h ic h I a m g r a t e fu l is v e r y lo n g b u t s o me o f t h e h ig h lig h t s in c lu d e t a k in g me in a s a N S F R E U s tu d e n t s e v e r a l y e a r s a g o, a c c e p ti n g m e i n to th e F u n c ti on a l M a te r i a l s L a b f or gr adu at e l ev el r esear ch sen di n g m e t o n at i o n al co n f er en ces, f un di n g r esear ch assi st an t sh i ps, i m p a rt i n g s k i l l s t h a t I w i l l u s e a s a re s e a rc h e r, e t c In a d d i t i o n t o m e n t o ri n g f rom D r. H a ri h a ra n Sr i k a n t h I a m a l s o g ra t e f u l f o r a l l t h e h e l p a n d a s s i s t a n c e f rom f o rm e r p o s t d o c D r. P a nk a j P o d d a r a nd c u r r e nt p o s t d o c D r S a ny a d a na m S r i na t h. D r S r i k a nt h, D r P o d d a r a nd Dr S r i n at h h ave al so co n t r i b u t ed t o m y d ee p er ap p r ec i at i o n o f I n d i an cu l t u r e a n d fo o d I w o u l d l i k e t o t ha nk D r M u k he r j e e a nd D r Ki l l i ng e r f o r be i ng o n m y t he s i s c o m m i t t e e a nd a l s o g i v i n g m e g r e a t a d v i c e a n d w i s d o m M u c h th a n k s g o e s to D r S M o r r i s o n D r. E. C a r p e nt e r D r P o d d a r a nd D r G e o r g e N o l a s f o r t he i r c o nt r i bu t i o n o f m a t e r i a l s a nd i n f or m a ti on f or th i s r e s e a r c h I w ou l d l i k e to th a n k J a m e s G a s s an d M a tt A l m a n d f or

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assi st an ce wi t h n an o par t i cl e sy n t h esi s an d o t h er aspect s o f t h i s re sear ch pr o j ect I wo ul d al so lik e t o t h a n k my o t h e r la b ma t e s w h ic h in c lu d e J e ff S a n d e r s R a n k o H e in d l, N a t a lie F r e y, Ch am i l a Si y am b al api t y a, Mo n et M o ral es a nd Suzan n e M o rr o w f o r l en di n g h el pi n g h an ds d u r i n g m y r e s e a r c h M u c h th a n k s g oe s to B ob b y H y d e f or a s s i s ta n c e w i th x r a y d i f f r a c ti on a n d e n e r g y d i s p e r s i v e x r a y s p e c t r o s c o p y M u c h t h a n k s a l s o g o e s t o S c o t t J a c k s on f or c r e a t i ng t he i l l u s t r a t i o ns d i s p l a y e d i n t hi s t he s i s L a s t bu t no t l e a s t I w o u l d l i k e t o t ha nk t he Nat i o n al Sci en ce F o un dat i o n f o r t h ei r suppo rt o f t h i s pro j ect t h ro ugh gran t CTS0408933.

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i T a bl e o f C o nt e nt s L is t o f T a b le s iii L is t o f F ig u r e s iv A b s t r a c t v ii C h a p te r 1 : T h e or y B a c k g r ou n d a n d M oti v a ti on 1 1 I n tr od u c ti on 1 1 .2 Th e o ry 1 1 3 B a c k g r o u n d : B u lk M a t e r ia ls 8 1. 4 Ba ckgro un d: Nan o m at eri al s 12 1. 5 Re s earch M o t i v at i o n an d L ay o ut 18 C h a p t e r 2 : M a t e r ia ls 2. 1 In t ro duct i o n 20 24 2. 2 Sy n t h es i s o f Co F e O Nan o part i cl es 20 2. 3 Nan o part i cl es Ob t ai n ed f ro m Co l l ab o rat o rs 25 2. 4 Bul k M at eri al s 25 C h a p te r 3 : S tr u c tu r a l a n d M a g n e ti c C h a r a c te r i z a ti on 3. 1 In t ro duct i o n 27 3. 2 M agn et i c C h aract eri zat i o n M et h o do l o gy 27 3. 3 Co b al t F err i t e Na n o part i cl e Sy s t em 28 3. 4 M an gan es e Z i n c F err i t e Na n o part i cl e Sy s t em 32 c o r e s h e ll 3. 5 Co b al t an d Co A g Nan o part i cl e Sy s t em 34 81 63 0 3. 6 E u Ga Ge T y pe I an d VII I Bul k Cl at h rat e Sy s t em s 37 C h a p te r 4 : M C E C h a r a c te r i z a ti on 4. 1 Ov erv i ew 40 4. 2 M CE Ch aract eri zat i o n M et h o do l o gy 40 4. 3 L ab VIE W Pr o gram 41 4. 4 Co b al t F err i t e Na n o part i cl es 46 4. 5 M an gan es e Z i n c F err i t e Na n o part i cl es 48 4. 6 Co b al t Nan o part i cl es 49 c o r e s h e ll 4. 7 Co A g Nan o part i cl es 51 81 63 0 4. 8 E u Ga Ge T y pe I an d VII I Cl at h rat es 53

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ii C h a p t e r 5 : S u m m a ry C o n c l u s i o n s a n d F u t u re W o rk 5. 1 Sum m ary 57 5. 2 Co n cl us i o n s 59 5. 3 F ut ure W o rk 62 Re f eren ces 65 A p p e n d ix Pub l i cat i o n s an d Pr es en t at i o n 73 Co n f eren ce A b s t ract 75 NSF Re co gn i t i o n 76 M CE Pr o gram Bl o ck Di agram 79

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iii Lis t of T abl es T a bl e 3 .1 A t o m i c perce n t ages f ro m E DS o f a co b al t f err i t e n an o part i cl e s am pl e. 29

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iv Lis t of F igur es F ig ur e 1 .1 Re duct i o n o f m agn et i c en t ro py b y an appl i ed f i el d. 2 F igur e 1.2 E n t ro py curv es f ro m Re f 1. 3. 6 5 22 F igur e 1.3 M CE i n Gd (S i Ge ) an d Gd f ro m Re f 1. 19. 12 C0 F igur e 1.4 P lo t o f T S / MH v s x f ro m Re f 1. 20. 14 0.85 0.15 F igur e 1.5 M CE i n Gd Y f ro m Re f 1. 22. 15 F igur e 1.6 E f f ect o f s i ze di s t ri b ut i o n o n m ax i m um en t ro py f ro m Re f 1. 24. 16 30 70 70 30 F igur e 1.7 M CE f o r Dy Z r an d Dy Z r f ro m Re f 1. 27. 17 F igur e 1.8 Pl o t o f t h e ef f ect o f an i s o t ro py o n M CE f ro m Re f 1. 29. 18 F igur e 2.1 Ol ei c ac i d. 22 F igur e 2.2 Re act i o n apparat us 22 F igur e 2.3 T E M i m ages o f po l y h edral n an o part i cl es 24 F igur e 3.1a XR D o f co b al t f err i t e n an o part i cl es 29 F igu r e 3.1b SE M i m ages o f E DS sa m pl e. 30 F igur e 3.1c T E M i m age o f co b al t f err i t e n an o part i cl es 31 F igu r e 3.1d F C a n d Z F C c urv es f o r co b al t f err i t e n an o part i cl es 32 F igur e 3.2a T E M i m age o f m an gan es e zi n c f err i t e n an o part i cl es 33 F igu r e 3.2b F C a n d Z F C c urv es f o r m an gan es e zi n c f err i t e. 33

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v F igur e 3.3a T E M i m age o f s urf act an t -co at ed co b al t n an o part i cl es 34 c o r e s h e ll F igu r e 3.3b T E M i m age o f Co A g n an o part i cl es 34 F igur e 3.3c F C a n d Z F C c urv es f o r co b al t n an o part i cl es 35 F igu r e 3.3d Il l us t rat i o n o f s urf ace /co re spi n 36 c o r e s h e ll F igur e 3.3e F C a n d Z F C c urv es f o r Co A g n an o part i cl es 37 F ig ur e 3 .4 a ,b Cl at h rat e s t ruct ures f ro m Re f 3. 8. 38 81 63 0 F igur e 3.4c M v s T f o r E u Ga Ge T y pe I an d VII I. 39 F igur e 4.1a M CE pr o gram i n put 43 F igu r e 4.1b M CE pr o gram f i l e s el ect /sa v e o pt i o n s 43 F igur e 4.1c M CE pr o gram o perat i o n 43 F igu r e 4.1d M C E p r o g r a m p lo t o f S v s T 4 4 F igur e 4.1e M C E p r o g r a m p lo t o f M v s H 4 4 Fi g u r e 4 1 f M C E p r o g r a m p lo t o f M v s T 4 5 F igur e 4.1g M C E p r o g r a m 3 D p lo t o f S ( H T ). 45 F igu r e 4.1h M C E p r o g r a m 3 D p lo t o f M ( H T ). 46 F igur e 4.2a P lo t o f M v s H f o r co b al t f err i t e n an o part i cl es 47 F igu r e 4.2b P lo t o f M v s T f o r co b al t f err i t e n an o part i cl es 47 F igur e 4.2c P lo t o f S v s T f o r co b al t f err i t e n an o part i cl es 47 F igur e 4.3 P lo t o f S v s T f o r m an gan es e zi n c f err i t e n an o part i cl es 49 F igur e 4.4a P lo t o f M v s H f o r co b al t n an o part i cl es 50 F igu r e 4.4b P lo t o f M v s T f o r co b al t n an o part i cl es 50 F igur e 4.4c P lo t o f S v s T f o r co b al t n an o part i cl es 50

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vi c o r e s h e ll F igur e 4.5a P lo t o f M v s H f o r Co A g n an o part i cl es 52 c o r e s h e ll F igu r e 4.5b P lo t o f M v s T f o r Co A g n an o part i cl es 52 c o r e s h e ll F igur e 4.5c P lo t o f S v s T f o r Co A g n an o part i cl es 52 81 63 0 F igur e 4.6a P lo t o f M v s H f o r E u Ga Ge cl at h rat es 54 81 63 0 F igu r e 4.6b P lo t o f S v s T f o r E u Ga Ge cl at h rat es 54 81 63 0 F igur e 4.6c P lo t o f d M /d T v s T f o r E u Ga Ge T y pe I cl at h rat e. 55

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v ii M agnetocal or ic E f f ect in Nanop ar ti cl es and B ul k Cl ath r ates Drew J Re b ar Abstr act Th e m a g n e to c a l o r i c e f f e c t ( M C E ) h a s b e e n a r e c o g n i z e d p h e n o m e n o n f o r o v e r a h u n d r e d ye a r s a n d is t yp ic a lly o b s e r v e d a s t h e h e a t in g o f a ma g n e t ic ma t e r ia l w h e n a ma g n e t ic f i e l d i s a p p l i e d a n d c ool i n g of th e m a te r i al wh e n th e f i e l d i s r e m ov e d B e s i d e s l ow t em perat ure ph y s i cs M CE h as b een ut i l i zed f o r co o l i n g at h i gh er t em perat ures f o r v ari o us a p p l i c a t i o n s H i g h e f f i c i e n c y a n d n o d e p e n d e n c e o n p re s s u ri z e d g a s e s h a v e p u s h e d f o rw a rd t h e r e s e a r c h o n M C E a n d ma g n e t ic c o o lin g s ys t e ms R e s e a r c h in t o t h e M C E in n a n o p a r t ic le s y s te m s h a s m os tl y b e e n w i th i n th e l a s t tw o d e c a d e s B e s i d e s n ov e l p r op e r ti e s i n h e r e n t to n a n o p a r t ic le s ys t e ms b e n e fit s s u c h a s e a s e o f t u n a b ili t y a n d ma t e r ia ls p r o c e s s in g fo r t h in film a p p l i c a t i o ns c o nt i nu e t o f u e l t he p u r s u i t o f M C E r e s e a r c h i n na no p a r t i c l e s y s t e m s We ch ar act er i zed t h e MCE i n co b al t f er r i t e n an o par t i cl es, m an gan ese zi n c f er r i t e n an o par t i cl es, c o r e s h e ll c ob a l t n a n op a r ti c l e s a n d C o A g n a n op a r ti c l e s I n a d d i ti on w e h a d th e op p or tu n i ty to 81 63 0 c h a r a c te r i z e th e M C E i n E u G a G e T y p e I a n d T y p e V I I I c l a th r a te s T h e c ob a l t f e r r i te n a n op a r ti c l e s we re s y n th e s i z e d i n ou r l a b u s i n g a d e c om p os i ti on of m e ta l or g a n i c s a l ts t e c h n i q u e T h e p r o d u c t p a r ti c l e s w e r e c a p p e d w i th o l e i c a c i d T h e s e p a r ti c l e s w e re c h a r a c te r i z e d w i th X R D E D S a n d T E M D C m a g n e ti c c h a r a c te r i z a ti on w a s p e r f or m e d on

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v iii a l l s a m p l e s C h a n g e i n e n tr op y w a s c a l c u l a te d u s i n g a M a x w e l l ’ s r e l a ti on a n d m a g n e ti z a ti on d a t a a s a f u nc t i o n o f a p p l i e d f i e l d a nd t e m p e r a t u r e M C E s t u d i e s i n c o ba l t f e r r i t e a nd m an gan es e zi n c f err i t e rev eal ed a b ro ad b ut l o w m agn i t ude ch an ge i n en t ro py o ve r a wi de c o r e s h e ll t e m p e r a t u r e s p a n. M C E s t u d i e s i n c o ba l t a nd C o Ag r e ve a l e d a l a r g e M C E c o r r e l a t i ng wi t h a sh ar p l o w t em per at ur e t r an si t i o n asso ci at ed wi t h t h e b l o cki n g o f sur f ace spi n s. MCE s tu d i e s i n b oth c l a th r a te s r e v e a l e d a l a r g e M C E a t l ow te m p e r a tu r e s T h e s e s tu d i e s i n d i c a te t h e po ssi b i l i t y o f d u a l f un ct i o n al i t y f o r t h ese cl at h r at es si n ce t h ese m at er i al s al so po sses s ex cel l en t t h er m o el ect r i c pr o per t i es.

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1 C h a p t e r 1 Theor y, B ack gr ound and M oti vati on 1. 1 I nt r oduct ion T he m a g ne t o c a l o r i c e f f e c t i s t he f a s c i n a t in g li nk be t w e e n m a g ne t i s m a nd t h erm o dy n am i cs It i s def i n ed as t h e ch an ge i n en t ro py an d t h e ch an ge i n t em perat ure due to or d e r i n g i n a m a g n e ti c m a te r i a l v i a a n a p p l i e d f i e l d T y p i c a l l y a m a g n e ti c m a te r i a l w i th a n o t i cea b l e m agn et o cal o ri c ef f ect (MC E ) wi l l ri s e i n t em perat ure upo n a p p l i cat i o n o f an a p p l i e d f i e l d a n d w i l l l ow e r i n te m p e r a tu r e u p on r e l a x a ti on of th a t f i e l d T h e p h e n om e n on w a s d i s c o v e r e d i n 1 8 8 1 by Wa r bu r g [ 1 1 ] a nd s t u d y o f t he M C E ha s c o nt i nu e d t o t he p r e s e n t. A p p l i c a ti on of M C E b e g a n w i th th e a c h i e v e m e n t of r e f r i g e r a ti on w e l l b e l ow 1 K f or l ow te m p e r a tu r e e x p e r i m e n tati on a n d on l y r e l a ti v e l y r e c e n tl y h a s b e e n p u r s u e d f or r e f r i g e r a t i o n u p t o r o o m t e m p e r a t u r e s B e c a u s e o f ne w r e s e a r c h p o s s i bi l i t i e s a nd p r o m i s i ng appl i cat i o n s M CE wi l l co n t i n ue t o b e pursue d f ar i n t o t h e f ut ure. 1 2 T h e o ry T h e ma g n e t o c a lo r ic e ffe c t is o b s e r v e d in ma g n e t ic ma t e r ia ls w h e n a ma g n e t ic fie ld is a p p lie d t o t h e ma t e r ia l. T h e me c h a n is m fo r t h e c h a n g e i n t e mp e r a t u r e a n d t h e c h a n g e in e nt r o p y a r i s e s f r o m t he m a ni p u l a t i o n o f t he m a g ne t i c m o m e nt s i n t h e ma t e r i a l Whe n a m a g n e ti c f i e l d i s a p p l i e d to a m a te r i a l c on ta i n i n g m a g n e ti c m om e n ts th e m om e n ts c ou p l e w i th t he f i e l d a nd a l i g n w i t h t he f i e l d d i r e c t i o n. T hi s a l i g n in g o f t h e mo m e nt s r e d u c e s t he

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2 r a n d om n e s s o r m a gn e ti c e n tr op y of th e s y s te m W h e n th e f i e l d i s r e m ov e d th e m om e n ts r e l a x a nd t he m a g ne t i c e nt r o p y i nc r e a s e s T hi s c o nc e p t i s i l l u s t r a t e d in Fi g u r e 1 1 I f t he s ys t e m is k e p t u n d e r is o t h e r ma l c o n d it io n s a n d c o n s t a n t p r e s s u r e t h e t o t a l e n t r o p y w ill decrea s e wh en a f i el d i s appl i ed an d i n creas e wh en t h e f i el d i s rem o v ed. If t h e s y s t em i s kept u nd e r a d i a ba t i c c o nd i t i o ns a nd c o ns t a nt p r e s s u r e t he t o t a l e nt r o p y r e m a i ns c o ns t a nt w hi l e t he t e m p e ra t u re i n c re a s e s w h e n t h e f i e l d i s a p p l i e d a n d d e c re a s e s w h e n t h e f i e l d i s re m o v e d T h e c h a n g e in ma g n e t ic e n t r o p y r e s u lt in g fr o m t h e a p p lic a t io n o f a ma g n e t ic fie ld u nd e r c o ns t a nt t e m p e r a t u r e a nd c o ns t a nt p r e s s u r e c a n be d e r i ve d f r o m t he fo llo w i ng M ax wel l ’ s rel at i o n s h i p: wh ere S i s t o t al ent r o p y H i s a p p l i e d f i e l d M i s m a g n e t i z a t i o n T i s t em perat ure, an d P is press ure. Iso l at i n g t h e di f f eren t i al o f en t ro py an d i n t egrat i n g, t h e ch an ge i n m agn et i c en t ro py i s g i ve n by F ig ur e 1 .1 R e d u c t io n o f ma g n e t ic en t ro py b y an appl i ed f i el d. W H = 0 H

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3 Th e a b o v e M a x w e l l ’ s re l a t i o n s h i p i s a l s o u s e d i n t h e d e ri v a t i o n o f t e m p e ra t u re c h a n g e To f i nd a f o r m u l a d e s c r i bi ng t e m p e r a t u r e c ha ng e t he d i f f e r e nt i a l o f t o t a l e nt r o p y m u s t f i r s t be co n s i dered an d i s gi v en b y t h e f o l l o wi n g: C h a r a c t e r iz a t io n o f M C E is c o n d u c t e d u n d e r c o n s t a n t p r e s s u r e c o n d it io n s s o t h e la s t t e r m in t he a bo ve d i f f e r e nt i a l i s d r o p p e d T he f o l l o w i ng s u bs t i t u t i o n i s m a d e i nt o t he f i r s t t e r m o f t he d iffe r e n t ia H,P wh ere C i s he a t c a p a c i t y g i ve n t he c o nd i t i o ns o f c o ns t a nt a p p l i e d f i e l d a nd p r e s s u r e T he e q u a t i o n o n t he r i g h t i s s u bs t i t u t e d w i t h t he a bo ve M a xw e l l ’s r e l a t i o ns hi p t o p r o d u c e t he fo llo w in g d if fe r e n t ia l o f t o t a l e n t r o p y.

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4 G i ve n a d i a ba t i c c o nd i t i o ns t he e nt r o p y o f t h e s y s t e m d o e s no t c ha ng e a nd t he a bo ve e q u a t i o n c a n be s e t t o z e r o T o f i n d t h e r e la t i o ns hi p f o r t e m p e r a t u r e c ha ng e t he a bo ve e q u a t i o n c a n b e re a rra n g e d i n t h e f o l l o w i n g m a n n e r. T h e f o rm ul a f o r t em perat ure ch an ge i n i n t egral f o rm i s t h us gi v en b y t h e f o l l o wi n g. H,P H e a t c a p a c i t y C a n d m a g n e t i z a t i o n M are b o t h d e p en den t o n t em perat ure an d appl i ed f i e l d [1 .2 ,3 ]. S e v e r a l ob s e r v a ti on s a b ou t th e M C E c a n b e a s c e r ta i n e d f r om th e a b ov e e q u a ti on s f or en t ro py ch an ge an d t em perat ure ch an ge. F i rst b o t h en t ro py ch an ge an d t em perat ure ch an ge are pro po rt i o n al t o t h e d e r i vat i v e o f m agn et i zat i o n wi t h respe ct t o t em perat ure. Si n ce m a g ne t i z a t i o n t y p i c a l l y d e c r e a s e s w i t h i nc r e a s i ng t e m p e r a t u r e i n f e r r o m a g ne t i c m a t e r i a l s t he d e r i vat i v e o f m agn et i zat i o n wi l l h av e a n egat i v e v al ue t h us m aki n g t h e en t ro py ch an ge a n e g a t iv e v a lu e a n d t h e t e mp e r a t u r e c h a n g e a p o s it iv e v a lu e W h e n t h e a p p lie d fie ld is 1 2 re m o v e d o r d e c re a s e d i .e H > H i n t h e i n t egral l i m i t s t h e v al ue f o r en t ro py ch an ge b eco m es p o s it iv e a n d t h e v a lu e fo r t e mp e r a t u r e c h a n g e b e c o me s n e g a t iv e T o s u mma r iz e a ma g n e t ic m a te r i a l u n d e r a d i a b a ti c c on d i ti on s w i l l e x p e r i e n c e a r i s e i n te m p e r a tu r e u p on a p p l i c a ti on of a m agn et i c f i el d an d a l o weri n g i n t em perat ure upo n rem o v al o f t h at f i el d.

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5 A s e c on d ob s e r v a ti o n t h a t c a n b e r e a c h e d f r om th e a b ov e e q u a ti on s i s th a t b oth e nt r o p y c ha ng e a nd t e m p e r a t u r e c ha ng e a r e p r o p o r t i o na l t o t he c ha ng e i n a p p l i e d f i e l d T he M C E me a s u r e d a t t w o d iffe r e n t a p p lie d fie ld s t r e n g t h s in t h e s a me ma t e r ia l s h o u ld r e s u lt in a h i g h e r v a l u e o f M C E a t t h e h i g h e r a p p l i e d f i e l d A t h i rd o b s e rv a t i o n i s t h a t t h e t e m p e ra t u re ch an ge i s pro po rt i o n al t o t h e i n i t i al t e m perat ure an d i n v erse l y pro po rt i o n al t o t h e h eat capa ci t y at co n s t an t appl i ed f i el d an d press ure. T y pi cal l y a l arge v al ue f o r en t ro py ch an ge wi l l co rr el at e wi t h a l arge v al ue f o r t em perat ure ch an ge b ut n o t n ece s s ari l y i f t h e m at eri al h as a re l a t i v e l y h i g h v a l u e f o r h e a t c a p a c i t y A f o u r t h o bs e r va t io n i s t ha t s i nc e l a r g e c ha ng e s i n m a g ne t i z a t i o n a c c o m p a ny tr a n s i ti on s i n m a g n e ti c m a te r i a l s th e M C E w i l l b e g r e a te s t a r ou n d th os e tr a n s i ti on s s i n c e b oth e n tr op y a n d te m p e r a tu r e c h a n g e a r e p r op or ti on a l to t h e d e r i v a ti v e of m a g n e ti z a ti on w i th r e s p e c t t o t e m p e r a t u r e An e xa m p l e o f a t r a ns i t i o n w hi c h w o u ld a f f e c t t he M C E i s t he f er r o m agn et i c t o par am agn et i c t r an si t i o n at t h e Cur i e t em per at ur e i n par am agn et i c m at er i al s. M a xi m u m s o r m i ni m u m s i n t e m p e r a t u r e c ha ng e c a n a l s o be i nf l u e nc e d by t r a ns i t i o ns i n t he h e a t c a p a c i t y o f t h e m a t e ri a l T o cl earl y el uci dat e t h e rel at i o n s h i p b et wee n en t ro py ch an ge an d t em perat ure ch an ge in ma g n e t ic ma t e r ia l, t w o e x a mp le e n t r o p y c u r v e s a t d iffe r e n t a p p lie d fie ld s a r e d is p la ye d in Fi g u r e 1 2 I n t he g r a p h, t he i s o t he r m a l d i f f e r e nc e i n e nt r o p y i s d r a w n a s a ve r t i c a l l i ne a nd th e i s e n tr op i c ( a d i a b a ti c ) d i f f e r e n c e i n te m p e r a tu r e i s d r a w n a s a h or i z on t a l l i n e B oth di f f er en ces b egi n at t h e sam e t em per at ur e an d o ccur du e t o t h e ch an ge i n app l i ed f i el d. T h ese l i n e s o f e n t ropy c a n b e c a l c u l a t e d f rom h e a t c a p a c i t y m e a s u re m e n t s w h i c h i n t u rn

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6 s imu lt a n e o u s ly g iv e e n t r o p y c h a n g e a n d t e m p e r a t u r e c h a n g e T h e fo r mu la u s e d fo r t h is purpo s e i s t h e f o l l o wi n g: 0,H wh ere S i s t he z e r o t e m p e r a t u r e e nt r o p y T he z e r o t e m p e r a t u r e e nt r o p y i s a s s u m e d t o be zero s o t h i s t e r m i s dro pped. Heat capa ci t y i s m eas ured at zero f i el d an d n o n zero f i el d, e n tr op y i s c a l c u l a te d f or b oth c on d i ti on s u s i n g th e a b ov e r e l a ti on a n d i s oth e r m i c p oi n ts on t h e t wo curv es are s ub t ract ed t o gi v e S [1 .2 ,3 ]. A n o t h er f act o r h as b een det erm i n ed t o i n f l uen ce t h e m ax i m um M CE a m at eri al can p o s s e s s T h e fo llo w in g fo r mu la s g iv e t h e ma g n e t ic e n t r o p y fo r t w o t yp e s o f ma g n e t ic J ma t e r ia ls b a s e d o n t o t a l a n g u la r mo me n t u m J C u r i e c on s ta n t C a p p l i e d f i el d H a nd t em perat ure T F ig ur e 1 .2 E n t ro py curv es f ro m Re f 1. 3.

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7 ( p a r a m a g n e t) C (f e rrom a g n e t T > T) M ax i m um en t ro py i n a m agn et i c m at eri al wo ul d co m e f ro m l et t i n g H = 0 an d T 4 Us ing t h ose conditions bot h of t h e above eq uat ions for ent ropy w ould reduce t o t h e f ollowing eq uat ion MA wh ere S i s t h e m a x i m u m e n t ropy p e r m o l e o f a t o m s c o n t a i n i n g m a g n e t i c m o m e n t s N is B A v o g a d ro’ s n u m b e r, k i s t h e B o l t zm an n co n s t an t an d R i s t h e id e a l g a s c o ns t a nt T he i m po rt an ce o f t h i s f o rm ul a i s t h e rev el at i o n t h at t h e m ax i m um en t ro py an d m ax i m um en t ro py c h a n g e a r e d e p e n d e n t o n t o t a l a n g u la r mo me n t u m a n d t h e r e fo r e t h e e ffe c t iv e ma g n e t ic m o m en t per at o m In addi t i o n m ax i m um m agn et i c en t ro py an d m ax i m um m agn et i c en t ro py c h a n g e b y d e f i n i t i o n h a v e f i n i t e l i m i t s [1 .2 ]. I n s u mma r y, a ma g n e t ic ma t e r ia l c o u ld e x h ib it a la r g e M C E v a lu e if o n e s o me o r a ll o f t he f o l l o w i ng c ha r a c t e r i s t i c s w e r e f o u nd i n t he m a t e r i a l A l a r g e va l u e f o r t he d e r i va t i ve

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8 o f m agn et i zat i o n wi t h r e s p e ct t o t em perat ure ( M M / M T ) is n e c e s s a r y fo r a la r g e c h a n g e in en t ro py an d t em perat ure. A l arge m agn e t i c m o m en t wo ul d al l o w f o r a l arge m ax i m um en t ro py an d resu l t i ng e nt ro py ch an ge. A s s o ci at ed wi t h a l arge m agn et i c m o m en t a h i gh sat u r at i o n m a g n e t i z a t i o n w o u l d a l l o w f o r t h e p o s s i b i l i t y o f a l a rg e v a l u e f o r M M / M T o v e r a wi der t em perat ure spa n T o m ax i m i ze t h e t em perat ure c ha ng e a l o w h eat capa ci t y at c o n s t a n t p re s s u re w o u l d n e e d t o b e c h a ra c t e ri s t i c o f t h e m a t e ri a l 1 3 B a c kg r o u n d : B u lk M a t e r i a ls M o s t m at eri al s t h at h av e b een ch aract eri zed f o r t h e m agn et o cal o ri c ef f ect h av e b een b u l k m a te r i a l s T h e d r i v i n g f or c e b e h i n d m u c h of th i s r e s e a r c h h a s b een th e a p p l i c a ti on of m agn et i c co o l i n g. A pp l i cat i o n o f MCE b egan wi t h l o w t em per at ur e co o l i n g. On e o f t h e f i r st r e p o r t s o f l o w t e m p e r a t u r e c o o l i ng c a m e f r o m W G ia u q u e a nd D M a c D o u g a l l a t t he Un i v er s i t y o f C a l i f o rn i a, Berkel ey i n 1933. Us i n g l i qui d h el i um t o b ri n g t h e i n i t i al 2 4 32 t e m pe rat ure o f Gd (SO ) C 8H O do wn t o 1. 5 K t h e l o w t em perat ure o f 0. 25 K was reach ed w i th a f i e l d of 8 0 0 0 G [ 1 4 ] T h e c h a r a c te r i z a ti on a n d a p p l i c a ti on of oth e r p a r a m a g n e ti c s a l ts s u c h a s i r on a l u m [1.5 ] c h r om i u m m e th y l a m i n e a l u m [ 1 6 ] a n d c e r i u m m a g n e s i u m n i tr a te [ 1 7 ] c o nt i nu e d i n t he p u r s u i t o f e ve r l o w e r t e m p e r a t u r e s M C E w a s f o u nd t o e xi s t o n t he n ucl ear l ev el an d a p p l i c a t i o n o f t h i s ef f ect h as co n t i n ued t o t h e pres en t day In 1970, 53 r e s e a r c h e r s K A n d r e s a n d E B u c h e r s t u d ie d P r P t a n d P r T l a n d fo u n d t h a t t h e a d ia b a t ic d e ma g n e t iz a t io n o f t h e n u c le a r ma g n e t ic mo me n t s in t h e s e V a n V le c k p a r a ma g n e t ic ma t e r ia ls pr o du ce d f i n a l te m p e ra tur e s of 0 .0 0 5 K a n d 0 .0 0 3 K re s p e c ti v e l y [1 .8 ]. In 2 0 0 0, J. T u o r i ni e m i a nd T Knu u t t i l a a c hi e ve d a nu c l e a r s p i n t e m p e r a t u r e o f 2 5 0 p K i n r ho d i u m u s i ng

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9 a t w o s t a g e n u c l e a r a d i a b a t i c d e m a g n e t i z a t i o n re f ri g e ra t o r w i t h c o p p e r a s t h e f i rs t s t a g e [1 .9 ]. In t h e s am e y ear, T K n uut t i l a repo rt ed i n h i s Sc. D. di s s ert at i o n t h e ach i ev em en t o f l es s t h an 100 pK i n a rh o di um n ucl ear s pi n s y s t em [1. 10]. W h i l e t h e ac h i ev em en t o f l o w t em perat ures h as co n t i n ued t o t h e pres en t i n t e r e s t i n m agn et i c co o l i n g h as al s o di v erged t o h i gh er te m p e r a tu r e s A p p l i c a ti on s of m a g n e ti c c ool i n g a t h i g h e r te m p e r a tu r e s i n c l u d e l i q u e f a c ti on o f gas es s uc h a s hy dro gen an d h el i um an d a m y ri ad o f appl i cat i o n s at ro o m t em perat ure. R e f r i g e r a t i o n t e c hni q u e s a l r e a d y e xi s t f o r a l l o f t he s e t e m p e r a t u r e r a ng e s M a g ne t i c c o o l i ng i s p u r s u e d be c a u s e o f t he hi g h e f f i c i e nc y i nhe r e nt t o s u c h s y s t e m s S e ve r a l g r o u p s a r o u nd t he g l o be ha ve d e m o ns t r a t e d t hi s hi g h e f f i c i e nc y i n a c t u a l d e vi c e s O ne d e vi c e w a s d e s i g ne d a nd b ui l t b y rese arch ers at t h e A m es L ab o rat o ry an d A s t ro n aut i cs Co rpo rat i o n o f A m eri ca. T h ey repo rt ed a c o o l i n g po wer o f 600 W us i n g an a p p l i ed f i el d o f 50 kOe. T h e ref ri geran t was b al l s o f t h e l an t h an i de m et al gado l i n i um an d a t em perat ure gr adi en t o f 10 K was ach i ev ed i n t h e s y s t em wi t h a Ca rn o t ef f i ci en cy n ear 75%. T h e dev i ce h ad a c o ef f i ci en t o f perf o rm an ce o f 10 wh i ch gr eat l y o ut r an ks t h e t y pi cal r at i n g o f 24 f o r co n v en t i o n al gas co o l i n g t ech n i qu es. Gr eat er ef f i ci e nc y t r an sl at es t o m o n ey sav i n gs an d r esear ch i m pet us f o r ex pl o r i n g t h ese m at eri al s [1. 11]. In addi t i o n t o m o n ey s av i n gs m o re ef f i ci en t m et h o ds o f l i quef y i n g h y dro gen us i n g m agn et i c ref ri gerat i o n b ri n g s o ci et y cl o s er t o t h e pract i cal appl i cat i o n o f h y dro gen as a f u e l [ 1 1 2 ] B e s i d e s t he be ne f i t o f hi g h e f f i c i e nc y m a g ne t i c c o o l i ng d o e s no t r e q u i r e a ny p r e s s u r e d g a s C on ta i n m e n t of a p r e s s u r i z e d g a s a n d p ote n ti a l h a r m of th e e n v i r on m e n t f r om c e r t a in g a s e s a r e e lim in a t e d in a ma g n e t ic c o o lin g s ys t e m. S i nc e m a g ne t i c r e f r i g e r a t i o n d r i ve s m o s t o f t he c u r r e nt r e s e a r c h o n M C E m u c h o f t he r e s e a r c h p u r s u e s c e r t a i n q u a l i t i e s t ha t w o u l d e nha nc e t he e f f i c i e nc y o f m a g ne t i c c o o l i ng T he

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10 f i rst o b v i o us ch aract eri s t i c i s a l a r g e m a gn i t ude o f M CE i n a m at eri al T o t h i s en d, la n t h a n id e s a n d c o mp o u n d s a n d a llo ys o f l a n t h a n id e s a r e p u r s u e d fo r t h e ir la r g e ma g n e t ic m o m en t A s des cri b ed ea rl i er, t h e m ax i m um t h eo ret i cal en t ro py ch an ge depe n ds o n t h e t o t al a n g u l a r m o m e n t u m J T he l a r g e a va i l a bl e s p i n i n l a nt ha ni d e s h a s p u s h e d t he m t o t he f o ref ro n t i n M CE r es earch [1. 3]. A s eco n d ch aract eri s t i c i s a l arge M CE o v er a wi de t e mp e r a t u r e r a n g e T h e fo ll o w ing fo r mu la is u s e d t o d e s c r ibe t h e r e fr ig e r a n t c a p a c it y o f a ma t e r ia l: wh ere q i s t h e h eat cal cul at ed f ro m t h e i n t egral o f en t ro py ch an ge o v er t h e t em perat ure r an ge b e t w e e n t h e h o t a n d c o ld s in k t e mp e r a t u r e s [ 1 1 3 ] I n lig h t o f t h is t h e mo s t d e s ir a b le b e h a v i o r f o r e n tr o p y c h a n g e i s a b r o a d l e v e l o f c h a n g e o v e r a w i d e te m p e ra t u re ra n g e A t hi r d c ha r a c t e r i s t i c i s a hi g h t he r m a l c o nd u c t i vi t y T hi s d e s i r a bl e c ha r a c t e r i s t i c f a c i l i t a t e s t he h e a t e x c h a n g e i n a m a g n e ti c r e f r i g e r a ti on s y s te m A f ou r th c h a r a c te r i s ti c i s ov e r a l l c os t of t h e ref ri geran t m at eri al W h i l e t h e l an t h an i des pro v i de a l a r g e t h eo ret i cal en t ro py ch an ge, t h ese m at er i al s al o n g wi t h t h e t r an si t i o n m et al s o f h i gh at o m i c m ass are a l so t h e m o st e x p e n s i v e m a te r i a l s pu rs ue d i n M C E r e s e a r c h T a k i n g a l l of th e s e c h a r a c te r i s ti c s i n to acc o un t m a te r i a l s a r e p u r s u e d th a t h a v e a g ood b a l a n c e of l a r g e b r oa d M C E w i th h i gh t h e rm a l c o n d u c t i v i t y a n d l o w c o s t s [1 .1 4 ]. A h os t of b u l k m a te r i a l s h a s b e e n c h a r a c te r i z e d f or th e m a g n e toc a l or i c e f f e c t. A g ood s u m m a r y o f m a ny o f t he s e m a t e r i a l s i s c o nt a i ne d i n R e f 1 2 As w a s m e nt i o ne d e a r l i e r t he ma x imu m M C E i n a m a t e r ia l is e x p e c t e d a r o u n d t r a n s it io n s s in c e t h is is w h e r e s lo p e s in

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11 m a g n e t i z a t i o n h a v e t h e i r l a rg e s t v a l u e Som e o f t h e s e t ra n s i t i o n s a n d e x a m p l e m a t e ri a l s a re t h e f o l l o wi n g. T h e f err o m agn et i c t o param agn et i c t ran s i t i o n i s t h e m o s t ch aract eri zed t r a ns i t i o n f o r t he M C E O ne m a t e r i a l w hi c h s ho w s t h is t r a ns i t i o n a nd a c c o m p a ny i ng m a x i m u m e n t ropy c h a n g e a t t h e t ra n s i t i o n i s c o b a l t Th i s m e t a l e x h i b i t s a C u ri e t e m p e ra t u re o f 1, 394 K an d a t em perat ure ch an ge o f appro x i m at el y 3. 3 K f o r an appl i ed f i el d o f 21. 6 kOe [ 1 1 5 1 6 ] A n oth e r tr a n s i ti on s e e n i n s om e c om p ou n d s i s a n a n ti f e r r om a g n e ti c to f e r r om a g n e ti c tr a n s i ti on O n e m a te r i a l w h i c h e x h i b i ts th i s tr a n s i ti on i s F e R h T h e tr a n s i ti on o c c u r s a t t h e t e mp e r a t u r e o f 3 1 6 K a n d t h e ma g n it u d e o f t h e t e mp e r a t u r e c h a n g e is app r o x i m at el y 8 K f o r an app l i ed f i el d o f 2. 5 T T h e un usual ph en o m en o n o f n egat i v e MCE o c c u r s w i t h t hi s m a t e r i a l m e a ni ng t ha t t he t e m p e r a t u r e l o w e r s u p o n a p p l i c a t i o n o f a f i e l d a nd r i ses wh en t h e f i el d i s r em o v ed [ 1. 17 ] A n o t h er t r an si t i o n pr ev al en t am o n g m an gan i t es i s a c ha r g e o r d e r t r a ns i t i o n be t w e e n a n a nt i f e r r o m a g ne t i c p ha s e a nd f e r r o m a g ne t i c p ha s e An 0.5 0.5 3 e x a m p l e of a m a n g a n i te w i th th i s tr a n s i ti on i s N d S r M n O T h e c h a r g e or d e r tr a n s i ti on o c c u rs a t a p p rox i m a t e l y 1 5 5 K a n d t h e e n t ropy c h a n g e f o r a n a p p l i e d f i e l d o f 1 0 k O e i s 2 .8 J /kgK T h i s m a t e r i al al s o po s s es s es a t ran s i t i o n b et wee n t h e f err o m agn et i c ph as e an d a param agn et i c ph as e wi t h a C uri e t em perat ure o f appro x i m at el y 240 K A s m al l er m ax i m um i n en t ro py ch an ge o ccurs at t h e C uri e t em perat ure wi t h a v al ue o f appro x i m at e l y 1 J /kgK [ 1 1 8 ] O n e la s t e x a mp le o f a t r a n s it io n is t h e s t r u c t u r a l t r a n s it io n b e t w e e n t w o fe r r o ma g n e t ic 5 22 ph as es f o un d i n t h e “ gi an t ” M CE m at eri al Gd (S i Ge ). T h i s s t ruct ural t ran s i t i o n o ccurs at 2 7 6 K a n d is in c lo s e p r o x imi t y t o t h e fe r r o ma g n e t ic t o p a r a ma g n e t ic t r a n s it io n a t t h e C u r ie t e mp e r a t u r e o f 2 9 9 K T h e ma x ima fo r e n t r o p y a n d t e mp e r a t u r e c h a n g e o c c u r a p p r o x ima t e ly at t h e s t ruct ural t ran s i t i o n t em perat ure an d h av e m agn i t udes o f 14 J /kgK a nd 7 4 K f o r an

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12 appl i e d f i el d o f 20 kOe. F i gure 1. 3 f ro m Re f 1. 19 d i s p l a y s t h e t e m p e ra t u re ch an ge o f t h i s m at eri al co m pared t o gado l i n i um wh i ch al s o po s s es s es a l arge M C E [1 .1 9 ]. 1 4 B a c kg r o u n d : N a n o ma t e r i a ls O n e o f t h e n e w e s t c la s s e s o f ma t e r ia ls t o b e c h a r a c t e r iz e d fo r t h e ma g n e t o c a lo r ic e ffe c t h a s b e e n n a n o ma t e r ia ls M a g n e t ic n a n o p a r t ic le s t h a t h a v e a s iz e b e lo w t h e s iz e d o ma in lim it a r e k n o w n t o e x h ib it s u p e r p a r a ma g n e t is m. E a c h n a n o p a r t ic le b y it s e lf is fe r r o ma g n e t ic w i t h a l a r g e m o me n t b u t t he r e i s no l o ng r a ng e c o r r e l a t i o n w i t h o t he r ne i g hbo r i ng na no p a r t i c l e s S o e a c h na no p a r t i c l e be ha ve s a s a “su p e r s p i n” w i t h t he w ho l e s y s t e m s ho w i ng a param agn et i c res po n s e. T h i s m agn et i c ch aract er i s t erm ed s uperparam agn et i s m A paper b y M c M ic h a e l, e t a l. e lu c id a t e d t h e p o s s ib ili t y o f e n h a n c e me n t o f M C E in s u p e r p a r a ma g n e t ic m a t e r i a l s o ve r p a r a m a g ne t i c m a t e r i a ls T hi s w a s d e r i ve d f r o m a c l a s s i c a l t r e a t m e nt o f t he 5 22 F ig ur e 1 .3 M CE i n Gd (S i Ge ) an d Gd f ro m Re f 1. 19.

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13 m agn et i c m o m en t s i n s uperparam agn et i c cl us t ers. T h e f o rm ul a f o r en t ro py i n t h e cl as s i cal cas e i s t h e f o l l o wi n g: wh ere N i s t h e n u m b e r of c l u s t e rs k i s B o l t z m a n n ’ s c o n s t a n t m i s th e m a g n e ti c m om e n t of C C a c l u s t e r, H i s appl i ed f i el d, an d T is t e mp e r a t u r e E n t r o p y c h a n g e is g iv e n b y S = S ( H ) CC – S (0 ) w h i c h p rodu c e s t h e f o l l o w i n g e x p re s s i o n f o r S Th i s e q u a t i o n w a s re w ri t t e n w i t h t h e s u b s t i t u t i o n x = mH / kT I n a d d i t i o n, bo t h s i d e s o f t he e q u a t io n w e r e d iv id e d b y x N an d k t h us gi v i n g t h e f o l l o wi n g f o rm ul a.

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14 C0 F ig ur e 1 .4 P l ot of T S / MH v s x f ro m Re f 1. 20. 0 Th e v ar i a b l e M i s t he s a tu r a ti on m a g n e ti c m om e n t. T he l e f t ha nd s i d e o f t he eq u at i o n was pl o t t ed as a fu n c t io n o f x ( Fi g u r e 1 4 ) a nd th e m a x i m u m of th e f u n c ti on wa s f ou n d a t x 3 5 G i v en t hi s i nf o r m a t io n, t he re l a t i o n s h i p f o r m a x i m u m e n t ropy c h a n g e c o u l d b e e v a l u a t e d C Th e e q u a t i o n f o r S c ou l d a l s o b e u s e d to d e s c r i b e a p a r a m a g n e t. G i v e n th e c on d i ti on of C l o w appl i ed f i el d m agn i t ude ( x n 1 ), t h e f o l l o w i n g i s t h e e q u a t i o n f o r S : wh ere N i s t h e n um b er o f m agn et i c m o m en t s an d m i s a s i ng l e m a g n e t ic m o m e nt i n t he m a t e r i a l Fr o m t he p r e vi o u s e q u a t i o ns s o m e r e l a t i o ns hi p s c a n be c o nc l u d e d Fr o m t he e q u a t i o n f o r x t h e v al ue f o r a cl us t er m agn et i c m o m en t wi t h regard t o m ax i m um en t ro py can C b e s o l v e d g i v e n a p a rt i c u l a r a p p l i e d f i e l d a n d t e m p e ra t u re F rom t h e e q u a t i o n s f o r S i n t he s u p e r p a r a m a g ne t i c a nd p a r a m a g ne t i c c a s e s u p e r p a r a m a g ne t i c e nt r o p y c ha ng e s ho u l d be

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15 0.85 0.15 F ig ur e 1 .5 M C E i n G d Y f rom R e f 1 .2 2 gr eat er at l o wer app l i ed f i el ds an d h i gh er t em per at ur es si n ce t h e dep en den ces o ccur as H/ T f o r t h e s uperparam agn et i c ca s e an d H / T f o r t h e p a ra m a g n e t i c c a s e [1 .2 0 ]. 22 In an o t h er earl y paper, R. Sh ul l di s cus s ed t h e adv an t ages o f go i n g t o t h e n an o s cal e. A s t udy o f en t ro py ch an ge i n a n an o co m po s i t e o f GGIG rev eal ed i t t o h av e a l arger en t ro py ch an ge t h an GGG wh i ch was t h e ref ri geran t o f ch o i ce i n t h e l o w t em perat ure regi m e at t h at 0.85 0.15 t i m e [1 .2 1 ]. S h a o e t a l c o n d u c t e d s t u d i e s o f v a ri o u s n a n o c o m p o s i t e ri b b o n s G d Tb 0.85 0.15 0.75 0.25 G d Y a nd G d Z n ba l l m i l l e d p o w d e r s w e r e p l a c e d i nt o s e p a r a t e c o p p e r t u be s a nd 0.85 0.15 press e d i n t o ri b b o n s Of t h e t h ree, o n l y t h e Gd Y n an o co m po s i t e ri b b o n ex h i b i t ed a l a r g e r M C E th a n th e p a r e n t b u l k m a te r i a l T h e e n tr op y c h a n g e a n d te m p e r a tu r e c h a n g e f or 0.85 0.15 Gd Y (Ref 1. 22) are di s pl ay ed i n F i gure 1. 5. T h i s s t udy pro v i ded reas o n ab l e ev i den ce th a t th e te n d e n c y of M C E e n h a n c e me n t in n a n om a te r i a l s i s n ot g u a r a n te e d s i n c e tw o of t h e t h r e e ma t e r ia ls d id no t s ho w e nha nc e m e nt o v e r t h e o r ig in a l b u lk m a t e r i a l [ 1 2 2 2 3 ] I n a s t u d y b y Ta n a k a e t a l ., si ze ef f ect s o f t h e MCE i n a n a n oc om p os i te 23 c o n s is t ing o f g Fe O

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16 n a n op a r ti c l e s i n a s i l v e r m a tr i x w e r e e v a l u a te d U s i n g a s i m i l a r p r oc e d u r e a s M c M i c h a e l e t. al di s cus s ed ab o v e, m a xi m um en t ro py was an al y zed w i t h t h e adde d f eat ure o f s i ze d is t r ib u t io n a mo n g t h e p a r t ic le s F ig u r e 1 6 fr o m R e f. 1 2 4 c o n t a in s t h e r e s u lt s o f t h is s t u d y. T h e co n cl us i o n o f t h e aut h o rs t h at can readi l y b e o b s erv ed i n F i gure 1. 6 i s t h at a n arr o w s i ze d i s t r i bu t i o n w i l l e nha nc e t he M C E i n a na no c o m p o s i t e T he na r r o w e s t s i z e d i s t r i bu t i o n i n t he f i g u r e a tta i n s th e h i g h e s t v a l u e a n d w i d e s t c u r v e [ 1 2 4 ] I n a r e l a te d p a p e r b y Y a m a m oto e t. 23 al t h e ent r o p y change o f g F e O n a n op a r ti c l e s i n a s i l v e r m a tr i x w a s c om p a r e d w i th i r on a m m o ni u m a l u m s i nc e bo t h m a t e r i a l s c o nt a i ne d Fe a nd w o u l d be i nf o r m a t i ve o f t he 3+ di f f eren ce i n M CE b et wee n s uperparam agn et i c an d param agn et i c m at eri al s T h ei r f i n di n gs s u p p o r t e d t he p r e m i s e o f M C E e nha nc e m e nt i n s u p e r p a r a m a g ne t i c m a t e r i a l s b e c a u s e t he n a n oc om p os i te w a s f ou n d to h a v e a n e n tr op y c h a n g e th a t w a s a p p r ox i m a te l y tw o or d e r s of 23 m agn i t ude l arger t h an t h e en t ro py ch an ge o f t h e b ul k al um [1. 25]. T h e g -F e O n an o part i cl es 34 i n a s i l ver m a t r i x we r e a g a i n co m p a r e d w i t h a s i m i l a r nano c o m p o si t e w hi c h co nsi st e d o f Fe O 34 an d go l d. T h e n an o co m po s i t e o f F e O an d go l d was f o un d t o h av e a l arger en t ro py ch an ge F ig ur e 1 .6 E f f ect o f s i ze di s t ri b ut i o n o n m ax i m um en t ro py f ro m Re f 1. 24.

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17 23 34 t ha n g F e O n a n o p a r t ic le s in a s ilv e r ma t r ix H o w e v e r t h e e n h a n c e d M C E in t h e F e O / g o ld n a n o c o m p o s i t e i s o n t h e o rd e r of 1 0 J /k g K [1 .2 6 ]. -2 O t h e r c o r r e la t io n s h a v e b e e n s t u d ie d w it h t h e M C E in n a n o ma t e r ia ls G ig u e r e e t a l. 70 30 30 70 c ha r a c t e r i z e d t h e D y Z r a nd D y Z r a l l o y s w hi c h be ha ve d a s na no c o m p o s i t e s s i nc e t he tw o m e ta l s w e r e i m m i s c i b l e T h e c l u s te r s of d y s p r os i u m w e r e m a g n e ti c a n d s e p a r a te d f r om e a c h o t h e r b y n on m a g n e ti c z i r c on i u m Z e r o f i e l d c oo l e d a n d f i e l d c oo l e d s u s c e p ti b i l i ty m e a s u r e m e n ts w e r e m a d e of b oth m at eri a l s r e v e a l i n g a b l oc k i n g tr a n s i ti on te m p e r a tu r e of 30 70 a p p r o xi m a t e l y 1 2 0 K. Fi g u r e 1 7 d is p la ys t he e nt r o p y c ha ng e f o r D y Z r ( o p e n) a nd 70 30 Dy Z r (so l i d) i n an appl i ed f i e l d of 7 T T h e e n tr opy 70 30 c h a ng e f o r D y Z r is p a r t i c u l a r ly l a r g e a nd t he m ax i m a f o r t h e t wo n a n o c o mp o s it e s a p p r o x ima t e ly al i gn T h i s r e s e a rch d e m on s tr a te s th e p os s i b i l i ty of t u n i n g t h e m a x i m u m M C E i n a m a t e ri a l w h i l e n o t s h i f t i n g t h e t e m p e ra t u re o f t h e p e a k In a d d i t i o n t h e m a x i m u m e n t ropy c h a n g e c o rre l a t e s w i t h t h e b l o c k i n g t ra n s i t i o n a t 1 2 0 K [1 .2 7 ]. T h e co rr el at i o n o f m ax i m um en t ro py ch an ge wi t h t h e b l o cki n g t ran s i t i o n h as b een o b s erv ed 12 i n o t h er sy s t em s T o r r e s e t a l repo rt ed a s t udy o n t h e h i gh s pi n cl us t er, M n wh i ch e xhi bi t e d a l a r g e e nt r o p y c ha ng e o f a p p r o xi m a t e l y 2 7 5 J / k g K i n a f i e l d o f 3 T a t t he bl o c k i ng te m p e r a tu r e of 3 K [ 1 2 8 ] E f f e c ts of a n i s otr op y h a v e a l s o b e e n c h a r a c te r i z e d i n r e g a r d s to 30 70 70 30 F ig ur e 1 .7 M C E fo r D y Z r a n d D y Z r fr o m R e f. 1. 27.

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18 F ig ur e 1 .8 Pl o t o f t h e ef f ect o f an i s o t ro py o n M CE f ro m Re f 1. 29. t he m a g ne t o c a l o r i c e f f e c t X Z ha ng e t a l c o nd u c t e d a s t u d y o n a ni s o t r o p i c e f f e c t s o n t he 8 M C E in F e c lu s t e r s F ig u r e 1 8 d is p la ys t h e r e s u l t s f r o m t h e ir s t u d y. F o r t h e a n is o t r o p ic s ys t e m, t h e e n t r o p y c h a n g e w a s ma x imu m fo r t h e a p p lie d fie ld p a r a lle l t o t h e e a s y a x is w h ile th e e n tr op y c h a n g e m i n i m i z e d f or th e a p p l i e d f i e l d p a r a l l e l to th e h a r d a x i s T h e c om p a r i s on i s o t r o p i c s y s t e m i s s i m u l a t e d i n t he f i g u r e a nd e xhi bi t s i nc r e a s i ng e nt r o p y c ha ng e w i t h no m a x i m u m a s t h e t e m p e ra t u re d e c re a s e s f o r t h e d i s p l a y e d ra n g e [1 .2 9 ]. 1.5 R e s e ar c h M otiv atio n and L ay out R e s e a r c h o f t he m a g ne t o c a l o r i c e f f e c t i n na no m a t e r i a l s i s va s t l y o u t f l a nk e d by t he r e s e a r c h o f b u lk s ys t e ms B u lk s ys t e ms c o n t in u e t o d o min a t e M C E r e s e a r c h b u t ma n y v ia b le rese arch pat h s s t i l l ex i s t f o r MC E re s earch i n t o n an o m at eri al s By go i n g t o t h e n an o s cal e, ma g n e t ic p r o p e r t ie s o f ma t e r ia ls c h a n g e a n d p r o v id e fr e s h g r o u n d s fo r r e s e a r c h in t o ma t e r ia ls p r e v io u s ly c h a r a c t e r iz e d in b u lk fo r m. M a g n e t i c p r o p e r t ie s a r e e a s ily ma n ip u la t e d in

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19 na no c o m p o s i t e s by c ha ng i ng p a r t i c l e s i z e a n d s p a c ing i ns t e a d o f s t o i c hi o m e t r y o f t he c o m p o u nd S u p e r p a r a m a g ne t i s m i s a no ve l p he no m e no n o bs e r ve d o nl y i n na no m a t e r i a l s a nd n o t i n b ul k m agn et i c m a t e r i a l s. F o r cer t ai n app l i cat i o n s such as o n si t e co o l i n g f o r ch i ps, M E M S N E M S e t c na no m a t e r i a l s m i g ht be t he be s t r o u t e f o r c o o l i ng s i nc e t he c o o l i ng ef f ect wo ul d b e l o cal i zed a n d eas i l y m a ni p u l at ed b y depo s i t i o n ro ut es s uch as L an gm ui rBl o dget t l ay eri n g o r spi n -co at i n g. F o r t h es e reas o n s n an o m at eri al s were pursue d f o r t h i s rese arch p r o j e c t Co b al t f err i t e, m an gan es e zi n c f err i t e, co b al t an d s i l v erco at ed c o ba lt na no p a r t i c l e s w e r e c ha r a c t e r i z e d f o r t he m a g ne t o c a l o r i c e f f e c t Al o ng t he w a y t he 81 63 0 o p p o r t u ni t y c a m e f o r M C E r e s e a r c h o f T y p e I a nd V I I I E u G a G e c l a t hr a t e s T he cl at h rat es were i n b ul k f o rm b ut co n t ai n ed m agn et i c i o n s o f euro pi um wh i ch were s eparat ed by r e l a t i ve l y l a r g e d i s t a nc e s a nd o nl y w e a k ly in t e r a c t e d w i t h t he i r c a g e s o f g a l l i u m a nd g e r m a n i u m B e c a u s e o f th i s th e s e m a te r i a l s w e r e a k i n to n a n oc om p os i te s th ou g h n ot n a n o s t ru c t u re d b y s t ri c t d e f i n i t i o n It i s t o b e em ph as i zed t h at M CE ex peri m en t s are n ew t o o ur F un ct i o n al M at eri al s L ab a t U S F I h a d th e r a r e op p or tu n i ty of p a r ti c i p a ti n g i n th i s p r oj e c t r i g h t f r om th e s ta g e of co n cept i o n M CE r es earch i n o ur l ab i s f un ded b y a gran t f ro m t h e Na t i o n al Sc i e n ce F o u n d a t io n I n t h e f o l l o w in g c h a p t e r s I w ill p r e s e n t o u r e ffo r t s in s yn t h e s is ma g n e t ic c h a ra c t e ri z a t i o n M C E m e a s u re m e n t s a n d a n a l y s i s o f M C E re s u l t s

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20 C h a p t e r 2 M a t e r i a ls 2. 1 I nt r oduct ion F o u r n a n o p a r t ic le s ys t e ms a n d t w o c la t h r a t e s a mp le s w e r e c h o s e n a n d t h e ir ma g n e t ic an d M CE pr o pert i es were ch aract eri zed. T h e n an o part i cl e s y s t em s are co b al t f e r r i t e, c o r e s h e ll m a ng a ne s e z i nc f e r r i t e c o ba l t a nd s i l ve r c o a t e d c o ba l t p a r t ic le s ( C o Ag ) w hi l e t he 8 16 30 cl at h r at e sy st em s ar e E u Ga Ge T y pe I an d VI I I T h ese m at er i al s r epr esen t a b r o ad cl ass o f s y s t em s l i kel y t o s h o w i n t eres t i n g M CE pro pert i es A m o n g t h es e, m y ef f o rt s o n ch em i cal s y n th e s i s w e r e f oc u s e d on th e c ob a l t f e r r i te s y s te m T h e oth e r s a m p l e s w e r e ob ta i n e d f r om v ar i o us co l l ab o rat o rs. A det ai l ed repo rt o f t h e s y n t h es i s f o r co b al t f err i t e wi l l b e gi v en f o l l o wed b y sum m ar i es o f t h e sy n t h eses f o r t h e o t h er m at er i al s f r o m ex t er n al co l l ab o r at o r s. 24 2. 2 Synt hes is of CoF e O Nanopar ti cl es O u r s y n th e s i s f or c ob a l t f e r r i te f ol l ow e d th e s p i n e l f e r r i te r e c i p e s g i v e n b y S on g e t. a l [ 2 1 ] a n d b y S u n e t. a l [ 2 2 ] T h e p a p e r b y S u n e t. a l m a i n l y d e s c r i b e d th e s y n th e s i s of i r o n f e r r i t e na no p a r t i c l e s by t he d e c o m p o s i t i o n o f m e t a l o r g a ni c s a l t s bu t a l s o br i e f l y no t e d t he s y n t h es i s o f co b al t f err i t e an d m an gan es e f err i t e us i n g a s i m i l ar t ech n i que. Gi v en t h e gen eral 24 fe r r it e fo r mu la o f M F e O t h e a u t h o r s o f t h e p a p e r n o t e d t h a t t h e ir s yn t h e s is t e c h n iq u e c o u ld 24 24 24 p o t e nt i a l l y p r o d u c e a va r i e t y o f f e r r i t e s s u c h a s M nFe O N i Fe O a nd M g Fe O [ 2 2 ] T he p a p e r b y S o n g e t a l. u s e d a v e r y s imi la r t e c h n iq u e a n d fo c u s e d o n t h e s yn t h e s is o f c o b a lt

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21 f err i t e [2. 1]. T h e reac t an t s us ed i n t h e s y n t h es i s were t h e f o l l o wi n g: M (I I) ace t y l ace t o n at e, i r o n( I I I ) a c e t y l a c e t o na t e a f a t t y a c i d a n a l k y l a m i ne a l o ng c ha i n a l c o ho l a nd a hi g h bo i l i ng p o i nt s o l ve nt Fo r t he p r o d u c t i o n o f c o ba l t f e r r i t e c o ba l t ( I I ) a c e t y l a c e t o na t e w a s u s e d a l o ng wi t h i ro n (I II ) acet y l ace t o n at e i n a 1:2 rat i o [2. 1-2] So m e ex peri m en t at i o n was perf o rm ed b y S u n e t. a l f or th e b e s t s u r f a c ta n ts l on g c h a i n a l c oh ol a n d h i g h b oi l i n g p oi n t s ol v e n t. T h e y f ou n d th a t b oth ol e i c a c i d a n d ol e y l a m i n e a r e n e e d e d i n th i s r e a c ti on f or th e g e n e s i s of t h e n a n o p a r t ic le s S e v e r a l lo n g c h a in a lc o h o ls w e r e a n a lyz e d fo r t h e ir e ffe c t o n n a n o p a r t ic le qu al i t y an d y i el d i n t h e r eact i o n an d 1 2h ex adeca n edi o l was di sco v er ed t o m ax i m i ze t h ese ch aract eri s t i cs Be n zy l et h er o r ph en y l et h er were t h e h i gh b o i l i n g po i n t s o l v en t s o f ch o i ce wi t h b o i l i n g po i n t s o f 298 C a n d 259 C, r es pect i v el y [2. 2]. T h e reac t i o n pro cedure di f f ered s l i g h tl y b e tw e e n th e tw o g r ou p s of r e s e a r c h e r s l i s te d S u n e t. a l d e s c r i b e d a n i n i ti a l a d d i ti on o f al l t h e n ece s s ary react an t s f o l l o w e d by he a t i n g t o 200 C w i t h s t i rr i n g un der a n i t ro gen a tm os p h e r e ( i r on f e r r i te s y n th e s i s ) D e p e n d i n g on th e s y n th e s i s tr i a l th e y h e a te d th e r e a c ta n ts at 200 C f r o m 3 0 m i nu t e s t o 2 ho u r s f o l l o w e d by he a t i ng t o t he bo i l i ng p o i nt o f t he s o l ve nt ( p h e n y l or b e n z y l e th e r ) T h e r e a c ta n ts w ou l d b e a l l ow e d to r e f l u x a t th e b oi l i n g p oi n t f r om 30 m i n ut es t o 1 h o ur depen di n g o n t h e s y n t h es i s t ri al [2. 2]. So n g, et al des cri b ed an i n i t i al a d d i ti on of th e n e c e s s a r y r e a c ta n ts e x c e p t f or i r on ( I I I ) a c e ty l a c e ton a te ( c ob a l t f e r r i te s y n t h es i s ) T he r e act an t s were h eat ed t o 140 C a t w h i c h p oi n t i r on ( I I I ) a c e ty l a c e ton a te di s s o l v ed i n addi t i o n al s o l v en t was added t o t h e reac t i o n v es s el T he r eac t an t s were t h en h eat ed t o t h e b o i l i n g po i n t o f t h e s o l v en t (ph en y l et h er i n t hi s p a p e r) an d ref l ux ed f o r 30 m i nu t e s [ 2 1 ] B o t h p a p e r s d e s c r i be d t he u s e o f e t ha no l a nd c e nt r i f u g i ng t o p r e c i p i t a t e a nd s e p a r a t e t h e p a r t ic le s fr o m t h e p r o d u c t s o lu t io n T h e p a r t ic le s w e r e c o a t e d w it h o le ic a c id

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22 F ig ur e 2 .1 Ol ei c ac i d. w it h t h e h yd r o p h o b ic t a il o f t h e o le ic a c id m o l e c u l e e x p o s e d t o t h e s o l v e n t (F i g u re 2 .1 ). F o r t h e p a rt i c l e s t o su s p e n d a n o n po l ar so l v en t o r o n e wi t h l i t t l e po l ar ch aract er m us t b e us ed. Bo t h rese a rch g r o u p s u s e d h e x a n e t o s u s p e n d t h e ir p a rt i c l e s [2 .1 -2 ]. T h e p r o c e d u r e fo r o u r s yn t h e s is c l o s e l y f o l l o w e d t he p r o c e d u r e o u t lin e d in t he p a p e r by S o ng e t a l [ 2 1 ] T he f o l l o w i ng re a c t a n t s w e re p u t i n t o a t h re e n e c k e d F l o re n c e f l a s k (F i g u re 2. 2): 0. 5143 g o f co b al t (I I) ace t y l ace t o n at e, 5. 1689 g o f 1, 2he xa d e c a ne d i o l 1 0 m l o f o l e i c a c i d 1 0 m l o f o l e y l a m i ne a nd 4 0 m l of p h e n y l e th e r T h i s s o l ut i on w a s h e a te d q u i c k l y to 156 C w h e r e 1. 4 1 2 7 g of ir o n ( I I I ) a c e t yla c e t o n a t e in 2 0 m l of p h e n y l e th e r w as p i p e te d i n to th e r e a c ti on v es s el T h e s o l ut i o n was f u rth e r h e a te d q u i c k l y to ~260 C (b .p o f p h e n y l e t h e r) a n d ref l ux e d f or appro x i m at el y 30 m i n ut es Hyd r opho bic T ail Hydr ophi l ic Head F ig ur e 2 .2 React i o n app ar at us. Re f l ux Co n den s er T he r m o c o u p l e P r o be Re act i o n Ves s el Hea t i n g Pad

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23 a f t e r w hi c h t he s o l u t i o n w a s a l l o w e d t o c o o l t o r o o m t e m p e r a t u r e T he na no p a r t i c l e s i n t he p r od u c t s ol u ti on w e r e r e m ov e d b y th e a d d i ti on of e th a n ol a n d c e n tr i f u g i n g a t 6 0 0 0 r p m f or 5 min u t e s fo llo w e d b y 9 0 0 0 r p m fo r 5 min u t e s T h e n a n o p a r t ic le s w e r e t h e n s u s p e n d e d in h ex an e. Due t o un des i red pr o duct rem ai n i n g wi t h t h e n an o par t i cl es af t er t h e i n i t i al s e p a r a t i o n t h e p a r t i c l e s w e r e w a s he d s e ve r a l t i m e s w i t h d e i o ni z e d w a t e r f o l l o w e d by m a g ne t i c dec an t at i o n T h e pro ces s o f m agn et i c dec an t at i o n i n v o l v ed t h e us e o f a s t r o ng perm an en t m agn et f o r h o l di ng t he nan o part i cl es wh i l e t h e f l ui d aro un d t h em was drai n ed. T h is p r o c e s s c a n o n ly b e a c c o mp lis h e d w h e n t h e p a r t ic le s w it h a n o n p o la r c o a t in g r e s id e in a po l ar s o l v en t In t h e en d, t h e part i cl es were o n ce agai n s us pen ded i n h ex an e. T h e f act t h at t h e p a rt i c l e s s u s p e n d e d a t a l l i n h exane p r o v i d e d g o o d e v i d e n c e t h a t t h e p a rt i c l e s w e re n an o si ze an d t h at t h e sur f act an t co at i n g, o l ei c aci d, r em ai n ed o n t h e par t i cl es. A n a t t e m p t w a s m a d e t o s y n t h e s i z e f e rri t e c u b e s a s d e s c ri b e d b y Son g e t a l [2 .1 ]. M a n g a n e s e ( I I ) a c e t yla c e t o n a t e w a s s u b s t it u t e d fo r t h e c o b a lt s a lt a n d a n o t h e r lo n g c h a in a lc o h o l, 1 o c t a d e c a n o l, w a s s u b s t it u t e d fo r 1 2 h e x a d e c a n e d io l. T h e p r ima r y s o lv e n t p h e n yl et h er, was n o t us ed i n t h i s react i o n T h e react an t s were h eat ed t o 210 C a t a s l o w he a t i ng ra t e o n t h e o rd e r of 0 .1 C pe r m i n ut e. T h e react an t s rem ai n ed at 210 C f o r 3 0 m i nu t e s a nd a f t e rw a rd s c o o l e d a t a ra t e o n t h e o rd e r of -0 .1 C per m i n ut e. T h e reac t i o n pro duced v ari o us po l y h edra wh i ch can b e o b s erv ed i n F i gure 2. 3. Be s i d e s c ub es t h e reac t i o n pro duced po l y h edr a h av i n g si l h o uet t es i n t h e T E M i m ages o f t r i an gl es, t r un cat ed t r i an gl es, h ex ago n s, an d n ear s ph eri cal s h apes On e o f o ur go al s f o r M CE ch aract eri zat i o n i s t o s t udy t h e ef f ect o f s h ape a ni s o t r o py o n en t ro py ch an ge i n f err i t e n an o part i cl es Si n ce o n e un i f o rm s h ape c o u l d n o t b e p rodu c e d t h e s e n a n o p a rt i c l e s w e re n o t c h a ra c t e ri z e d f o r M C E. F u rt h e r

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24 F ig ur e 2 .3 T E M i m ages o f po l y h edral n an o par t i cl es.

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25 r ef i n em en t o f t h e pr o cedu r e i s n eeded t o pr o du ce uni f o r m po l y h edr al n an o par t i cl es. 2 3 N a n o p a rt i c le s O b t a i n e d fro m C o lla b o ra t o rs T he o t h er n an o part i cl e s y s t em s prese n t ed i n t h i s rese arch were s y n t h es i zed by 0.68 0.25 2.07 4 c o l l a b o ra t o rs M a n g a n e s e z i n c f e rri t e M n Z n F e O (M Z F O ), w a s re c e i v e d f rom D r. S M o r r is o n a n d D r E C a r p e n t e r fr o m t h e V ir g in ia C o mmo n w e a lt h U n iv e r s it y. T h e ir m et h o d o f s y n t h es i s was t h e v ersa t i l e rev erse m i cel l e t ech n i que. T h e s urf act an t b i s -( 2et h y l h ex l ) so di um s ul f o s ucci n at e (A OT) was us ed i n t h e reac t i o n t o f o rm po cket s o f di s s o l v ed me t a l s a lt s w h ic h w e r e r e d u c e d t o fo r m n a n o p a r t ic le s T h e r e v e r s e mic e lle s w e r e fo r me d in i s oo c ta n e a n d th e s i z e of th e r e v e r s e m i c e l l e s w a s c on tr ol l e d b y th e m ol a r r a ti o o f w a te r to c o r e s h e ll su r f ac t ant [ 2 3 4 ] Co b al t and C o A g n ano p ar t i cl es we r e sy n t h esi ze d b y Dr P P o d d ar ’s gro up at t h e Nat i o n al Ch em i cal L ab o rat o ry i n Pun e, I n di a. Co b al t n an o part i cl es were f o rm ed b y t h e reduct i o n o f m et al s al t s i n v es i cl es f o rm ed b y t h e s urf act an t s o l ei c ac i d an d s o di um d o d e c y l s u l f a t e (S D S) i n a n a q u e o u s s o l u t i o n Th e o n l y s o l v e n t i n t h e re a c t i o n w a s w a t e r, a n d t h e p a rt i c l e s i z e a n d s t a b i l i t y w e re c o n t rol l e d b y t h e m o l a r ra t i o o f s u rf a c t a n t s [2 .5 ]. c o r e s h e ll Co b al t n an o part i cl es were f o rm ed i n t o Co A g n an o part i cl es b y pl aci n g t h em i n t o a s i l v er s a l t s o l u t i o n. S u r f a c e c o ba l t a t o m s w e r e r e p la c e d w it h r e d u c e d s i l ve r i o ns i n a t ra n s m e t a l l a t i o n re a c t i o n Th e f i n a l p rodu c t w a s n o t c a p p e d w i t h a s u rf a c t a n t [2 .6 ]. 2 4 B u lk M a t e r i a ls T h e t wo b ul k cl at h rat e s am pl es prese n t ed i n t h i s rese arch were s y n t h es i zed b y Dr. G. No l as i n t h e No v el M at eri al s L ab o rat o ry at t h e Un i v ersi t y o f So ut h F l o ri da. T h e f i rst s am pl e,

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26 8 16 30 E u G a G e T yp e I w a s fo r me d b y r e a c t in g s t o ic h io me t r ic a mo u n t s o f e u r o p iu m, g a lli u m, a nd g e r m a ni u m i n a f u r na c e a t hi g h t e m p e r a t u r e u nd e r a n a r g o n a t m o s p he r e [ 2 7 ] Fo r t he 81 63 0 s e c o n d s a m p l e Eu G a G e Ty p e I w a s f u rt h e r a n n e a l e d t o p rodu c e t h e Ty p e V III s t ru c t u re a s d e s c r i be d i n R e f 2 8 [ 2 8 9 ] T he m a g ne t i c c ha r a c t e r i z a t io n o f t he s e m a t e r i a l s a nd t he n a n o m a t e ri a l s w i l l b e p re s e n t e d a n d d i s c u s s e d i n t h e n e x t c h a p t e r.

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27 C h a p t e r 3 St r uctu r al and M agneti c Char acter iz ati on 3. 1 I nt r oduct ion St ruct ural an d m agn et i c ch aract eri zat i o n was perf o rm ed o n al l s am pl es St ruct ural c h a r a c te r i z a ti on w a s p e r f or m e d b y e x te r n a l c ol l a b or a tor s on a l l s a m p l e s e x c e p t c ob a l t f e r r i te s i n c e th i s m a te r i a l w a s s y n th e s i z e d i n ou r l a b M a g n e ti c c h a r a c te r i z a ti on w a s p e r f or m e d on s a m p l e s i n ou r l a b T h e c l a th r a te m a te r i a l s h a v e b e e n s tu d i e d b y oth e r g r ou p s i n a d d i ti on to o ur wo r k. T h e o v er al l m agn et i c pr o per t i es ar e i n go o d a g r eem en t i n al l t h ese st ud i es. 3. 2 M agneti c Char acter iz ati on M ethod ol ogy T h e m agn et i c ch aract eri zat i o n f o r al l m at eri al s i n t h i s pro j ect was perf o rm ed i n o ur l ab us i n g t h e A C M eas urem en t Sy s t em ( AC M S) pro b e o pt i o n o n o ur P h y s i cal Pr o pert i es M e a s u r e m e n t S y s te m ( P P M S ) b y Q u a n tu m D e s i g n T h e m a g n e ti z a ti on c h a r a c te r i z a ti on i n v o l v ed s t udy i n g t h e DC m agn et i c res po n s e o f a s am pl e as a f un ct i o n o f t em perat ure. S a m p l e s w ou l d f i r s t b e c ool e d to a l ow te m p e r a tu r e ty p i c a l l y 1 0 K T h e m a g n e ti z a ti on of t h e s am pl es wo ul d t h en b e m eas ured i n a f i el d o f 100 Oe us i n g a s wee p m o de as t h e s am pl es wer e war m ed. T y pi cal r at es f o r war m i n g we r e 23 K per m i n ut e. F o r n an o par t i cl e sy st em s, a n a d d it io n a l ma g n e t iz a t io n me a s u r e me n t w a s t a k e n w it h t h e s a mp le b e in g c o o le d in a fie ld o f 1 0 0 O e Th e t w o t y p e s o f m a g n e t i z a t i o n m e a s u re m e n t s i n n a n o p a rt i c l e s y s t e m s a re ty p i c a l l y d e s i g n a te d z e r o f i e l d c ool e d ( Z F C ) a n d f i e l d c ool e d ( F C ) m a g n e ti z ati on

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28 m e a s u r e m e nt s T he p u r p o s e i n t he m a g ne t i c c ha r a c t e r i z a t i o n ha d t he d u a l r o l e s o f f u r t he r i ng k n o w le d g e c o n c e r n in g t h e s p e c ific ma t e r ia ls w e s t u d ie d a n d r e v e a lin g t r a n s it io n s in ma g n e t ic p ha s e s w hi c h m i g ht c o r r e l a t e w i t h t he m a g ne t o c a l o r i c e f f e c t I t i s w e l l k no w n t ha t Z FC a nd F C ma g n e t iz a t io n d a t a p r o v id e in fo r ma t io n a b o u t t h e ma g n e t ic o r d e r d is o r d e r t r a n s it io n in na no p a r t i c l e s y s t e m s k no w n a s t he bl o c k i ng t r a ns i t i o n. 3. 3 Cobal t Fer r it e Nanopar ti cl e Sys tem Co b al t f err i t e n an o part i cl es were ch aract eri zed us i n g x -r ay di f f ract i o n (XRD), en ergy di s pers i v e x r a y s p e c tr os c op y ( E D S ) tr a n s m i s s i on e l e c tr on m i c r os c op y ( T E M ) a n d DC m agn et i zat i o n m eas urem en t s T h e XR D pat t ern di s pl ay ed i n F i gure 3. 1a, rev eal s t h e m at eri al t o ha ve t h e m o rph o l o gy o f t h e i n v erse s pi n el cry s t al l i n e s t ruct ure o f co b al t f err i t e. T he m easur em en t was t aken f o r app r o x i m at el y f o ur t een h o ur s b ut st i l l pr o du ced l o w co un t n o i sy peaks due t o t h e n an o part i cul at e n at ure o f t h e s y s t em T h e h um p s een at l o wer an gl es i s due t o t h e am o rph o us gl as s s ub s t rat e o n w hi c h t h e n an o part i cl es were pl ace d f o r t h e x -r ay p r o c e d u r e F o r fu r t h e r c h a r a c t e r iz a t io n o f t h e ma t e r ia l, E D S w a s p e r fo r me d o n t h e t h in film o f n a n o p a r t ic le s fr o m t h e X R D s c a n t o d e t e r min e i f i m p u r i t i e s e x is t e d w it h in t h e ma t e r ia l. T a bl e 3 1 c o nt a i ns a l i s t o f a t o m i c p e r c e nt a g e s o f e l e m e nt s i n t he na no p a r t i c l e s y s t e m T he v a l u e s i n th e ta b l e a r e a c tu a l l y th e a v e r a g e s of m e a s u r e m e n ts ta k e n a t f ou r d i f f e r e n t s p ots on t h e t h i n f i l m o f t h e n an o part i cl es T h e s t an dard dev i at i o n s o f t h e perce n t ages are l i s t ed b es i de t h e av erages F ro m t h e n o rm al i zed C o /F e rat i o o f 1:2. 96, i t i s ev i den t t h at s o m e i m puri t i es e x is t in t h e ma t e r ia l d u e t o t h e e le v a t e d ir o n c o n t e n t H o w e v e r a s a n o t e o f c a u t io n it s h o u ld b e me n t io n e d t h a t t h e E D S s a mp le s a la r g e v o lu me ( t yp ic a lly m ) a n d h e n c e i s n o t a v e ry 3

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29 T a bl e 3 .1 A t o mic p e r c e n t a g e s fr o m E D S o f a c o b a lt f err i t e n an o part i cl e s am pl e. F igur e 3.1a XRD o f co b al t f er r i t e n an o par t i cl es.

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30 p r e c i s e p r o be w he n e va l u a t i ng t he c he m i c a l c o m p o s i t i o n o f na no p a r t i c l e s T he o t he r s e e m i ng i m pu r i t i es such as si l i co n sul f ur an d cal ci um ar e m o st l i kel y co n t ai n ed w i t h i n t h e gl ass s u b s t ra t e N o n i t roge n w a s f o u n d w i t h i n t h e s a m p l e g i v i n g e v i d e n c e t h a t t h e p a rt i c l e s w e re c a p p e d e x c lu s iv e ly w it h o le ic a c id w it h n o r e ma in in g o le yla min e o n t h e s u r fa c e O x yg e n is s h ow n to h a v e a h i g h p e r c e n ta g e d u e to th e oc c u r r e n c e of th e e l e m e n t n ot o n l y i n th e f e r r i te s t ruct ure b ut al s o i n t h e s urf act an t an d i n t h e gl as s s ub s t rat e. T h e carb o n co n t en t i s al s o h i gh d u e t o t he l o ng c a r bo n t a i l o n t he o l e i c a c i d m o l e c u l e s s u r r o u nd i ng e ve r y na no p a r t i c l e T he ex act n at ure o f t h e ex t ra i ro n co n t en t h as n o t b een det erm i n ed. T h e i m puri t y m i gh t ex i s t as p a r t ic le s in a d d it io n t o c o b a lt fe r r it e n a n o p a r t ic le s o r t h e imp u r it y mig h t o c c u r in e v e r y c o b a lt fe r r it e n a n o p a r t ic le I n a n y c a s e t h e X R D s c a n ma k e s c le a r t h a t t h e p r e d o m i n a n t p h a s e is cry s t al l i n e an d t h at t h e i den t i t y o f t h e cry s t al l i n e ph as e i s co b al t f err i t e. S i n ce t h e EDS used fo r t h is c h a r a c t e r iz a t io n w a s a t t a c h e d t o a s c a n n in g e le c t r o n mic r o p r o b e ( S E M ) t h e t h in film s urf ace was al s o i m aged a s s een i n t wo pi ct ures o f v ary i n g m agn i f i cat i o n (F i gure 3. 1c). On t h e m i c r on l e v e l th e f i l m a s s e m b l e d i n to a f a i r l y s m ooth th i n f i l m S i n c e th e n a n op a r ti cl es w e r e s i m p l e d r o p c a s t o n a g l a s s s u bs t r a t e w i t h no c a r e g i ve n f o r s m o o t hi ng t he s u r f a c t a nt F igur e 3.1b SE M i m ages o f E DS sa m pl e.

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31 c o a t in g is s h o w n o n c e a g a in t o b e a n e x c e lle n t t o o l in s e lf a s s e mb lin g n a n o p a r t ic le s fo r t h in f i l m appl i cat i o n s A T E M i m age o f t h e co b al t f err i t e n an o part i cl es i s di s pl ay ed i n F i gure 3. 1d. T h e a v e r a g e p a r ti c l e s i z e w i th s ta n d a r d d e v i a ti on ob ta i n e d f r om T E M i s 5 3 n m A l s o f r om t h e TE M pi ct ure, t h e part i cl es are s eparat e wi t h o ut cl us t eri n g wh i ch l en ds m o re ev i den ce t h at t h e o le ic a c id s u r fa c t a n t c o a t in g r e ma in e d o n t h e n a n o p a r t ic le s F ig u r e 3 1 e c o n t a in s t h e fie ld c ool e d ( F C ) a n d z e r o f i e l d c ool e d ( Z F C ) m a g n e ti z a ti on m e a s u r e m e n ts on th i s s y s te m F r om t h is d a t a t h e ma g n e t ic c h a r a c t e r o f t h e p a r t ic le s ys t e m is c le a r ly s e e n t o b e s u p e r p a r a ma g n e t ic a bo ve 2 2 3 K s i nc e t he FC a nd Z F C ma g n e t i z a t i o n m e a s u r e m e nt s d i ve r g e a t t he bl o c k i ng B t em perat ure ( T ) o f 223 K T h e b ro ad pea k i n Z F C i s a s i gn at ure o f f reezi n g (o r b l o cki n g) o f s p i n s a n d th e l a r g e w i d th i s c on s i s te n t w i th a b r oa d d i s tr i b u ti on i n p a r ti c l e s i ze. M o no d i s p e r s e s a m p l e s w o u l d r e s u l t i n a m u c h s ha r p e r bl o c k i ng t r a ns i t i o n. F igur e 3.1c T E M i m age o f co b al t f er r i t e n an o par t i cl es.

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32 3. 4 M anganese Z inc F er r it e Nanopar ti cl e Sys tem M an gan es e zi n c f err i t e n an o part i cl es were s t ruct ural l y ch aract eri zed b y us as wel l as t he c o l l a bo r a t o r s w ho c o nt r i bu t e d t he s y s t e m f o r o u r s t u d y X R D w a s p e r f o r m e d o n t he n an o part i cl e s y s t em rev eal i n g t h e s y s t em t o b e i n ex cel l en t agreem en t wi t h b ul k M Z F O us ed f o r co m pari s o n F o r el em en t al an al y s i s t h e t ech n i que o f i n duct i v el y co upl ed pl as m a o pt i cal em i s s i o n s pect ro m et ry was us ed. A s s um i n g t h at al l t h e m an gan es e, zi n c, i ro n an d o x y gen a t o m s w e r e c o nt a i ne d w i t hi n t he f e r r i t e s t r u c t u r e t he c he m i c a l f o r m u l a f o r m a ng a ne s e z i nc 0.68 0.25 2.07 4 f e r r i t e i s M n Z n Fe O A T E M i m a g e o f t he s y s t e m i s d i s p l a y e d i n Fi g u r e 3 2 a T he part i cl e s i ze w as det erm i n ed t o b e appro x i m at el y 15 n m wi t h a s i ze di s t ri b ut i o n o f l es s t h an 2 n m [ 3 1 ] T h e ima g e is b lu r r e d s o me w h a t d u e t o e x c e s s s u r fa c t a n t le ft o n t h e p a r t ic le s y s t em T h e s urf act an t co ul d h av e b een rem o v ed b ut was al l o wed t o rem ai n o n t h e part i cl es t o p r e ve nt c l u m p i ng w hi c h w o u l d a f f e c t p a r t i c l e s i z e s i z e d i s t r i bu t i o n, a nd s u bs e q u e nt l y t he F igur e 3.1d F C a n d Z F C c u r v e s f or c ob a l t f e r r i te n an o par t i cl es.

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33 F igur e 3.2b F C a n d Z F C c urv es f o r m an gan es e zi n c f err i t e. F igur e 3.2a T E M i m a g e o f m a ng a ne s e z i nc f er r i t e n an o par t i cl es. na t u r e a nd t e m p e r a t u r e o f t he bl o c k i ng t r a n s it i o n. I n a d d i t i o n, a l l o w i ng t he s ur f a c ta n t to r e m a i n on th e p a r ti c l es p r ov i d e d a c om m on a l i ty b e tw e e n th r e e of t h e n a n o p a r t ic le s ys t e ms s t u d ie d in t h is p r o j e c t Fi g u r e 3 2 b c o nt a i ns t he FC a nd Z F C ma g n e t iz a t io n me a s u r e me n t s o n t h is s ys t e m. F r o m t h is d a t a t h e ma g n e t ic c h a r a c t e r o f t h e p a r t ic le s y s t e m is o bs e r ve d t o be s u p e r p a r a m a g ne t i c a t hi g h t e m p e r a t u r e w i t h a bl o c k i ng t e m p e r a t u r e o f 4 6 K. Due t o t h e m o n o di s perse n at ure o f t h e s y s t em t h e m ax i m um o f t h e Z F C c urv e i s m uch na r r o w e r i n c o m p a r i s o n w i t h t he c o ba l t f e r r i t e s y s t e m I n t hi s m a t e r i a l, s in c e t he bl o c k i ng t r ansi t i o n i s a t a m u c h l o w e r t e m p e ra t u re o n e c a n o b s e rv e t h e C u ri e l a w (~ 1 /T ) rep res e n t a t i ve o f s up e rpa ram a gn e t i s m f ro m 50 K t o 30 0 K De v i a t i o n s f ro m a n i de a l 1/T

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34 d e p e nd e nc e i s e xp e c t e d i n r e a l s ys t e ms w he r e d i p o l a r i nt e r a c t i o ns be t w e e n ne i g hbo r i ng n a n o p a rt i c l e s a re u n a v o i d a b l e co r e s hell 3 5 Co b a lt a n d Co Ag Na n o p a r t i cle S y st ems c o r e s h e ll C o ba l t a nd C o Ag na no p a r t i c l e s w e r e s t r u c t u r a l l y c ha r a c t e r i z e d by t he c o lla b o r a t o r s w h o c o n t r ib u t e d t h e s ys t e m. T h e T E M ima g e s o f t h e s ys t e ms a r e d is p la ye d in Fi gure 3. 3a an d F i gure 3 3 b, r espect i v el y [ 3. 2] F r o m t h ese i m a g e s t h e c o b a l t n a n o p a rt i c l e s w e re f o u nd t o be 4 9 5 nm w hi l e t he c o r e s h e ll C o Ag na no p a r t i c l e s w e r e f o u nd to b e a p p r ox i m a te l y 9 7 n m F u rt h er c o r e s h e ll m e a s u r e m e nt s o f t he C o Ag n a n o p a r t ic le s r e v e a l t h e m t o c o n t a in a c o ba l t c o r e o f a p p r o xi m a t e l y 4 0 nm a n d a s ilv e r s h e ll o f a p p r o x ima t e ly 28. 5 n m T h e i m ages al s o rev eal t h at t h e c o b a lt n a n o p a r t ic le s a r e n o t in c o n t a c t w i t h e a c h o t h e r d u e t o t h e ir s u r f a c t a nt c o a t in g a n d t he c o r e s h e ll C o A g n a n o p a r t ic le s a r e in c o n t a c t w it h e a c h o t h e r d u e t o t h e ir c o r e s h e ll F igur e 3.3b T E M i m a g e o f C o Ag n an o part i cl es F igur e 3.3a T E M ima g e o f s u r fa c t a n t c o a t e d c o b a lt n an o par t i cl es.

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35 l a c k of s u r f a c ta n t c oa ti n g F i g u r e 3 3 c c on ta i n s th e F C a n d Z F C m a g n e ti z a ti on m e a s u r e m e n ts fo r t h e c o b a lt s ys t e m. F r o m t h i s d a t a t h e ma g n e t ic c h a r a c t e r o f t h e p a r t ic le s ys t e ms is B ob s e r v e d to b e s u p e r p a r a m a g n e ti c a t h i g h te m p e r a tu r e w i th a b l oc k i n g te m p e r a tu r e ( T ) of 1 3 5 K f o r t h e c o b a l t n a n o p a rt i c l e s In a d d i t i o n t o t h e b l o c k i n g t ra n s i t i o n a t 1 3 5 K a s h a rp t r a ns i t i o n i s o bs e r ve d a t t he l o w e r t e m p e r a t u r e o f 1 6 K. Fr o m w ha t w a s o bs e r ve d e a r l i e r t he s urf act an t o l ei c aci d, act i n g as a cappi n g agen t do es n o t part i ci pat e i n t h e m agn et i c pro pert i es of n a n op a r ti c l e s y s te m s oth e r th a n to s e p a r a te p a r ti c l e s f r om e a c h oth e r a n d p r e v e n t ox i d a ti on p ro ces s es f ro m o ccurri n g o n t h e s urf ace o f t h e part i cl e. T h e b es t ex pl an at i o n f o r t he o ccurren ce o f t h i s l o w t em perat ure t ran s i t i o n i s s urf ace an i s o t ro py gi v i n g t h e s urf ace m agn et i c m o m en t s ch ar act er i st i cal l y di f f er en t b eh av i o r t h an t h e b ul k co r e m agn et i c m o m en t s. T h i s di s t i n ct b eh av i o r h as b een o b s erv ed an d m o del ed i n o t h er n an o part i cl e s y s t em s s uch as F igur e 3.3c F C an d ZF C cur v es f o r co b al t n an o par t i cl es.

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36 0.26 0.74 50 50 0.25 0.75 65 35 ( F e N i ) B ( C o N i ) B a n d N iO [ 3 3 6 ] A b o v e 1 3 5 K a ll o f t h e mo me n t s in a na no p a r t i c l e e xhi bi t r a nd o m i z e d be ha vi o r d u e t o t he r m a l e ne r g y e xc e e d i ng t he e f f e c t i ve a n i s otr op y e n e r g y of th e b u l k of th e m om e n ts B e l ow 1 3 5 K th e b u l k of th e m om e n ts d o n ot h a v e t h e e n e r g y t o fl ip b e c a u s e o f t h e e n e r g y b a r r ie r p r e s e n t e d b y t h e e ff e c t iv e a n is o t r o p y. H o w e v e r t h e s u r fa c e mo me n t s a r e n o t s u r r o u n d e d o n a ll s id e s b y a d jo in in g mo me n t s a n d s t ill h a v e th e f r e e d om to f l i p ra n d om l y b e l ow 135 K F i gure 3. 3d i l l us t rat es t h i s co n cept d e p i c ti n g a s l i c e of a n a n op a r ti c l e w i th d i f f e r i ng s u r f a c e / c o r e s p i n c ha r a c t e r As s een i n t h e F i gure 3. 3c, t he m o m en t s do ev en t ual l y f i n d t h ei r en ergy b arr i er at 16 K a n d b e g in t o fr e e z e r a n d o mly T h e s a me b e h a v io r o f s u r fa c e a n d c o r e m o m e n t s is c o r e s h e ll ob s e r v e d i n th e C o A g s y s te m F i g u re 3 3 e d i s p l a y s th e F C a n d Z F C m a g n e ti z a ti on c o r e s h e ll me a s u r e me n t s fo r C o A g n a n o p a r t ic le s T h e ma g n e t ic c h a r a c t e r o f t h e p a r t ic le s ys t e ms i s o b s erv ed t o b e s uperparam agn et i c at h i gh t em perat ure wi t h a b l o cki n g t em perat ure o f 150 c o r e s h e ll K f o r t h e Co A g n an o par t i cl es. A s wi t h t h e co b al t n an o par t i cl es, a sh ar p t r an si t i o n i s al so o bs e r ve d w i t h t hi s s y s t e m a t 2 7 K. D e s p i t e t he i r s i m i l a r i t i e s t he r e a r e s o me im p o r t a nt d i f f e r e n c e s b e t w e e n t h i s s y s t e m a n d t h e c o b a l t s y s t e m B o t h b l o c k i n g t e m p e r a t u r e s f or c o r e s h e ll C o Ag na no p a r t i c l e s i nc r e a s e d w he n c o m p a r e d w i t h t he c o ba l t na no p a r t i c l e s Fo r t he b u lk m o m e n t s t h is is s o me w h a t u n u s u a l d u e t o t h e r e d u c t io n in v o lu me o f t h e c o b a lt n a n op a r ti c l e w i th i n th e s i l v e r s h e l l T h i s i m p l i e s a n i n c r e a s e of th e e f f e c ti v e a n i s otr op y or F igur e 3.3d I l l u s t r a t i o n o f s u r f a c e / c o r e s p i n.

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37 so m e o t h er ph en o m en o n un kn o wn at t h i s t i m e. I n o r der t o det er m i n e co r r el at i v e ef f ect s, f u rt h e r s t u d i e s w i l l n e e d t o b e c o n d u c t e d i n w h i c h b o t h p a rt i c l e s i z e a n d s h e l l t h i c k n e s s a re v ari ed s eparat el y an d perh aps s i m ul t an eo us l y A n i n di rect RK K Y m ech an i s m was co n s i dered c o r e s h e ll f o r t h i s s y s t em s i n ce t h e C o A g n an o part i cl es were o b s erv ed t o b e i n co n t act wi t h eac h o t h er. A n y ef f ect s f ro m t h i s wo ul d pro b ab l y b e n egl i gi b l e due t o t h e rat h er l en gt h y di s t an ce o f 2 8 5 nm f o r t h e s ilv e r s he l l T hi s a s s e s s m e nt c o u l d a l s o be r i g o r o u s l y t e s t e d by t he v a ri a t i o n o f t h e s h e l l t h i c k n e s s 8 16 30 3 6 Eu G a G e Ty p e I a n d VI II B u lk Cla t h r a t e S y st ems 81 63 0 T he c l a t hr a t e s y s t e m s E u G a G e T yp e I a n d V I I I ha ve be e n s y nt he s i z e d a nd e x t e n s i v e l y s t u d i e d b y D r. G N o l a s w h o c o n t ri b u t e d t h e m a t e ri a l s f o r our s t u d y [3 .7 ]. F i g u re 3 4 a b d is p la ys t h e s t r u c t u r e s fo r T y p e I a n d T yp e V I I I r e s p e c t iv e ly. W h ile b o t h c o n t a in c o r e s h e ll F igur e 3.3e F C an d ZF C cur v es f o r Co A g n an o par t i cl es.

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38 euro pi um i o n s i n t h e 2+ s t at e, t h e cage s t ruct ures s urr o un di n g t h e i o n s di f f er. F o r t h e Ty pe I c l a t hr a t e t w o t y p e s o f c a g e s s u r r o u nd t he e u r o p i u m i o ns a nd a r e g i ve n t he fo l l o w i ng co n f i gurat i o n s : E u1 i s cen t ered i n t h e pen t ago n al do deca h edro n cage wh i l e Eu2 i s cen t ered i n t h e t et rakai deca h edro n cage [3. 9, 10]. F o r t h e Ty pe VIII cl at h rat e, o n l y o n e s h ape c age s urr o un ds t h e euro pi um i o n s an d t h i s i s gi v en as a pen t ago n al do deca h edro n [3. 10]. Si n ce th e e u r op i u m e m b e d d e d w i th i n th e c l a th r a te i s a m a g n e ti c i on i t w a s i m m e d i a te l y s tu d i e d f or i t s m a g ne t i c p r o p e r t i e s T he E u i o n i m be d d e d w i t hi n t he g a l l i u m / g e r m a ni u m c a g e s ha s t he 2+ m a g ne t i c g r o u nd s t a t e s he l l c o nf i g u r a t i o n o f 4 f w hi c h w a s e xp e r i m e nt a lly c o nf i r m e d by 7 Pach eco et al [3. 11]. T h i s s am e s h el l co n f i gurat i o n i s f o un d wi t h t h e G d i o n w hi ch 3+ e x h ib it s a la r g e c h a n g e in ma g n e t ic e n t r o p y. T h e ma g n e t ic mo me n t h a s b e e n r e p o r t e d in B s ev eral papers as b ei n g v ery cl o s e t o t h e f ree i o n v al ue o f 7. 93 f or E u [ 3 1 0 1 2 ] B oth 2+ c l a t h ra t e s w e re f o u n d t o b e v e ry s o f t m a g n e t s w i t h a c o e rc i v i t y o n t h e o rd e r o f 1 G [3 .1 0 ,1 2 ]. In F i gure 3. 4c, m agn et i zat i o n as a f un ct i o n o f t em perat ure m eas urem en t s (per f o rm ed i n o ur l a b) a r e d i s p l a y e d T he t y p e I c l a t hr a t e s ho w s s o m e t a p e r i ng a t l o w t e m p e r a t u r e f o l l o w e d by a s h or t, s h a rp t ran s i ti on a t th e C u r i e te m p e r a tu r e of 3 3 9 K w h i l e th e ty p e V I I I c l a th r a te s ho w s a s ha r p t r a ns i t i o n a t i t s r e s p e c t i ve C u r i e t e m p e r a t u r e o f 1 2 4 K O t he r g r o u p s ha ve F ig ur e 3 .4 a ,b Cl at h rat e s t ruct ures f ro m Re f 3. 8.

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39 al s o m e a s u r e d th e C u r i e te m p e r a tu r e f or th e s e m a te r i a l s a n d ou r n u m b e r s m a tc h kn o wn v al ues t o wi t h i n appro x i m at e l y 2 K [3. 10-12] T h e ex pl an at i o n o f t h e s po n t an eo us f e r r o m a g ne t ic o r d e r i ng be l o w t he C u r i e t e m p e r a t u r e ha s be e n g i ve n a s t he i nd i r e c t R KKY i n t eract i o n s i n ce t h e s m al l es t i n t eri o n di s t an ces are 5. 23 f o r t h e Ty pe I cl at h rat e an d 5. 562 f o r t he T y p e V I I I c l a t hr a t e [ 3 1 0 ] I n t he ne x t c h a p t e r M C E m e a s u r e m e nt s w i l l be p re s e n t e d a l o n g w i t h d i s c u s s i o n o f t h e re s u l t s 81 63 0 F igur e 3.4c M v s T f o r Eu G a G e Ty p e I a n d V III

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40 C h a p t e r 4 M CE Char acter iz ati on 4. 1 Over view T h e m a g n e toc a l or i c e f f e c t w a s s tu d i e d b y d e r i v i n g e n tr op y c h a n g e f r om m a g n e ti z a ti on dat a o v er a wi de ran ge i n t em perat ures a n d m agn et i c f i el ds M eas urem en t s o n al l t h e s am pl es w e r e c a r r i e d ou t i n th e P P M S d e s c r i b e d i n c h a p te r 3 T h e s a m p l e s w e r e w e i g h e d l oa d e d i n to g e l c a p s a n d m ou n te d i n th e A C M S p r ob e op ti on f or th e P P M S M a g n e ti z a ti on a s a f u n c ti on of a p p l i e d f i e l d a n d te m p e r a tu r e wa s m e a s ur e d a n d th i s d a ta w a s th e n u s e d to c a l c u l a te en t ro py ch an ge ( S ) T h e p r oc e s s i n g w a s f i r s t p e r f or m e d u s i n g M i c r os of t E x c e l a n d / or O ri g i n b u t l a t e r a p rogra m w a s w ri t t e n i n L a b V IEW t o f a c i l i t a t e t h i s w o rk O n c e a g a i n I w o u ld lik e t o e mp h a s iz e t h a t t h e s t a r t o f M C E e x p e r ime n t s in o u r l a b c o in c id e d w it h my j o i ni n g t h e gro up f o r graduat e rese arch T h us I h ad t h e o ppo rt un i t y t o co n t ri b ut e t o t he m e a s u r e m e n ts a n d a n a l y s i s f r om th e b e g i n n i n g of th i s on g oi n g p r oj e c t. 4. 2 M CE Char acter iz ati on M ethod ol ogy T h e PPMS was pro gram m ed t o m eas ure m agn et i zat i o n f o r a ran ge o f appl i ed f i el ds at a co n s t an t t em perat ure v al ue. On ce t h e m eas urem en t s f o r t h e en t i re r an ge o f appl i ed f i el ds w e re c o m p l e t e d t h e t e m p e ra t u re w o u l d b e a d j u s t e d t o t h e n e x t v a l u e i n t h e t e m p e ra t u re r an ge, an d t he c y c l e w o ul d b egi n agai n T h i s pr o gr am m ed p at t er n ser v ed t wo pu r po ses. F i r st b y keep i n g t he t e m per at ur e an d p r essur e co n st an t t h e der i v ed ent r o py ch an ge i s a

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41 f un ct i o n o f o n l y appl i ed f i el d. Seco n d, appl i ed f i el ds can b e cy cl ed f as t er t h an t em perat ures s o th i s p a r ti c u l a r p a tte r n s a v e s ti m e a n d u l ti m a te l y m on e y d u e to th e u s e of l i q u i d h e l i u m f or t h e s u p e r c on d u c ti n g m a g n e t i n s i d e th e P P M S U s i n g E x c e l a n d / or O r i g i n a n d l at er L ab VIE W en t ro py ch an ge wa s deri v ed us i n g t h e M ax wel l equat i o n di s cus s ed i n ch apt er 1. A f i n i te d i f f e r e n c e m e th od w a s e m p l oy e d to d e te r m i n e th e d e r i v a ti v e of m a g n e ti z a ti on w i th r e s p e c t to te m p e r a tu r e T h i s r e q u i r e d ta k i n g c oor d i n a ti n g s u c c e s s i v e v a l u e s f or m a g n e ti z a ti on ( M ) an d t em perat ure ( T ) an d respe ct i v el y s ub t ract i n g t h em T h e t wo di f f eren ces wo ul d t h en be d i vi d e d a nd t he p r o c e s s r e p e a t e d f o r t he ne xt s e t o f va l u e s Fo r t he i nt e g r a l t he f o l l o w i ng a pp ro x i m a t i o n wa s us e d: wh ere H i s t h e s t ep di f f eren ce i n t h e appl i ed f i el d dat a s equen ce. T h e ca l cul at ed en t ro py c ha ng e w a s p l o t t e d w i t h t e m p e r a t u r e f o r c o nve nt i o na l t w o d i m e ns i o na l g r a p hs a nd w i t h t he L ab VIE W pro gram was pl o t t ed wi t h b o t h t em perat ure an d a p p l i e d f i e l d t o o b t ai n a t h ree di m en s i o n al perspe ct i v e. 4. 3 LabVI EW P r ogr am T h e L a b V I E W p r o g r a m f o r t h e d e r iva t io n o f S f ro m m agn et i zat i o n dat a wa s wri t t en t o s a v e t h e u s e r la r g e a mo u n t s o f t ime t h a t w o u ld n o r ma lly b e n e c e s s a r y t o ma n u a lly m an i pu l at e t h e dat a i n E x cel T h e user b egi n s b y gi v i n g t h e pr o gr am i n pu t o f sam pl e m ass a n d lo c a t io n o f t h e r a w d a t a file fr o m t h e P P M S ( F i g u r e 4 1 a ) I f d e s ir e d t h e u s e r c o u ld

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42 s e l e c t o p t i o ns t o na m e a nd s a ve s p r e a d s he e t d a t a f o r m a g ne t i z a t i o n ( M ) v s appl i ed f i el d ( H ), m a g ne t i z a t i o n ( M ) v s t em perat ure ( T ), an d n egat i v e en t ro py ch an ge ( S ) v s t e m p e ra t u re ( T ) ( F i g u r e 4 1b ). W h en a l l n e c e s s a r y i n f or m a ti on i s e n te r e d a n d op ti on s s e l e c te d or d e s e l e c t e d t he u s e r r u ns t he p r o g r a m by s e l e c t i ng t he r u n bu t t o n l o c a t e d ne a r t he t o p o f t he scr een i n Lab VI E W ( F i gu r e 4. 1c) T h e pr o gr am o ut pu t s an y o pt i o n al l y sav ed f i l es an d al so o u t p u t s o n s c r e e n s w it h in t h e p r o g r a m t h e 2 D g r a p h s o f S v s T (F i g u re 4 .1 d ), M v s H (F i gure 4. 1e), an d M v s T ( F i g u r e 4 1 f ) A l s o i n c l u d e d w i th th e ou tp u t a r e th e 3 D g r a p h of S a s a fu n c t io n o f H an d T ( F ig u r e 4 1 g ) a n d t h e 3 D g r a p h o f M a s a fu n c t io n o f H a nd T ( F i gu r e 4. 1h ) T h e Lab VI E W sub VI t h at m akes t h e t h r ee di m en si o n al gr aph s po ssi b l e al so al l o ws t h e us er t o ro t at e an d/o r zo o m i n o n t h e graph i n an y di rect i o n On e co ul d eas i l y v i ew t h e 3 -D g ra p h s f rom 2 -D p e rs p e c t i v e s o r c l o s e l y e x a m i n e i n t e re s t i n g s e c t i o n s o f t h e 3 -D s u r f a c e Al l o f t he g r a p hs d i s p l a y e d i n t hi s r e s e a r c h ha ve be e n d i s p l a y e d w i t h S i n s te a d of f S Th e v a l u e s f o r S a re n e g a t i v e s i n c e t h e m a g n e t i z a t i o n w a s m e a s u re d f o r H f ro m 0 6 H wh i ch pro duces n egat i v e v al ues acc o rdi n g t o t h e M ax wel l rel at i o n By graph i n g t h e en t ro py f c ha ng e d a t a a s p o s i t i ve va l u e s t hi s p r e s e nt s t he d a t a a s o c c u r r i ng w he n H d e c r e a s e s fr o m H 6 0 w h i c h w ou l d b e th e c ool i n g c y c l e i n a n a d i a b a ti c p r oc e s s T h i s c on v e n ti on s te m s f r om t h e appl i cat i o n o f M CE f o r co o l i n g purpo s es an d predo m i n an t l y o ccurs i n l i t erat ure. Si n ce f t h e dat a was o n l y m easur ed as H go es f ro m 0 6 H t h i s co n v en t i o n as s um es t h at t h e en t ro py c h a n g e o c c u r s r e v e r s ib ly i n t h e ma t e r ia l.

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43 F igur e 4.1b MCE p r o gr am f i l e sel ect /sav e o pt i o n s. F igur e 4.1c M C E p r o g r a m o p e r a t i o n. F igur e 4.1a M C E p r og r a m i n p u t.

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44 F igu r e 4.1d M C E p r o g r a m p lo t o f S v s T F igur e 4.1e M C E p r o g r a m p lo t o f M v s H

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45 F igur e 4.1f M C E p r o g r a m p lo t o f M v s T F igur e 4.1g M C E p r o g r a m 3 D p lo t o f S ( H T ).

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46 4. 4 Cobal t Fer r it e Nanopar ti cl es T he f o l l o w i ng a r e t he r e s u l t s o f o u r s t u d i e s o n M C E u s i ng m a g ne t i z a t i o n d a t a a nd t he M a x w e ll r e la t io n a p p r o x im a t io n F ig u r e 4 2 a c o n t a ins t h e g r a p h o f M v s H f or c ob a l t f e r r i te n a n o p a rt i c l e s a n d F i g u re 4 .2 b c o n t a i n s t h e c o rre s p o n d i n g c u rv e s f o r M v s T T h e M v s H c u r v e s s h ow a q u i c k r e s p on s e i n m a g n e ti z a ti on a t l ow a p p l i e d f i e l d s w h i c h i s ty p i c a l of s u p e r p a ram agn et i c n an o part i cl es F ro m t h e M v s H c u r v e s th e c h a n g e i n e n tr op y was d e r iv e d I n F ig u r e 4 2 c t h e c a lc u la t e d e n t r o p y c h a n g e fo r c o b a lt fe r r it e n a n o p a r t ic le s is d i s p l a y e d Th e c h a n g e i n e n t ropy i s f o u n d t o b e g ra d u a l l y i n c re a s i n g o v e r a w i d e t e m p e ra t u re ch an ge wi t h t h e ab s o l ut e v al ue ran gi n g f ro m l es s t h an 510 J /kgK t o appro x i m at el y 1410 -2 -2 J /kgK f o r t h e h i gh es t appl i ed f i el d o f 30 kOe. T h i s t ren d i s di s pl ay ed f o r a t em perat ure r an ge F igur e 4.1h M C E p r o g r a m 3 D p lo t o f M ( H T ).

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47 F igur e 4.2b P lo t o f M v s T fo r c o b a lt f er r i t e n an o par t i cl es. F igur e 4.2a P lo t o f M v s H fo r c o b a lt f er r i t e n an o par t i cl es. F igur e 4.2c P lo t o f S v s T f o r co b al t f er r i t e n an o par t i cl es.

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48 o f 5 0 2 9 0 K. At t he l o w e r a p p l i e d f i e l d s o f 0 7 5 3 k O e a br o a d m a xi m u m i s f o u nd i n t he v i ci n i t y o f t h e b l o cki n g t ran s i t i o n at 223 K T h e b ro ad m ax i m um i s cen t ered aro un d 235 K w i t h a m a g ni t u d e o f a p p r o xi m a t e l y 1 5 1 0 J / k g K f o r a n a p p l i e d f i e l d o f 3 k O e T he t r e nd -2 o f en t ro py ch an ge i n creas i n g o v er a b ro ad t em perat ure r an ge i n n an o part i cl e s y s t em s h as b een o bs e r ve d p r e vi o u s l y by o t he r r e s e a r c he r s P e d e r s e n, e t a l o bs e r ve d t he s a m e be ha vi o r i n a 1-x x n an o co m po s i t e co n s i s t i n g o f F e Hg n an o part i cl es i n m ercury It s h o ul d b e po i n t ed o ut t h at 1-x x t h e S v al ues i n o ur st udi es are an o rder o f m agn i t ude h i gh er t h an t h at f o r t h e F e Hg s y s t em r e p or te d b y P e d e r s e n e t. a l T h e or e ti c a l m od e l i n g p r ov i d e d e v i d e n c e l i n k i n g th i s ty p e of 1-x x t ren d i n en t ro py ch an ge f o r t h e F e Hg s y s t em t o m agn et i c i n t eract i o n s am o n g t h e part i cl es [4 .1 ]. 4. 5 M anganese Z inc F er r it e Nanopar ti cl es In F i gure 4. 3, t h e cal cul at ed en t ro py ch an ge f o r m an gan es e zi n c f err i t e n an o part i cl es i s di s pl ay ed. L i ke t h e co b al t f err i t e s y s t em t h e ch an ge i n en t ro py i s f o un d t o b e v ery b ro ad w i th n o s h a r p f e a tu r e s ov e r a w i d e te m p e r a tu r e r a n g e N o m a x i m a w e r e ob s e r v e d d ow n to t he l o w e s t a p p l i e d f i e l d o f 1 k O e M o s t l i k e l y a m a xi m u m w o u l d ha ve be e n o bs e r ve d i n t he pro x i m i t y o f t h e b l o cki n g t ran s i t i o n h ad t h e ap p l i e d f i e l d b een l o wer. T h e o v eral l en t ro py ch an ge f o r t h i s s y s t em i s h i gh er t h an t h e en t ro py ch an ge f o r co b al t f err i t e wi t h t h e m ax i m um i n t h e ran ge ex t en di n g up t o appro x i m at el y 2010 J /kgK f o r an appl i ed f i e l d o f 3 0 kOe. -2 T h i s o b s erv at i o n i s i n go o d agreem en t wi t h t h eo ret i cal m o del s t h at h av e s ugges t ed t h at l arger M C E i s t o b e e x p e c t e d i n n a n o p a rt i c l e s y s t e m s w i t h u n i f o rm p a rt i c l e s i z e d i s t ri b u t i o n [4 .2 ]. I n a r e c e n t m a n u s c r i p t, w e r e p or te d ou r c h a r a c te r i z a ti on of th e M C E i n c ob a l t f e r r i te

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49 n a n o p a rt i c l e s a n d m a n g a n e s e z i n c f e rri t e n a n o p a rt i c l e s [4 .3 ]. 4. 6 Cobal t Nan opar ti cl es I n F ig u r e 4 4 a t h e g r a p h o f M v s H i s d i s p l a y e d f or c ob a l t n a n op a r ti c l e s a l on g w i th t h e co rr es po n di n g M v s T curv es i n F i gure 4. 4b A l arge n egat i v e s l o pe at l o w t em perat ures can b e v i s ual l y o b s erv ed i n t h e M v s T curv es gi v i n g go o d i n i t i al ev i den ce f o r a l arge ch an ge in e n t r o p y in t h is s ys t e m. F ig u r e 4 4 c c o n t a i n s t h e c a l c u la t e d e n t r o p y c h a n g e fo r c o b a lt n a n o p a r t ic le s T h e s h a p e o f t h e e n t r o p y c h a n g e c u r v e s is c h a r a c t e r is t ic a lly d iffe r e n t fo r t h is n a n op a r ti c l e s y s te m th a n w h a t w a s s h ow n f or c ob a l t f e r r i te a n d m a n ga n e s e z i n c f e r r i te na no p a r t i c l e s A w e l l d e f i ne d p e a k w i t h a n a bs o l u t e va l u e o f 2 3 J / k g K e xi s t s a t t he t e m p e ra t u re o f 1 3 K a n d a p p l i e d f i e l d o f 3 0 k O e Th i s m a x i m u m i n e n t ropy c h a n g e o c c u rs F ig ur e 4 .3 P lo t o f S v s T f o r m an gan ese zi n c f er r i t e n an o par t i cl es.

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50 F igur e 4.4b P lo t o f M v s T fo r c o b a lt n a n o p a rt i c l e s F igur e 4.4a P lo t o f M v s H fo r c o b a lt n an o par t i cl es. F igur e 4.4c P lo t o f S v s T f o r co b al t n an o par t i cl es.

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51 m uch l o wer t h an t h e b l o cki n g t em perat ure o f 135 K f o r t h i s s y s t em A s di s cus s ed i n ch apt er 3 th i s a d d i ti on a l e v i d e n c e of a s e c on d tr a n s i ti on a t v e r y l ow te m p e r a tu r e c a n b e u n d e r s tood in t h e fr a me w o r k o f a c o r e s h e ll s t r u c t u r e o f t h e n a n o p a r t ic le s T h e c o r e u n d e r g o e s s p in S b l o c k i n g a t h i g h t e m p e ra t u re w h e re a s t h e s u rf a c e s p i n s i n t h e s h e l l re g i o n f re e z e a t 1 6 K (T ) wh i ch co rr e l a t e s w e l l w i t h t h e S m a xi m u m a t 1 3 K. As e vi d e nc e d by Fi g u r e 4 4 c t he e n t ropy c h a n g e a t t h e s u rf a c e b l o c k i n g t ra n s i t i o n f a r o u t w e i g h s t h e e n t ropy c h a n g e a t t h e c o re b l o cki n g t ran s i t i o n In a rece n t m an us cri pt we h av e repo rt ed t h e o b s erv at i o n o f t h i s l arge M C E a s s oc i a te d w i th th e or d e r d i s or d e r tr a n s i ti on of th e s u r f a c e s p i n s [ 4 4 ]. A n o t h er ob s e r v a ti on c on c e r n i n g e n tr op y c h a n g e i n th i s s y s te m i s th a t h i g h e r a p p l i e d f i e l d s a p p e a r to s hi f t t he w e l l d e f i ne d p e a k t o hi g he r t e m p e r a t u r e s Fo r t he l o w e r a p p l i e d f i e l d o f 2 0 k O e t he peak, n o w at 1. 7 J /k g K, ha s s hi f t ed do wn t o 11 K F o r t h e n ex t l o wer appl i ed f i el d o f 10 k O e th e p e a k n ow a t 1 0 J / k g K d oe s n ot s h i f t d ow n b u t th i s m i g h t b e d u e to th e r e s ol u ti on o f t h e m e a s u re m e n t co r e s hell 4. 7 Co Ag Nanopar ti cl es c o r e s h e ll I n F ig u r e 4 5 a t h e g r a p h o f M v s H i s d i s p l a y e d f o r C o Ag na no p a r t i c l e s a l o ng wi t h t h e co rr es po n di n g M v s T c u rv e s i n F i g u re 4 .5 b A s w i t h t h e c o b a l t n ano p art i c l e s a l arge n egat i v e s l o pe at l o w t em perat ures c an b e v i s ual l y o b s erv ed i n t h e M v s T c u r ve s g i vi ng g o o d i ni t i a l e vi d e nc e f o r a l a r g e c ha ng e i n e nt r o p y i n t hi s s y s t e m Fi g u r e 4 5 c c o nt a i ns t he c o r e s h e ll c a lc u la t e d e n t r o p y c h a n g e fo r C o A g n a n o p a r t ic le s T h e a d d it io n o f t h e s ilv e r s h e ll d id n o t af f ect t h e o v eral l s h a p e o f t h e en t ro py ch an ge curv es A wel l def i n ed pea k wi t h an a bs o l u t e va l u e o f 2 3 J / k g K e xi s t s a t t he t e m p e r a t u r e o f 1 5 K a nd a p p l i e d f i e l d o f 3 0 k O e As

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52 F igur e 4.5b P lo t o f M v s T f or c o r e s h e ll Co A g n an o par t i cl es. F igur e 4.5a P lo t o f M v s H f or c o r e s h e ll Co A g n an o par t i cl es. c o r e s h e ll F igur e 4.5c P lo t o f S v s T f o r Co A g n an o par t i cl es.

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53 wi t h t h e co b al t n an o part i cl es t h i s m ax i m um en t ro py ch an ge o ccurs at a t em perat ure m uch l o wer t h an t h e b l o cki n g t em perat ure o f 150 K f o r t h i s s y s t em Ho wev er, s i n ce t h e s urf ace S s p in s fr e e z e a t t h e h ig h e r t e mp e r a t u r e o f 2 7 K ( T ) t h e ma x imu m S a t 1 5 K d oe s n ot c o r e s h e ll co rr el at e as s t ro n gl y as wh at was o b s erv ed i n t h e co b al t s y s t em T h e C o A g s y s t em f ol l ow s th e p e a k s h i f t b e h a v i or e x h i b i te d b y th e c ob a l t n a n op a r ti c l e s F or a n a p p l i e d f i e l d of 20 kOe, t h e peak, n o w at 1. 7 J /kgK h as s h i f t ed do wn t o 13 K an d f o r an appl i ed f i el d o f 10 k O e t he p e a k no w a t 0 9 J / k g K, ha s s hi f t e d d o w n t o 1 1 K. 8 16 30 4. 8 Eu Ga Ge Type I and VII I Cl ath r ates 81 63 0 I n F ig u r e 4 6 a t h e g r a p h o f M v s H i s d i s p l a y e d f o r b u l k Eu G a G e Ty p e I a n d V III c l a t h ra t e s Th o u g h h a v i n g t h e s a m e s t o i c h i o m et r y p er un i t c e l l t h e d i f f e re n c e i n s t ru c t u re pl ay s a ro l e i n t h e m agn et i c pro pert i es o f b o t h m at eri al s as ev i den ced b y t h e M v s H cur v es. T h e Ty pe VIII c l a t h rat e reac h es i t s s at urat i o n m agn et i zat i o n o f 65 em u/g at a m uch l o wer a p p l i e d f i e l d t h a n t h e Ty p e I c l a t h ra t e w h i c h h a s a s a t u ra t i o n m a g n e t i z a t i o n o f 5 2 e m u /g In a d d i t i o n t h e Ty p e V III c l a t h ra t e e x h i b i t s c o n v e n t i o n a l s h a p e s f o r M v s H c u r ve s w hi l e t he cur v es f o r t h e T y pe I cl at h r at e di spl ay t wo o r t h r ee di f f er en t sl o pes as M appro ach es sat ur at i o n Gi v en t h e di f f er en ces i n cl at h r at e cages b et ween t h e T y pe I an d VI I I cl at h r at es, t h e di f f eren ces b et wee n t h ei r r es pect i v e M v s H g ra p h s m o s t l i k e l y re l a t e t o t h e u n i q u e n a t u re o f t he E u m o m e nt s t r a p p e d i n t he c a g e s a nd t he i r c o u p l i ng t o t he l a t t i c e I n Fi g u r e 4 6 b, t he 2+ 81 63 0 c a l c u l a t e d e nt r o p y c ha ng e s f o r E u G a G e T y p e I a nd V I I I a r e d i s p l a y e d At 3 0 k O e t he Ty p e I c l a t h ra t e s h o w s a m a x i m u m e n t ropy c h a n g e o f 6 .0 J /k g K a t 9 K w h i l e t h e Ty p e V III cl at h rat e s h o ws a m ax i m um en t ro py ch an ge o f 9. 3 J /kgK at 14 K T h e m ax i m um i n en t ro py

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54 81 63 0 F igur e 4.6b P lo t o f S v s T f o r E u Ga Ge cl at h r at es. 81 63 0 F igur e 4.6a P lo t o f M v s H f o r E u Ga Ge cl at h r at es.

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55 c h a n g e f o r t h e Ty p e V III c l a t h ra t e s h o w s g o o d c o rre l a t i o n w i t h t h e C u ri e t e m p e ra t u re a t 1 2 .4 K Ho wev er, t h e m ax i m um i n en t ro py ch an ge do es n o t co rr el at e wel l wi t h t h e t ran s i t i o n at th e C u r i e te m p e r a tu r e f or th e T y p e I c l a th r a te F i g u r e 4 6 c r e v e a l s s om e f u r th e r i n f or m a ti on c o nc e r ni ng t he e nt r o p y c ha ng e f o r t he T y p e I c l a t hr a t e t ha t i s no t e a s i l y s e e n i n Fi g u r e 4 6 b. Th e f i g u re c o n t a i n s t h e g ra p h o f t h e d e ri v a t i v e o f m a g n e t i z a t i o n w i t h re s p e c t t o t e m p e ra t u re wh i ch i s t h e f i rst s t ep i n cal cul at i n g en t ro py ch an ge. T wo m ax i m a are o b s erv ed i n t h i s graph at 9 K an d 33 K A cl o s e l o o k at t h e en t ro py ch an ge o f t h e T y pe I cl at h rat e i n F i gure 4. 6b r e v e a ls t h a t a le s s e r ma x imu m d o e s in d e e d o c c u r a t a p p r o x ima t e ly 3 3 K b u t is g r e a t ly di m i n i s h ed co m pared t o t h e m ax i m um at 9 K part i cul arl y at h i gh er f i el ds T h e o ccurren ce o f d u a l ma x ima in e n t r o p y c h a n g e h a s b e e n o b s e r v e d f o r o t h e r s ys t e ms s u c h a s b u lk 0 .5 0.5 3 N d S r M n O m e n t i o n e d i n c h a p t e r 1 [ 4 5 ] M u l t i p l e m a x i m a t y p i c a l l y c o r r e l a t e w i th m ul t i pl e t ran s i t i o n s o f ei t h er st ruct u r a l o r m a g net i c ch aract er. In t h e ca s e o f t h e Ty pe I c l a th r a te th e n a tu r e of th e l a r g e e n tr op y c h a n g e a t l ow te m p e r a tu r e i s n ot w e l l u n d e r s tood at t h i s t i m e an d i s t h e s ub j ect o f o n go i n g s t udy It i s n o t ab l e t h at t h e en t ro py ch an ge o b s erv ed 81 63 0 F igur e 4.6c P lo t o f d M /d T v s T f o r E u Ga Ge T y pe I cl at h rat e.

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56 i n t h ese cl at h r at e m at er i al s i s o n l y ab o ut an o r der o f m agn i t ud e l o wer t h an t h at o f gi an t MCE ma t e r ia ls I n a r e c e n t ma n u s c r ip t w e r e p o r t e d t h e s e s t u d ie s o f t h e ma g n e t o c a lo r ic e ffe c t in 81 63 0 E u G a G e T y p e I a nd V I I I c l a t hr a t e s [ 4 6 ] T he ne xt a nd f i na l c h a p t e r s u m m a r i z e s t he ch aract eri zat i o n an d an al y s i s o f t h e n an o part i c l e a nd cl at h rat e s y s t em s an d di s cus s es c o nc l u s i o ns a nd f u t u r e w o r k r e g a r d i ng t hi s r e s e a r c h.

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57 C h a p t e r 5 S u mm a ry C o n c lu s i o n s a n d F u t u re W o rk 5 1 S u mm a ry Fo u r na no p a r t i c l e s y s t e m s a nd t w o bu l k s y s t e m s w e r e c ha r a c t e r i z e d f o r t he m a g ne t o c a l o r ic e ffe c t C o ba l t f e r r i t e na no p a r t i c l e s w e r e s y nt he s i z e d i n o u r l a b by d e c o mp o s in g me t a lo r g a n ic s a l t s a n d w e r e c a p p e d d u r in g t h e r e a c t io n b y t h e fa t t y a c id s urf act an t o l ei c ac i d. T h i s n an o part i cul at e m at eri al was f o un d t o b e po l y di s perse wi t h a s i ze di s t ri b ut i o n o f 5 3 n m an d a b l o c k i ng t e m perat ure o f 223 K T h e ch an ge i n en t ro py was o b s e r v e d t o b e g r a d u a lly in c r e a s in g in t h e t e mp e r a t u r e r a n g e o f 5 0 2 9 0 K a n d a p p lie d fie ld o f 3 0 k O e w it h t h e ma x im u m v a lu e r e a c h ing a p p r o x im a t e ly 1 4 1 0 J / k g K. F o r fi e ld s o f 3 -2 k O e a n d l ow e r th e c h a n g e i n e n tr op y w a s ob s e r v e d to f or m a m a x i m u m th a t c or r e l a te d w i th t h e t ran s i t i o n at t h e b l o cki n g t em perat ure. M a ng a ne s e z i nc f e r r i t e na no p a r t i c l e s w e r e s y nt he s i z e d by c o l l a bo r a t o r s a nd ha d t he 0.68 0.25 2.07 4 s t o ic h io me t r y o f M n Z n F e O T h e s ys t e m w a s mo n o d is p e r s e w it h a n a v e r a g e p a r t ic le s i z e of 1 5 n m a n d a s i z e d i s tr i b u ti on of l e s s th a n 2 n m T h e s e p a r ti c l e s w e r e a l s o c a p p e d w i th a s urf act an t A OT, duri n g t h ei r f o rm at i o n T h e b l o cki n g t em perat ure f o r t h i s s y s t e m was a r ou n d 4 6 K a n d d i s p l a y e d a m u c h n a r r ow e r Z F C p e a k th a n th e c ob a l t f e r r i te s y s te m d u e to t h e mo n o d is p e r s it y o f t h e s ys t e m. T h e c h a n g e in e n t r o p y w a s o b s e r v e d t o a ls o b e g r a d u a lly i nc r e a s i ng i n t he t e m p e r a t u r e r a ng e o f 5 0 2 9 0 K a nd a p p l i e d f i e l d o f 3 0 k O e w i t h t he m a x i m u m v a l u e re a c h i n g 2 0 1 0 J /k g K -2

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58 C o ba l t na no p a r t i c l e s w e r e s y nt he s i z e d by c o l l a bo r a t o r s T he p a r t i c l e s ha d a n e xc e l l e nt s p h e ri c a l s h a p e a n d w e re c a p p e d w i t h o l e i c a c i d Th e p a rt i c l e s i z e w a s a p p rox i m a t e l y 4 9 .5 nm a nd t he s y s t e m w a s ve r y m o no d i s p e r s e T he s y s t e m w a s f o u nd t o h a v e a bl o c k i ng t e mp e r a t u r e o f 1 3 5 K b u t a ls o d is p la ye d a s h a r p t r a n s it io n in ma g n e t iz a t i o n a t 1 6 K T h is s e c o n d t ra n s i t i o n i s a t t ri b u t e d t o t h e b l o c k i n g o f s u rf a c e s p i n s a t a m u c h l o w e r t e m p e ra t u re th a n th e b l oc k i n g of c or e s p i n s T h e c h a n g e i n e n tr op y w a s f ou n d to s tr on g l y c or r e l a te w i th t he l o w e r t e m p e r a t u r e t r a ns i t i o n w i t h a p e a k va l u e o f a p p r o xi m a t e l y 2 3 J / k g K a t 1 3 K a nd 30 kOe. I n addi t i o n t h e pea k i n en t ro py ch an ge appe ared t o s h i f t t o h i gh er t em perat ures a t h i gh er app l i ed f i el ds. c o r e s h e ll C o A g n a n o p a r t ic le s w e r e r e c e iv e d fr o m t h e s a me s o u r c e a s t h e c o b a lt n a n o p a r t ic le s T h e ma jo r d iffe r e n c e b e t w e e n t h e t w o s ys t e m s w a s t h a t t h e c o b a lt co re shell n a n o p a rt i c l e s w e re s o l e l y c o v e re d w i t h o l e i c a c i d w h i l e t h e C o A g n a n o p a rt i c l e s w e re s o l e l y c o ve r e d w i t h a s i l ve r s he l l w i t ho u t t he s u r f a c t a nt c o a t i ng Ano t he r d i f f e r e nc e w a s t he c o r e s h e ll f a c t t ha t t he c o ba l t c o r e i n t he C o Ag s y s t e m w a s s m a l l e r w i t h a d i a m e t e r o f 4 0 nm T he ov e r a l l a v e r a g e p a r ti c l e s i z e w a s 9 7 n m w i th a co b al t co re o f 4 0 n m a n d a s i l v e r s h e l l of a p p r o xi m a t e l y 2 8 5 nm T he s y s t e m w a s f o u nd t o ha ve a bl o c k i ng t e m p e r a t u r e o f 1 5 0 K a nd a s eco n d t ran s i t i o n as wi t h t h e co b al t n an o part i cl es at 27 K T h i s s y s t em h as t h e un us ual ch aract eri s t i c o f h av i n g h i gh er t ran s i t i o n t em perat ures wi t h a s m al l er co b al t co re v o l um e t h an t h e co m par ab l e co b al t n an o par t i cl es co at ed wi t h o l ei c aci d. A s wi t h t h e co b al t n an o par t i cl es, t h e en t ro py ch an ge was f o un d t o co rr el at e wi t h t h e l o wer t em perat ure t ran s i t i o n wi t h a pea k v al ue o f appro x i m at el y 2. 3 J /kgK at 15 K an d 30 kOe. T h i s s y s t em al s o di s pl ay s t h e pea k s h i f t f ro m ch an gi n g appl i ed f i el d.

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59 8 1 63 0 E u Ga G e T y p e I an d VI I I cl at h r at es wer e sy n t h esi zed by Dr G. No l as. T h ese c l a t hr a t e s c o ns i s t o f g a l l i u m a nd g e r m a ni u m c a g e s w i t h e u r o p i u m i o ns e m be d d e d w i t hi n t he cage s It h as b een f o un d t h at t h e euro pi um i o n s h av e n earl y t h e s am e m agn et i c m o m en t as i f t h ey w er e f r ee i o n s an d t h at t h e spo n t an eo us f er r o m agn et i sm at l o wer t em per at ur e m o st lik e ly o c c u r s d u e t o R K K Y in t e r a c t io n b e t w e e n t h e mo me n t s [ 5 1 3 ] T h e C u r ie t e m p e ra t u re s o f t h e s e m a t e ri a l s o c c u r a t 3 3 .9 K f o r t h e Ty p e I a n d 1 2 .4 K f o r t h e Ty p e V III T h e ch an ge i n en t ro py f o r t h e Ty pe I cl at h rat e h as a m ax i m um o f 6. 0 J /kgK at 9 K an d 30 kOe wh i l e t h e en t ro py ch an ge f o r t h e Ty pe VIII h as a m ax i m um o f 9. 3 J /kgK at 14 K an d 30 k O e T h e p e a k e n tr op y c h a n g e f or th e T y p e V I I I c l a th r a te s h ow s e x c e l l e n t c or r e l a ti on w i th t h e f err o m agn et i c t o param agn et i c t ran s i t i o n at t h e C uri e t em perat ure wh i l e t h e peak e n t ro py ch an ge f o r t h e Ty pe I cl at h rat e do es n o t co rr el at e wi t h t h e t ran s i t i o n at t h e C uri e t em perat ure. Si n ce m ax i m um s i n en t r o p y ch an ge are t y pi cal l y o b s erv ed aro un d t ran s i t i o n s f urt h er i n v es t i gat i o n o f t h e pea k en t r o p y c ha ng e f o r t h e Ty pe I cl at h rat e i s n ece s s ary f o r a b et t er u n d e r s t a n d in g o f t h e M C E in t h is ma t e r ia l. 5.2 C onc l us ion s T h e m a te r i a l s i n v ol v e d i n th i s s tu d y h a d s om e d e g r e e s of s i m i l a r i ty w h i c h a l l ow e d f or c om p a r i s on b e tw e e n th e m C ob a l t f e r r i te n a n op a r ti c l e s a n d m a n g a n e s e z i n c f e r r i te na no p a r t i c l e s w e r e s i m i l a r d u e t o t he i r i nve r s e s p i ne l s t r u c t u r e s u r f a c t a n t c o a t i ng a nd n a n o p a r t ic u la t e n a t u r e A p o s s ib le c o n c lu s io n w h ic h c a n b e d r a w n fr o m t h e s e t w o ma t e r ia ls is t h a t t h e mo n o d is p e r s e n a t u r e o f t h e ma n g a n e s e z in c fe r r it e e n h a n c e d t h e M C E o f t h is ma t e r ia l. T h is p a r t ic u la r c h a r a c t e r is t ic h a s a lr e a d y b e e n o b s e r v e d in o t h e r n a n o p a r t ic le

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60 s y s t e m s [ 5 4 5 ] H o w e ve r t he e nha nc e m e nt w a s no t l a r g e a nd o t he r f a c t o r s s u c h a s d i f f e r e nt s urf act an t wei gh t s an d di f f eren t cat i o n o ccupa n cy di d h a v e an ef f ect o n en t ro py ch an ge c a l c u l a t i o n B o t h o f t h e s e m a t e ri a l s s h o w a b road e n t ropy c h a n g e o v e r a w i d e t e m p e ra t u re ran ge wh i ch i s a cruci al ch aract eri s t i c f o r ef f i ci en t m agn et i c ref ri gerat i o n wi t h a l arge ran ge o f us ef ul o perat i n g t em perat ure. Un f o rt un at el y t h es e m at eri al s do n o t ex h i b i t a l arge ch an ge i n e n tr op y a n d f u tu r e w or k s h ou l d f oc u s on e n h a n c i n g th e e f f e c t. C o b a lt fe r r it e n a n o p a r t i c l e s ma n g a n e s e z in c fe r r it e n a n o p a r t ic le s a n d c o b a lt n a n op a r ti c l e s w e r e c om p a r a b l e d u e to e a c h h a v i n g a s u r f a c ta n t c oa ti n g a n d a n a n op a r ti c u l a te n a t u re Th e c o b a l t f e rri t e a n d m ang a n e s e z i n c f e rri t e n a n o p a rt i c l e s b o t h d i s p l a y e d a v e ry b ro ad b ut wea k ch an ge i n en t ro py o v er a wi de t em perat ure ran ge. L o wer appl i e d f i el ds r ev eal ed a m ax i m um en t ro py ch an ge i n co b al t f err i t e aro un d t h e b l o cki n g t ran s i t i o n b ut c o mp a r a t iv e a p p lie d fi e ld s d id n o t r e v e a l a ma x imu m in ma n g a n e s e z in c fe r r it e M o s t lik e ly, t h i s b eh av i o r wo ul d h av e b een o b s erv ed i n m an gan es e zi n c f err i t e h ad l o wer m agn i t ude appl i ed f i el ds b een us ed. T h e co b al t n an o pa r t i c l e s pro b ab l y al s o ex h i b i t a we ak m ax i m um aro un d t h e b l o cki n g t ran s i t i o n at wea k f i el ds Ho wev er, t h e pea k en t ro py c han ge l o cat ed appro x i m at el y at t h e t ran s i t i o n as s o ci at ed wi t h s urf ace s pi n f reezi n g h as a l arge m agn i t ude s u c h th a t i t p r ob a b l y w a s h e d ou t tr a n s i ti on s on th e or d e r of 1 0 to 1 0 J / k g K A c on c l u s i on -1 -2 t h a t c a n b e d r a w n fr o m t h is a n a lys is is t h a t t h e t r a n s it io n a s s o c ia t e d w it h t h e s u r fa c e s p in s in a n a n o p a r t ic le s ys t e m p r o d u c e s a la r g e r M C E t h a n w h a t t h e b lo c k in g t r a n s it io n a lo n e w o u ld p rodu c e i n t h e s a m e s y s t e m c o r e s h e ll Co b al t n an o part i cl es an d Co A g n an o part i cl es were co m parab l e b eca us e ea ch co n t ai n ed a c o b al t co re t h at was s y n t h es i zed i n ex act l y t h e s am e m an n er. T h e o n l y di f f eren ce

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61 b e t w e e n t h e s e t w o s ys t e m s w a s t h e o u t e r s h e lls o f o le ic a c id a n d s ilv e r B o t h s ys t e ms d i s p l a y e d a l ow te m p e r a tu r e tr a n s i ti on a s c r i b e d to s u r f a c e s p i n b l oc k i n g a n d th e tr a n s i ti on h ad t h e s am e ch aract eri s t i c s h ape, m agn i t ude i n creas e wi t h appl i ed f i el d i n creas e, an d peak s hi f t w i t h appl i ed f i el d i n creas e. Ho wev er, as dem o n s t rat ed i n ch apt er 4, an i n t ere s t i ng c o r e s h e ll ph en o m en o n o ccurs wi t h t h e C o A g n an o part i cl es A l t h o ugh po s s es s i n g a s m al l er vo l u m e t ha n t he o r i g i na l c o ba l t na no p a r t i c l e s t he t e mp e r a t u r e o f bo t h t r a ns i t i o ns i n t he c o r e s h e ll C o A g s ys t e m s h ift t o h ig h e r t e mp e r a t u r e s A t t h is p o in t n o r ig o r o u s e x p la n a t io n fo r t h is c o r e s h e ll p he no m e no n e xi s t s Fr o m a n a p p l i c a t i o n p e r s p e c t i ve t he m e t a llic s he l l o f t he C o Ag p a r t i c l e s w o u l d p r o vi d e be t t e r he a t e xc ha ng e w i t h t he s u r r o u nd i ng l a t t i c e w hi c h i s i m p o r t a nt f or r e f r i g e r a ti on S i n c e th e m a x i m u m s f or e n tr op y c h a n g e f a l l a t l ow te m p e r a tu r e s f or b oth o f t h ese m at er i al s, t h ei r m ax i m um ef f i ci en cy i n co o l i n g wo ul d o n l y b e at l o wer t em per at ur es. 81 63 0 E u G a G e T y p e I a nd V I I I w e r e c o m p a r a bl e s i nc e t he y a r e bo t h c l a t hr a t e s w i t h t he s am e s t o i ch i o m et ry T h o ugh h av i n g t h e s am e s t o i ch i o m et ry t h e t wo cl at h rat es co n t ai n ed d i f f e r e nt c a g e s t r u c t u r e s w hi c h e f f e c t e d d i f f e r e nc e s be t w e e n t he m a g ne t i c p r o p e r t i e s o f t he t wo s ub s t an ces T h e pro b e o f en t ro py ch an ge i n t h e Ty pe VIII cl at h rat e rev eal ed a m ax i m um at appro x i m at el y t h e Curi e t em perat ure wh i ch i s t y pi cal am o n g o t h er m at eri al s wi t h s i m i l ar t ran s i t i o n s T h e pro b e o f en t ro py ch an ge i n t h e T y pe I cl at h rat e rev eal ed a m ax i m um t h at was c l o s e i n t e m p e ra t u re p l a c e m e n t t o t h e m a x i m u m i n t h e Ty p e V III c l a t h ra t e a n d o n l y a v e ry s m a l l m a x i m u m a t i ts ow n C u r i e te m p e r a tu r e T w o p os s i b i l i ti e s e x i s t f or th e e x p l a n a ti on of t he s e o bs e r va t i o ns O ne t he M C E i n t he T y p e V I I I c l a t hr a t e f o l l o w s t y p i c a l be ha vi o r a r o u nd i ts f e r r om a g n e ti c to p a r a m a g n e ti c tr a n s i ti on a n d th e T y p e I c l a th r a te p os s e s s e s a tr a n s i ti on c u r r e nt l y u nd e s c r i be d a t t he m a xi m u m f o r i t s e nt r o p y c ha ng e T he o t he r p o s s i bi l i t y i s t ha t t he

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62 M C E i n b oth c l a th r a te s i s c on tr ol l e d b y a t ran s i t i o n oth e r th a n th e f e r r om a g n e ti c to p a r a m a g n e ti c tr a n s i ti on a n d th e m a x i m u m i n e n tr op y c h a n g e f or th e Ty p e V I I I c l a th r a te h a p p e n s to f a l l c l os e to t h e C u r i e te m p e r a tu r e T h e m os t l i k e l y s c e n a r i o i s th e f i r s t p os s i b i l i ty b u t th e oth e r p os s i b i l i ty c a n n ot b e r u l e d ou t a t th i s p oi n t. F u r th e r s tu d y w i l l b e n e c e s s a r y to an s wer t h es e ques t i o n s Co n cl us i o n s co n cern i n g pract i cal appl i cat i o n o f t h es e m at eri al s can b e r e a d ily s e e n S in c e b o t h o f t h e s e ma t e r ia ls a r e a ls o e x c e lle n t t h e r mo e le c t r ic ma t e r ia ls t h is s tu d y r e v e a l s f or th e f i r s t ti m e th e p os s i b i l i ty of d u a l f u n c ti on a l i ty w i th th e s e m a te r i a l s N ot o n l y c o u l d s o m e a p p l i c a t i o n u s e t h e s e m a t e ri a l s f o r p o w e r g e n e ra t i o n f rom t e m p e ra t u re g r a d i e nt s o r vi c e ve r s a bu t t he s e m a t e r i a l s c o u l d a l s o be u s e d i n t he s a m e d e vi c e f o r he a t i ng o r co o l i n g v i a a m ag ne t i c fi e l d S h o ul d a d ev i ce b e r eal i zed f o r ut i l i zi n g t h ese m at er i al s, co o l i n g at l o wer t em perat ures wo ul d b e t h e o n l y pract i cal ran ge s i n ce t h e m ax i m a i n en t ro py ch an ge l i e at l o w t em per at ur es. 5 3 F u t u re W o rk Fu t u r e c ha r a c t e r i z a t i o n i s ne c e s s a r y f o r a f u l l u nd e r s t a nd i ng o f t he M C E i n a l l o f t he ma t e r ia ls in v o lv e d w it h t h is s t u d y. A s me n t io n e d in c h a p t e r 1 c h a n g e in e n t r o p y is t yp ic a lly c o r r e l a t e d w i t h t e m p e r a t u r e c h a n g e bu t no t ne c e s s a r i l y d u e t o t he d e p e nd e nc y o f t he t em perat ure ch an ge o n h eat capa ci t y E i t h er h eat capa ci t y dat a o r di rect t em perat ure ch an ge m e a su r e m e nt i s ne c e ss a r y f o r a f u l l k no w l e d g e o f MC E c o nce r ni ng a p a r t i c u l a r m a t e r i a l. M o re part i cul ar r eas o n s f o r f urt h er ch aract eri zat i o n are t h e f o l l o wi n g. T h e cl at h rat e s am pl es i n t hi s s t u d y r e q u i r e m o r e s t u d y d u e t o t he p o s s i b ilit ie s o p e ne d by t he i r p o s s e s s i ng t h e r m o e l e c t r i c a nd m a g ne t o c a l o r i c p r o p e r t i e s Fu r t he r s t u d y i s ne e d e d p a r t i c u l a r l y f o r t he

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63 T y pe I cl at h rat e i n o rder t o as cert ai n t h e n at ure o f t h e t ran s i t i o n wh ere t h e pea k i n en t ro py c o r e s h e ll ch an ge f al l s T h e co b al t an d Co A g n an o part i cl es requi re f urt h er st udy f o r a b e t t er u n d e r s t a n d in g o f t h e s u r fa c e s p in b e h a v io r in t h e s e ma t e r ia ls S o me s t e p s t a k e n t o w a r d t h is o b je c t iv e h a v e b e e n s t u d ying me mo r y e ffe c t s t h a t e x is t in t h e s e t r a d it io n a l s u p e r p a r a ma g n e t ic c o r e s h e ll p a r t i c l e s I n a d d i t i o n, p a r t i c l e s i z e s a nd s he l l t hi c k ne s s e s f o r t he C o Ag s y s t e m a nd part i cl e s i zes wi t h t h e co b al t s y s t em n eed t o b e s y s t em at i ca l l y s t udi ed f o r f urt h er u nd e r s t a nd i ng o f t he R KKY i nt e r a c t i o n be t w e e n p a r t i c l e s a nd t he e nha nc e m e nt o f e f f e c t i ve a n i s o t ropy O th e r p os s i b i l i ti e s f or f u tu r e w or k h a v e s t em m ed f r om th i s p r oj e c t. A v a r i e ty of fe r r it e s c a n b e s yn t h e s iz e d u s in g v ir t u a lly t h e s a me p r o c e d u r e lis t e d in c h a p t e r 2 T h e o n ly s eem i n g h i n dran ce t o t h i s en deav o r wo ul d b e f i n di n g t h e ace t y l ace t o n at e s al t wi t h t h e des i red cat i o n A l s o t h i s part i cul ar r eac t i o n can b e us ed t o m ake s ev eral po l y he d r a l s h apes as d e m o n s t ra t e d b y o t h e rs [5 .6 ,7 ]. Th i s e n d e a v o r h a s b e e n s u c c e s s f u l l y a t t e m p t e d b y t h e a u t h o r. H ow e v e r s y n th e s i z i n g on l y on e p ol y h e d r on h a s b e e n a d i f f i c u l ty n ot o v e r c om e a t th i s p oi n t. Pr o c e d u ra l re f i n e m e n t b e t t e r s e p a ra t i o n t e c h n i q u e s a n d p o s s i b l y p u re r re a c t a n t s a re n e c e s s a ry f o r t h e s e re a c t i o n s t o g row on l y o n e p o l y h e d ron A n oth e r a s p e c t of f u tu r e s tu d y r e g a r d i n g c h e m i s tr y i s th e v a r i a ti on of s u r f a c ta n t. Co r r el at i o n b et ween i n t er par t i cl e d i s t a nc e an d t h e MCE n eeds t o b e an al y zed i n t h ese na no p a r t i c l e s y s t e m s T hi s i nvo l ve s s i m p l y u s i ng a l o ng e r o r s ho r t e r c ha i ne d f a t t y a c i d i n t he s y n t h es i s o r r em o v i n g t h e s urf act an t af t er t h e react i o n an d reco at i n g wi t h an o t h er f at t y aci d. R e m o va l a nd r e p l a c e m e nt o f t he s u r f a c t a nt w o u l d be a r i s k y p r o c e d u r e s i nc e t hi s c o a t i ng p rot e c t s t h e s u rf a c e o f t h e n a n o p a rt i c l e f rom re a c t i n g w i t h t h e s u rroun d i n g e n v i ron m e n t A

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64 f i n al area o f f ut ure s t udy regardi n g ch em i s t ry rel at ed t o t h e s y n t h es i s i n ch apt er 2 i s ex ch an ge bi a s O ne o f t he p o s s i bl e u n d e s ir e d p r o d u c t s f r o m t he r e a c t i o n i s a n o xi d e o f o ne o f t he cat i o n s Gi v en t h e co rr ect ph as e o f t h e o x i de, t h i s pro v i des a m e ch an i s m f o r gr o wi n g an an t i f err o m agn et i c m at eri al W h i l e n o t n o rm al l y des i red, t h i s perh aps co ul d b e m an i pul at ed f o r t h e p u rp o s e o f g rowi n g an ant i f e rrom a g n e t i c s h e l l o n a f e rri m a g n e t i c n a n o p a rt i c l e A s i m i l ar pr o c es s h as al ready b een dem o n s t rat ed wi t h gro wt h o f a M n O l ay er surro un di n g a 24 Co F e O n an o part i cl e al t h o ugh di f f i cul t y was ex peri en ced i n ach i ev i n g di s t i n ct an t i f err o m agn et i c an d f err i m agn et i c l ay ers [5. 8]. If po s s i b l e wi t h t h e react i o n i n ch apt er 2, e x c h a n g e b ia s e d n a n o p a r t ic le s mig h t s h o w a la r g e t u n a b le M C E d u e t o la r g e c h a n g e s in m agn et i zat i o n asso ci at ed w i t h t h e b r eaki n g o f t h e ex ch an ge b i as. Las t l y f ut ur e st ud y i n o ur l ab i nv o l ve s t h e co n st r uct i o n o f a pr o b e f o r t h e P P MS w hi c h w o u l d d i r e c t l y m e a s u r e t e m p e r a t u r e c ha ng e f o r t he p u r p o s e o f d i r e c t l y c ha r a c t e r i z i ng t h e M CE i n a m a t eri al T h i s wo ul d i n v o l v e co n s t ruct i n g an i n s ul at ed s am pl e h o l der wh i ch w o u l d be a t t a c he d t o a c o l u m n. T he s a mp le h o ld e r w o u l d c o nt a i n a hi g hl y s e ns i t i ve t h erm o co upl e wh i ch wo ul d run wi res up t h e c o l u m n t o t h e pro b e h ead. T h e pro b e h ead w o u l d s i t o n t o p o f t he P P M S s e a l i ng t he i nne r c ha m be r f o r t he p u r p o s e o f e n a bl i ng t he P P M S to f or m a v a c u u m i n th e c h a m b e r A L a b V I E W p r og r a m w ou l d b e w r i tte n to m on i tor an d s av e t h e t em perat ure dat a. I n addi t i o n t h e L ab VIE W pro gram wo ul d b e pro gram m ed t o c o n t r o l t h e P P M S s o t h a t t h e u s e r c o u ld s e t p a r a me t e r s fo r t e mp e r a t u r e a n d a p p lie d fie ld a nd l e a ve t he m a c hi ne t o w o r k a u t o no m o u s l y t i l l t he d a t a r u n f i ni s he d O nc e t hi s p r o be a nd s o f t w a r e a r e c o ns t r u c t e d f u l l c ha r a c t e r i z a t i o n o f t he M C E w i l l be c o nd u c t e d by o u r l a b.

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65 Ref er ences C h a p t e r 1 1 .1 E. W a rb u rg A nn. P hys v .1 3 p .1 4 1 1881 1 .2 A Ti s h i n Y Sp i c h k i n The Magnetocaloric Ef f ect and Its Applications In s t i t u t e o f Ph y s i c s Pu b l i s h i n g Ph i l a d e l p h i a 2003 1 .3 V Pe c h a rs k y K G s c h n e i d n e r. J MMM v .2 0 0 p .4 4 1999 1 .4 W G i a u q u e D M a c D o u g a l l P hys R ev v .4 3 p .7 6 8 1933 1 .5 J Sa u e r. P hys R ev v .6 4 p .9 4 1943 1 .6 D d e K l e rk R H u d s o n P hys R ev v .9 1 p .2 7 8 1953 1 .7 J W h e a t l e y T. Es t l e P hys R ev v .1 0 4 p .2 6 4 1956 1 .8 K A n d re s E. B u c h e r. P hys R ev. Let v .2 4 p .1 1 8 1 1970 1 .9 J Tu o ri n i e m i T. K n u u t t i l a P hys ica B v .2 8 0 p .4 7 4 2000 1 .1 0 T. K n u u t t i l a Nuc lear Magnetism and Super conductivity in Rhodium d i s s e rt a t i o n H e l s i n k i U n i v e rs i t y o f Te c h n o l o g y Es p o o F IN 2000 1 .1 1 K G s c h n e i d n e r, V Pe c h a rs k y A Ts o k o l R ep. Pr og. P hys v 68. p. 1479. 2005 1. 12. M Bo wm an “ T h e B i g Ch i l l ” I nq ui ry A m e s L a b o ra t o ry 1997 1 .1 3 V Pe c h a rs k y K G s c h n e i d n e r. J AP v .9 0 p .4 6 1 4 2001

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66 1 .1 4 B Y u Q G a o B Z h a n g X M e n g Z C h e n Int. J R e f r ig v 26. p. 622. 2003 1 .1 5 W H i rs c h l e r, W R o c k e r. Z. angew P hys v .2 1 p .3 6 8 1966 1 .1 6 R K o h l h a a s Magnetismus : St r uctur und E igens chaf ten Magnetisher F e s tk or pe r V EB Deu ts c he r V e r lag f ur Gr un ds tof f ind us tr ie (L e i p z i g ). p .1 3 4 1967 1 .1 7 M A n n a o ra z o v H G v e n K B rn e r. J. A ll. Com p v .3 9 7 p .2 6 2005 1. 18 P. San de, L. Hues o D. Mi gun s J Ri v as F Ri v adul l a, M. L pezQ u i n t e l a A PL v .7 9 p .2 0 4 0 2001 1 .1 9 V Pe c h a rs k y K G s c h n e i d n e r. P hys R ev. Let v .7 8 p .4 4 9 4 1997 1 .2 0 R M c M i c h a e l R Sh u l l L Sw a rt z e n d ru b e r, L B e n n e t t J MMM v 111. p .2 9 1992 1 .2 1 R Sh u l l IE E E Trans actions on Magnetics v .2 9 p .2 6 1 4 1993 1 .2 2 Y Sh a o J Z h a n g J L a i C Sh e k J AP v .8 0 p .7 6 1996 1 .2 3 Y Sh a o J L a i C Sh e k J MMM v .1 6 3 p .1 0 3 1996 1. 24 M. T an aka, Y. Mi saka, K S h i o m i T Yam am o t o T Nak agaw a, M. K a t s u ra T. N u m a z a w a T. N a k a y a m a K N i i h a ra Scr ipta M ater v 44. p .2 1 4 1 2001 1. 25 T Yam am o t o M. T an aka, T Nak ay am a, K Ni sh i m aki T Nak agaw a, M. K a t s u ra K N i i h a ra Jpn. J. A ppl. P hys v .3 9 p .4 7 6 1 2000 1. 26 T K i n o s h i t a, S Sei n o H. Maruy am a, Y. Ot o m e, K Oki t s u, T Naka y am a, K N i i h a ra T. N a k a g a w a T. Y a m a m o t o J. A ll. Com p v .3 6 5 p .2 8 1 2004

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67 1 .2 7 A G i g u e re M F o l d e a k i R D u n l a p R C h a h i n e P h y s Re v B v 59. p .4 3 1 1999 1 .2 8 F To rre s J H e rn n d e z X B o h i g a s J Te j a d a A PL v .7 7 p .3 2 4 8 2000 1 .2 9 X Z h a n g H W e i Z Z h a n g L Z h a n g P hys R ev. Let v 87. p. 157203-1. 2001 C h a p t e r 2 2 .1 Q Son g a n d Z Z h a n g JA CS v .1 2 6 p .6 1 6 4 2004 2. 2 Sh o uh en g Sun Hao Z en g, Dav i d Ro b i n s o n Si m o n e R ao ux Ph i l i p Ri ce, Sh a n W a n g a n d G u a n x i o n g L i JA CS v .1 2 6 p .2 7 3 2004 2 .3 S. M o rri s o n C C a h i l l E. C a rp e n t e r, S C a l v i n V H a rri s J AP v 93. p .7 4 8 9 2003 2 .4 P. P o d d a r, H Sr i k a n t h S. M o rri s o n E. C a rp e n t e r. J MMM v 288. p. 443. 2005 2. 5 D. S i dh ay e, T Bal a, S S r i n at h H. S r i kan t h P P o dd ar M. S ast r y B. Pr a s a d C he mi s tr y of Mate r ial s i n pr ess 2006 2. 6 P. Po ddar, S. Sr i n at h D. Reb ar, D. S i dh ay e, R. Pas ri ch a, B. Pr as ad, H. Sr i k a n t h P h y s Re v B. s u b m i t t e d 2006 2 .7 G N o l a s T. W e a k l e y J C o h n R Sh a rm a P h y s Re v B v 61. p. 3845. 2000

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68 2 8 S P a s c he n, W. C a r r i l l o C a br e r a A. B e nt i e n, V T r a n, M B a e ni t z Y G r i n, F Steg l i c h P h y s Re v B v .6 4 p .2 1 4 4 0 4 -1 2001 2. 9 G. No l as pri v at e co m m un i cat i o n 2005. C h a p t e r 3 3 .1 S. M o rri s o n C C a h i l l E. C a rp e n t e r, S C a l v i n V H a rri s J AP v 93. p .7 4 8 9 2003 3. 2 D. S i dh ay e, T Bal a, S S r i n at h H. S r i kan t h P P o dd ar M. S ast r y B. Pr a s a d C he mi s tr y of Mate r ial s i n pr ess 2006 3 .3 E. B i a s i C R a m o s R Z y s l e r, H R o m e ro. P h y s Re v B v 65. p. 144416. 2002 3 .4 R Z y s l e r, H R o m e ro, C R a m o s E. D e B i a s i D F i o ra n i J MMM v 266. p .2 3 3 2003 3 .5 E. D e B i a s i R Z y s l e r, C R a m o s H R o m e ro, D. F i o ra n i P h y s Re v B v .7 1 p .1 0 4 4 0 8 2005 3 .6 E. W i n k l e r, R Z y s l e r, M M a n s i l l a D F i o ra n i P h y s Re v B v 72. p .1 3 2 4 0 9 2005 3. 7 G. No l as pri v at e co m m un i cat i o n 2005. 3 .8 S. L e o n i W C a rri l l o -C a b re ra Y G ri n J. A ll. Com p v .3 5 0 p .1 1 3 2003

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69 3 .9 G N o l a s T. W e a k l e y J C o h n R Sh a rm a P h y s Re v B v 61. p. 3845. 2000 3 1 0 S P a s c he n, W. C a r r i l l o C a br e r a A. B e nt i e n, V T r a n, M B a e ni t z Y G r i n, F Steg l i c h P h y s Re v B v .6 4 p .2 1 4 4 0 4 2001 3. 11 V. P ach eco A Be n t i en W Ca rr i l l o -Cab rer a, S Pas ch en F St egl i ch Y. G ri n P h y s Re v B v .7 1 p .1 6 5 2 0 5 2005 3 .1 2 B Sa l e s B C h a k o u m a k o s R J i n J Th o m p s o n D M a n d ru s P hys R ev. B v .6 3 p .2 4 5 1 1 3 2001 C h a p t e r 4 4 .1 M Pe d e rs e n S. M ru p S. L i n d e rot h C J o h a n s s o n M H a n s o n J P h y s .: Cond ens. M at ter. v .9 p .7 1 7 3 1997 4. 2 M. T an aka, Y. Mi saka, K S h i o m i T Yam am o t o T Nak agaw a, M. K a t s u ra T. N u m a z a w a T. N a k a y a m a K N i i h a ra Scri pt a M at er. v 44. p .2 1 4 1 2001 4 .3 P. P o d d a r, J G a s s D R e b a r, S Sr i n a t h H Sr i k a n t h S. M o rri s o n E. C a rp e n t e r. J MMM i n p re s s 2006 4. 4 P. Po ddar, S. Sr i n at h D. Reb ar, D. S i dh ay e, R. Pas ri ch a, B. Pr as ad, H. Sr i k a n t h P h y s Re v B. s u b m i t t e d 2006

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70 4. 5 P. San de, L. Hues o D. Mi gun s J Ri v as F Ri v adul l a, M. L pezQ u i n t e l a A PL v .7 9 p .2 0 4 0 2001 4 .6 S. S ri n a t h J G a s s D R e b a r, G W o o d s H Sr i k a n t h G N o l a s J AP v 99. p .0 8 K 9 0 2 2006 C h a p t e r 5 5 1 S P a s c he n, W. C a r r i l l o C a br e r a A. B e nt i e n, V T r a n, M B a e ni t z Y G r i n, F Steg l i c h P h y s Re v B. v .6 4 p .2 1 4 4 0 4 2001 5. 2 V. P ach eco A Be n t i en W Ca rr i l l o -Cab rer a, S Pas ch en F St egl i ch Y. G ri n P h y s Re v B v .7 1 p .1 6 5 2 0 5 2005 5 .3 B Sa l e s B C h a k o u m a k o s R J i n J Th o m p s o n D M a n d ru s P hys R ev. B. v .6 3 p .2 4 5 1 1 3 2001 5. 4 M. T an aka, Y. Mi saka, K S h i o m i T Yam am o t o T Nak agaw a, M. K a t s u ra T. N u m a z a w a T. N a k a y a m a K N i i h a ra Scr ipta M ater v 44. p .2 1 4 1 2001 5. 5 T K i n o s h i t a, S Sei n o H. Maruy am a, Y. Ot o m e, K Oki t s u, T Naka y am a, K N i i h a ra T. N a k a g a w a T. Y a m a m o t o J. A ll. Com p v .3 6 5 p .2 8 1 2004 5 .6 Q Son g Z Z h a n g JA CS v .1 2 6 p .6 1 6 4 2004 5 .7 H Z e n g P. R i c e S. W a n g S. S u n JA CS v .1 2 6 p .1 1 4 5 8 2004

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71 5 .8 O M a s a l a R Se s h a d ri JA CS v .1 2 7 p .9 3 5 4 2005

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72 App e ndix

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73 App e ndix J o u r na l P u bl i c a t i o ns S S r i na t h, J G a s s D J R e ba r G T Wo o d s H S r i k a nt h, G S N o l a s “G i a nt M agn et o cal o ri c E f f ect i n Cl at h rat es ” Jour nal of A pplied P hys ics v 99. p. 08K 902. 2006 P. P o d d a r, J G a s s D J R e b a r, S Sr i n a t h H Sr i k a n t h S. A M o rri s o n E. E. Ca rpen t er. “ M agn et o cal o ri c Ef f ect i n F err i t e Na n o part i cl es ” Jou rnal of M ag net ism an d Mag ne tic Mate r ial s i n pr ess 2006 P. Po ddar, S. Sr i n at h, D J R e ba r, D. S i dh ay e, R. Pas ri ch a, B. L V. P rasa d, H. S r i k a n th “ M a g n e ti c T r a n s i ti o n a n d L a r g e M a g n e toc a l or i c E f f e c t A s s oc i a te d w i th c o r e s h e ll Sur f ace Spi n Di s o rder i n Co an d Co A g Nan o part i cl es ” P h y s Re v B. s ub m i t t ed, 2006 C on f e r e n c e P r e s e n ta ti on D J R e b a r J G a s s S S r in a t h H S r ik a n t h G S N o la s “ M a g n e t o c a lo r ic E ffe c t in N a no p a r t i c l e S y s t e m s a nd C l a t hr a t e s ” S e s s i o n H 2 2 0 0 0 0 1 M a r c h M e e t i ng o f T he A m e ri c a n Ph y s i c a l Soc i e t y B a l t i m o re 2006

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74 N S F R e c og n i ti on “ E v al uat i n g Re f ri gerat i o n P ote n ti al o f M agn et i c Na n o s t ruct ures.” CTS P ro gram 1415 N u g g e t s h t t p:/ /w ww .n s f .go v /e n g/c t s /n ug ge t s /14 15 /1 4 1 5 s r i k a n th h tm 23 Fe b 2006

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75 Co n f eren ce A b s t ract

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76 N S F R e c og n i ti on CT S Aw ard Ac hieve men ts No t abl e Acco mpl i shme nt s f r o m CT S Awar ds Evaluating Refrigera tion Poten tial of M agnetic N anostru ctures H ar i hara n S r i kant h – U ni ver si t y o f S o ut h Fl o r i da O utl i ne: Lo cal coo li ng an d/or heat ing have become imp or tan t in a n ever -shrinki ng world of electro nic devi ces f a br ic a t e d u si n g M ic r o -E le ct r o -M e ch a n ic a lSy st e m s (M E M S) t e ch n o lo gy N e w a n d in n o v a t iv e m e t h o ds t o a ch ie v e t h is h a v e t o b e e x pl o r e d. Wh ile m a gn e t ic r e f r ig e r a t io n it se lf h a s be e n a r o u n d f o r s e v e r a l d e ca de s, c o n v e n t io n a l m a t e r ia ls have li mitations in terms o f th e o perating tempera ture Unfor tuna tely, so me o f th e mo st pro mis ing candi date ma t er ials fo r ma gn et ic r ef rige ra t io n ca nn o t be e a sily s yn t he size d o r pr o ces sed in t hin film o r na no st ru ct ur ed fo rm s. An a t t r a ct iv e a lt e r n a t iv e is t o c o n si de r c h e m ic a lly s y n t h e si z e d a ss e m bl ie s o f m a gn e t ic n a n o pa r t ic le s. T h is p r o j e ct is a im e d a t c o n du ct in g a s y st e m a t ic a ss e ss m e n t o f t h e p o t e n t ia l a n d v ia bi lit y o f u si n g f e r r o m a gn e t ic a n d f e r r im a gn e t ic nanop arti cl es for ref ri g erat i on. M e t h od ol og y: Ma gnetic refrigeration poten tial can be deduced fro m measur ing the en tro py change a s an externa l m a gn e t ic f ie ld is c y cl e d u p a n d do wn f r o m z e r o t o f ie ld s u p t o 3 T e sl a C o o lin g is p o ss ib le in t h e d e m a gn e t iz in g cy cl e w hen the magnetic momen ts un dergo a chan ge fro m a satur ated o rdered stat e t o a disordered sta te. T his causes absorption o f hea t e nergy fro m th e surr ou nding medium in w hich the magnetic particles ar e embedded th us re su ltin g in co o ling o f t he la t t ice. T his ph en o me no n des cr ibed a s t he m a gn et o -ca lo ric e ffe ct (M CE ) is e mplo ye d by the PI a nd his team in this proj ect t o study the entr opy cha nge in vario us con fig ura tions o f na no particl es sy nthe si ze d in the PI’s l ab oratory R es u l ts : Sys temat ic MCE experiments don e by th e P I’ s grou p on na no particl es have yiel ded i mport ant results t hat po int t o war ds c er t a in ben ef its o f u sing n a no pa rt icleba sed m a t er ials fo r ma gn et ic r ef rige ra t io n. T he e nt ro py ch a ng e its elf is lo w co mpa re d t o so me o f t he “ gia nt -M CE ” bu lk m a t er ials t ha t a re cu rr en t ly k no wn. Ho weve r, t he siz e dispe rs io n o f t he n a no pa rt icles a nd br o a d v a ria t io n in t he blo ckin g t em per a t ur e re su lte d in m a int a inin g t he e nt ro py ch a n ge c o n st a n t o v e r a b r o a d t e m pe r a t u r e r a n ge T h is is p r o m is in g f o r a pp lic a t io n s wh e r e o pe r a t in g t e m pe r a t u r e s in t he 1 0 0 K t o 3 0 0 K r a ng e a re r equ ire d. Sma ll ch a ng es in int er -pa rt icle sep a ra t io n ha ve a sign ifica nt e ffe ct o n t he M CE and t he P I’ s grou p is curr ently exploring this in chemic all y assembl ed nan opar ticl e a rra ys w ith different surfa ctant coat ings o n t he n ano particl es. A fut ure goal is to deposit na no particl e a rra ys on a high therma l conductive substrate and di rec tl y me as ure the tem p erat ure chan g e und er f i el d cy cl i ng by attac hi ng a therm ocoup l e to suc h a de vi ce

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77 T ra n smi ssi on El ect ron M i cr oscope i mage of ch emi ca l l y syn th es i ze d pol ydi sp er se coba l t f er ri te na no p ar t ic les and th e en t ro p y c ha ng e as a f unc t io n o f te m p e ra t ur e fo r va rio us ex t e rn al m ag ne t ic fie ld cycl es Credit: H arihar an Sri kanth – Universi ty o f South F lorida S ci en ti fi c U n i qu en es s: In convent ional bulk and t hin fil m ferr oma gnets, the coo li ng capacity is maximum ar ou nd t h e m a gn e t ic o r de r in g t e m pe r a t u r e ( a ls o k n o wn a s t h e C u r ie t e m pe r a t u r e ). T h e C u r ie t e m pe r a t u r e is ma t er ialdep en den t a nd is no t e a sy t o t un e fo r a pplica t io ns t ha t r equ ire a br o a d r a ng e o f o per a t ing t em per a t ur es. T he inno vative idea behind this proj ect is to ta ke advant age o f a versatile or der-di sorder tra nsiti on te mperatur e (a ls o k n o wn a s t h e b lo ck in g t r a n si t io n ) in n a n o si z e m a gn e t ic p a r t ic le s t h a t s e pa r a t e s t h e p h a se s wh e r e t h e m a gn e t ic m o m e n t s a r e f lu ct u a t in g (s u pe r pa r a m a gn e t ) a n d f r o z e n ( bl o ck e d st a t e ). T h is b lo ck in g t r a n si t io n c a n b e s e le ct iv e ly t un ed in na no pa rt icles by co nt ro lling t he pa rt icle size a nd in t er -pa rt icle sep a ra t io n, t hu s le a ding t o a br o a der r a ng e i n term s of op erat i ng temp erat ure I mp a ct on I n d u s t r y a n d /o r S oc i e t y: T h e p r o j e ct h a s a b r o a d im pa ct f r o m a s ci e n t if ic p e r sp e ct iv e a s we ll a s in trai ni ng stud ent s i n the c utti ng -e d g e fi el d of nanotec hnol ogy St ud ent s i n PI’s g roup have de ve l ope d hand son ex p erti se in al l asp ec ts of the p roj ec t. T hi s c ove rs a w i d e rang e of ex p eri me ntal sk i l l s f rom c hem i ca l sy nthe si s of n a n o pa r t ic le s, e le ct r o n ic in st r u m e n t a t io n c r y o ge n ic s, a n a ly si s o f r e su lt s a n d pr e se n t a t io n in p e e r -r e v ie we d j o u r n a ls and co nferences. LabVi ew interfa ce and so ftware fo r a nalysi s o f th e M CE were developed compl etely in-house by the st ud en t s. F ro m a glo ba l pe rs pec t ive, r ef rige ra t io n gen er a lly in vo lves ga s c o mpr ess io n t ech niqu es t ha t u se ozone -d ep l eti ng gas es Magn eti c re fri g erat i on is env i ronm ent al l y fr i end l y and prov i d es an al tern ati ve meth od for cool i ng /h eati ng This wo rk is no t ab le be ca us e it is a n a t t em pt t o se a rc h fo r ne w ma t er ials ba sed o n na no st ru ct ur es t ha t co uld be se aml es sl y i nte g rate d in to the tec hnol ogy of mag net i c re fri g erat i on to achi ev e su p eri or pe rfor man ce for on-c hi p sp ot co o lin g a pp lic a t io n s. T h e wo r k a dd r e ss e s bo t h t h e f u n da m e n t a l s ci e n t if ic a sp e ct s o f m a gn e t ic n a n o st r u ct u r e s a s we ll as t he t argeted a ppl ic ation o f buil di ng a prot ot ype coo li ng devic e based o n ma gnetic entro py change a ssociated w ith ex tern al fi el d cy cl e. A d van ce me nt of S & E i n thi s are a w oul d res ul t i n new env i ronm ent al l y fr i end l y re fri g erat i on me thod s. T hi s work i nvolves m ul ti di sci pl i na ry research T he P I is a physic is t t raining graduate students seeking degrees in Applied P hy sics. Re sea rc h is m ult idisciplin a ry a s it s c o mpo ne nt s in clud e ch em ica l sy nt he sis o f n a no pa rt icles, ch a r a ct e r iz in g t h e p h y si ca l p r o pe r t ie s u si n g v a r io u s a n a ly t ic a l i n st r u m e n t s, d e v ic e f a br ic a t io n a n d e n gi n e e r in g. T h is i nv ol ve s A p p l i ed Phys i cs Materi al s S ci enc e and Chem i ca l Eng i nee ri ng Pro gram Offi cer: Judy Raper N SF Aw ard N umber: 040 893 3 Aw ard T itl e: Magnet ocalo ric Effect in N ano particl e Ass embl ies for Refrig erat ion Appl ic ation s P I N a me s: H arihar an Sri kanth

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78 Institution N ame: Un iver sit y o f So ut h F lo rida Pro gram Element: 141 5 T o p o f P a ge T his Nu gget w as Updated o n 2 3 F ebruary 200 6.

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79 M CE Pr o gram Bl o ck Di agram C on ti n u e s to n e x t page

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80 A tta c h m e n t to prev i o us page