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Siyambalapitiya, Chamila S.
Growth and physical properties of magnetite thin films
h [electronic resource] /
by Chamila S. Siyambalapitiya.
[Tampa, Fla] :
b University of South Florida,
ABSTRACT: This project focused on two aspects of magnetite thin films. The first was to find optimum parameters and conditions for deposition of stoichiometeric Magnetite films using pulsed laser deposition (PLD). The second aspect was the characterization of the magnetic and electrical properties in order to broaden the spectrum of understanding of PLD Magnetite films. These properties were also investigated in terms of the substrates on which the films were deposited. Discussed in this thesis are deposition parameters, structural characteristics, magnetic and electrical characteristics of the films in terms of different substrates and film thicknesses. The discussion consists of structural parameters obtained using X- ray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), and electric properties such as resistance as a function of temperature and voltage dependence on the applied current. The magnetic properties measured were the magneto-resistance, M-H hysteresis loop, and magnetization as a function of temperature. The results obtained are then compared with pre-existing literature data. It will be shown that there is an impurity phase that may be seen when magnetite films are deposited on Sillicon dioxide substrates.
Thesis (M.A.)--University of South Florida, 2006.
Includes bibliographical references.
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Adviser: Srikanth Hariharan, Ph.D.
t USF Electronic Theses and Dissertations.
Growth and Physical Properties of Magnetite Thin Films by Chamila S. Siyambalapitiya A thesis submitted in partial fulfillment of the requirements for the dual degrees of Master of Science Department of Physics College of Arts and Science and Master of Science in Engineering Science Department of Electrical Engineering College of Engineering University of South Florida Co-Major Professor: Srikanth Hariharan, Ph.D. Co-Major Professor: Sarath Witanachchi, Ph.D. Myung K. Kim Ph.D. Andrew M. Hoff, Ph.D. Don Morel, Ph.D. Date of Approval: July 12, 2006 Keywords: PLD, Fe 3 O 4 Verwey, M-H loops, Magneto-resistance, IV Copyright 2006, Chamila S. Siyambalapitiya
ACKNOWLEDGEMENTS First and foremost I offer my sincere gr atitude to my supervisor, Dr. Srikanth Hariharan, who has supported me throughout my th esis and, especially for his support in my pursuit of dual masters degree. One simply could not wish for a better or friendlier supervisor. Also, I am indebted to Dr Srinath Sanyadanam for his guidance and continuous support throughout my project. My work could not be completed without th e help of my graduate co-advisor Dr. Sarath Witanachichi. His supervision of my work and access to his laboratory allowed me to perform my experiments and to write th is thesis. I would also like to thank my committee members, Dr. Myung Kim, Dr. Andrew Hoff, and Dr. Don Morel, for their time, effort, and for serving on my committee. I would like to thank the members of Dr. Hariharan s research group: Natalie Fray, James Gass, Ranko Heindl, Drew Rebar, Jeff Sanders, Marienette Morales. Their cheerful and supportive nature helped me tr ough many difficulties. The help and support from the members of Dr. Witanachchis group with the PLD system was essential to my success; for that, I am very thankful. Esp ecially I would thank R obert Hyde and Marek Merlak for being good friends and supportive colleagues. I would like to thank the staff members of the Department of Physics for their support. Especially I am thankf ul for all the time I have spen t with the late Sue Wolfe. Finally, I want to thank my family and es pecially my parents and aunts for raising me to become the person that I am, and for teaching me the importance of education. Without their support and confidence in me, I would not be able to reach my goals. I am blessed with many friends and I like to thank them with all of my heart.
i TABLE OF CONTENTS LIST OF TABLES iii LIST OF FIGURES iv ABSTRACT vii CHAPTER ONE: BACKGROUND AND MOTIVATION 1 1.1. Introduction 2 1.1.1. Verwey Transition 3 1.1.2. Magnetic Properties of Ferromagnets 4 1.2. Overview of Research on Fe 3 O 4 6 1.3. Conclusion 14 CHAPTER TWO: GROWTH OF Fe 3 O 4 FILM BY PULSED LASER DEPOSITION 15 2.1. Overview of the Deposition Setup 17 2.1.1. Excimer Laser 18 2.1.2. Laser Target Interaction 18 2.1.3. Plume 19 2.2. Film Preparation 20 2.3. Process Flow 22 2.4. Conclusion 25 CHAPTER THREE: STRUCTURAL CHARACTERIZATION 26 3.1. X-Ray Diffraction (XRD) 28 3.1.1. X-Ray Diffraction of Fe 3 O 4 on SiO 2 Buffer Layered Si Substrate 29 3.1.2. X-Ray Diffraction of Fe 3 O 4 on MgO Substrate 31 3.1.3. X-Ray Diffraction of Fe 3 O 4 on Si (>50 -cm) Substrate 34 3.1.4. X-Ray Diffraction of Fe 3 O 4 on Si (<.005 -cm) Substrate 34 3.2. Scanning Electron Microscopy (SEM) 37 3.3. Energy Dispersive Spectroscopy (EDS) 39 3.4. Thickness Measurements 41
ii CHAPTER FOUR: ELECTRICAL AND MAGNETIC CHARACTERIZATION 42 4.1. Overview of the PPMS System 43 4.2. Electrical Resistance Measurements 45 4.2.1. Temperature Dependence of the Resistance in Fe 3 O 4 Films Grown on SiO 2 46 4.2.2. Temperature Dependence of the Resistance in Fe 3 O 4 Films Grown on MgO 47 4.2.3. Temperature Dependence of the Resistance in Fe 3 O 4 Films Grown on High Resistive Si (resistivity > 50 cm) 49 4.2.4. Temperature Dependence of the Resistance in Fe 3 O 4 Film Grown on Low Resistive Si (resistivity <0.005 cm) 51 4.2.5. Comparisons and Conclusions 52 4.3. Magneto-Resistance 55 4.3.1. MR for Fe 3 O 4 on SiO 2 55 4.3.2. MR for Fe 3 O 4 on MgO 57 4.3.3. Fe 3 O 4 on High Resistive Si (resistivity > 50 cm) 58 4.4. Magnetization as a Function of the App lied Magnetic Field (M-H) 60 4.4.1. M-H for Fe 3 O 4 on SiO 2 61 4.4.2. M-H for Fe 3 O 4 on MgO 62 4.4.3. M-H for Fe 3 O 4 on Si (resistivity > 50 cm) 64 4.4.4. M-H for Fe 3 O 4 on Si (resistivity > 50 cm) 65 4.5. Magnetization Versus Temperature (M-T) 67 4.5.1. M-T for Fe 3 O 4 on SiO 2 68 4.5.2. M-T for Fe 3 O 4 on MgO 69 4.5.3. M-T for Fe 3 O 4 on Si (<0.005 cm) 70 4.6. Current -Volatag e Characteristics 70 4.6.1. Current and Voltage Dependence of Fe 3 O 4 on SiO 2 71 CHAPTER FIVE: SUMMARY, CONCLUSIONS AND FUTURE PLANS 73 5.1. Future Plans 76 REFERENCES 77 BIBLIOGRAPHY 79
iii LIST OF TABLES Table 3.1. Elements, weight and Atomic weight percentage for the target. 40 Table 3.2. Average thickness measurements for the Fe 3 O 4 films. 41 Table 4.1. Verwey transition temperatures for films on SiO 2 MgO and high resistive Si (> 50 cm). 52 Table 4.2. Coercivity, saturation magnetization, remanence values for the Fe 3 O 4 films on SiO 2 MgO, Si (>50 cm) and Si (< 0.005 cm) substrates. 67
iv LIST OF FIGURES Figure 1.1. Fe 3 O 4 structure (adopted from Friedrich, 2002). 2 Figure 1.2. Hysteresis loop show ing the saturation magnetization M s the remanent magnetization M r and the coercive Field H c 5 Figure 1.3. The resistivity as a function of temperature for 1500 and 6600 thick Fe 3 O 4 films in the range of 60350 K. 8 Figure 1.4. Resistance vs. temperature curves for 1500 of Fe 3 O 4 deposited on a Si substrate and on Ta, Ti and SiO 2 buffer layer. 9 Figure 1.5. Resistance as a f unction of temperature for Fe 3 O 4 films with different thicknesses. 10 Figure 1.6. Magneto-resistance at H =1500 Oe as a function of temperature for a polycrystalline (850 ) a nd an epitaxial (1500 ) Fe 3 O 4 film. 11 Figure 1.7. Magnetization hysteresis loops at 90 K and 300 K for the 6600film with field scans up to 5.5 T. 12 Figure 2.1. Schematic diagram of pulsed laser deposition system. 17 Figure 2.2. Substrate mounting configur ation. 21 Figure 2.3. Laser spot sample. 21 Figure 2.4. Film deposition process flow. 22 Figure 3.1. X-Ray diffraction scans for Fe 3 O 4 target. 28 Figure 3.2. XRay diffraction scan for the Fe 3 O 4 on SiO 2 buffer layered Si substrate. 30 Figure 3.3. Zoomed X-ray diffraction scans around 43 for the MgO substrate (top), Fe 3 O 4 on MgO substrate (bottom). 32
v Figure 3.4. Zoomed X-Ra y diffraction scan around angle 94 for the MgO substrate (top), Fe 3 O 4 on MgO substrate (bottom). 33 Figure 3.5. X-Ray diffraction scan for the Fe 3 O 4 on Si substrate. 35 Figure 3.6. X-Ray diffraction scan for Fe 3 O 4 film on Si (<.005 -cm) substrate. 36 Figure 3.7. SEM images for Fe 3 O 4 film on SiO 2 substrate (a) and Fe 3 O 4 film on MgO substrate (b). 38 Figure 3.8. SEM images for Fe 3 O 4 film on high resistive Si (>50 -cm) substrate. 39 Figure 3.9. EDS graph for Fe 3 O 4 target. 40 Figure 4.1. (a) Schematic of Physical Property Measurement System (PPMS), (b) The different probes for different measurements. 44 Figure 4.2. Temperature dependence of resistance for Fe 3 O 4 film on SiO 2 substrate. 47 Figure 4.3. Temperature depende nce of resistance for Fe 3 O 4 film on MgO substrate. 48 Figure 4.4. Temperature dependence of resistivity for Fe 3 O 4 film on MgO substrate. 49 Figure 4.5. Temperature depende nce of resistance for Fe 3 O 4 Film on Si substrate. 50 Figure 4.6. Temperature depende nce of resistance for Fe 3 O 4 film on high conducting Si substrate. 51 Figure 4.7. Temperature dependence of the resistance of Fe 3 O 4 film on SiO 2 MgO and high resistive Si. 52 Figure 4.8. Temperature dependence of the resistance of Fe 3 O 4 film on SiO 2 MgO and high resistive Si. 53 Figure 4.9. Temperature dependenc e of magneto-resistance for Fe 3 O 4 /SiO 2 film. 56 Figure 4.10. Temperature dependenc e of magneto-resistance for Fe 3 O 4 /MgO film. 57
vi Figure 4.11. Resistance depende nce on temperature of Fe 3 O 4 film on Si (>50 cm) substrate in the presence of applied field of 5 T and absence of field. 58 Figure 4.12. M-H loops for Fe3O4 film on SiO 2 at 10 K and 300 K. 61 Figure 4.13. M-H loops for film on MgO at 10 K, 120 K and 300 K. 62 Figure 4.14. M-H loops for film on high resistive Si substrate (resistivity > 50 cm) at 10 K, 120 K and 300 K. 64 Figure 4.15. M-H loops for film on low re sistivity Si substrate at 10 K and 300 K. 65 Figure 4.16. M vs.T for the film on SiO 2 under applied field of 300 Oe and 5000 Oe.68 Figure 4.17. M vs.T for the film on MgO Under applied field 300 Oe, 5000 Oe. 69 Figure 4.18. M vs.T for the film on low resi stivity Si substrate under applied field of 300 Oe and 5000 Oe. 70 Figure 4.19. I-V characteristics of the Fe 3 O 4 film on SiO 2 for the temperature range of 125 K T 300 K. 72 Figure 4.20. I-V characteristics of the Fe 3 O 4 film on SiO 2 for temperature 70 K and 65 K. 72
vii GROWTH AND PHYSICAL PROPERTIES OF MAGNETITE THIN FILMS Chamila S. Siyambalapitiya ABSTRACT This project focused on two aspects of ma gnetite thin films. The first was to find optimum parameters and conditions for deposition of stoichiometeric Fe 3 O 4 films using pulsed laser deposition (PLD). The second aspect was the char acterization of the magnetic and electrical propertie s in order to broaden the sp ectrum of understanding of PLD Fe 3 O 4 films. These properties were also inves tigated in terms of the substrates on which the films were deposited. Discussed in this thesis are deposition parameters, structural characteristics, magnetic and electri cal characteristics of the films in terms of different substrates and film thicknesses. The discussion consists of structural parameters obtained using Xray diffraction (XRD), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), a nd electric properties such as resistance as a function of temperature and voltage dependence on the applied current. The magnetic properties measured were the magneto-resistance, M-H hysteresis loop, and magnetization as a function of temperature. The results obtai ned are then compared with pre-existing literature data. It will be shown that there is an impurity phase that may be seen when magnetite films are deposited on SiO 2 substrates.
1 CHAPTER ONE BACKGROUND AND MOTIVATION Magnetite, Fe 3 O 4 shows unique magneto-transport properties that has potential application in spintronics devices. Thin films made of Fe 3 O 4 draws attraction in devices dependent on magnetic tunneling junctions an d spin valve configuration. The use of Fe 3 O 4 films in magneto-electronic devices is al so a growing interest. Ferromagnetic oxide material may be used as memory storage mate rial as a part of a magnetic multilayer to store information. The magnetizations of domain parallel and anti parallel to an applied external magnetic field are used as distinct states for binary logi c. Another interesting aspect of a storage material is to exploit it s anisotropy in magnetism. A material that has fairly large switching field in one crystal orientation and a smalle r switching field in another crystal orientation may be tailor ed to meet desired specifications.
1. 1. Introduction Magnetite, Fe 3 O 4 is one of the best known minerals on earth and is famous for its half metallic nature, 100% spin polarizability and high Curie temperature (858 K). The signature property of Fe 3 O 4 is the Verwey transition which is an unusual metal to insulator transition at a temperature of about 120 K and named after E. J. W. Verwey on behalf of his model and work in 1939. Fe 3 O 4 has spinel structure and the ionic formula can be written as Fe 3+ A [Fe 2+ Fe 3+ ] B O 4 According to this formula, 2+ and 3+ Fe ions coexist on the same crystal site (S. Jain and A. O. Adeyeye, 2005). Figure 1. 1. Fe 3 O 4 structure (adopted from Friedrich, 2002). 2
3 1. 1. 1. Verwey Transition The fundamental lattice for Fe 3 O 4 can be considered as a face centered cube of O 2ions, as shown in Figure 1. 1. Per unit cell, this compilation contains 64 interstices of the tetrahedral type (surrounded by 4 oxygen i ons) and 32 interstices of the octahedral type (surrounded by 6 oxygen ions). 8 Fe 3+ ions are located at the 8 tetrahedral A interstices, and 8 Fe 2+ + 8 Fe 3+ together at the 16 octahedral B interstices (E. J. W. Verwey, 1941). The Fe 2+ ion is responsible for th e conduction. The electrical conductivity of Fe 3 O 4 is expected to be much higher than that of Fe 2 O 3 for example, which contains the Fe 3+ ion only (N. Tsuda 2000). The conduc tion occurs due to the fact that Fe 3+ interchanges with Fe 2+ at B-sites. Thus, Verwey proposed the Fe 2+ and Fe 3+ ions, above T v to be randomly distributed over the B -sites, permitting relatively easy valency exchange according to (Fe 2+ e Fe 3+ ) by means of thermally activated fast electron hopping. Upon cooling below the critical temperature ( T v ), together with the reduction of the crystal symmetry from cubic to tetragonal (as he originally assumed), charge ordering was proposed in a way that successive, a/ 4-spaced (100) lattice planes would be occupied, alternating, by two and thre e fold Fe ions (Friedrich, 2002). Although it is generally accepte d that the transition is due to the ordering of the Fe 3+ and Fe 2+ ions, the mechanism governing the tr ansport and magnetic properties of this material still remains unclear (G. Q. Gong 1997). However, unde r theoretical aspects two concepts, going back to Mott and IhleL orenz, presently appear most promising. Motts view of the Verwey transition, as corresponding to the phase changing of a Wigner glass (T > T v ) into a Wigner crystal (T < T v ) describes most adequately the various low-temperature mechanisms in Fe 3 O 4 in terms of tunneling and variable range
4 hopping of small polarons. On the other hand, the well-elaborated IhleLorenz model, assuming a superposition of polaron-ba nd and hopping conductivity, is in better agreement with the high-temperature data ( T v < T < 600 K) (Friedrich, 2002). 1. 1. 2. Magnetic Properties of Ferromagnets Magnetic properties of a material have direct relationship wi th electrons and their Spin. Each unpaired spin has a moment of one Bohr magnetron b Fe 3+ with a electronic structure of (1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 )3 d 5 has 5 b Similarly Fe 2+ has 4 b The moments of the Fe 3+ ions on octahedral and tetrahedral site s are opposite to each other and the net moment arises only from the Fe 2+ ions; the arrangement being termed as ferrimagnetic (S. B. Ogale 1998). Ferrimagnetism is similar to fe rromagnetism. It exhibits all the hallmarks of ferromagnetic behavior spontaneous magnetization, Curie temperatures, hysteresis, and remanence (Bruce M. Moskowitz, 1991) The most common way to represent th e magnetic properties of a ferro or ferrimagnetic material is by a plot of magnetization ( M) for various field strengths (H) This is referred as the hy steresis loop. The term hyste resis corresponds to lagging of M behind H A typical hysteresis loop with important parameters is shown in Figure 1. 2. These parameters include saturation magnetization M s remanent magnetization M r and the coercivity field H c From the plot it can be seen that the ferromagnetic in its initial states is not magnetized. Application of a field H causes the magnetic induction to increase in the field direction. If H is increased indefinitely, the magnetization eventually
reaches saturation at a value denoted as M s This represents the condition where all the magnetic dipoles within the material are aligned in the direction of the magnetic field H. Figure 1. 2. Hysteresis loop showing the saturation magnetization M s the remanent magnetization M r and the coercive field H c The next characteristic quantity is the remanence. When the field is reduced to zero after magnetizing the magnetic material the remaining magnetic induction is called remanent or remanenceM r The magnetization can be reduced to zero by applying a reverse magnetic field of strength H c This field strength is known as the coercivity. When the coercive field of a ferro or ferrimagnet is large, the material is said to be a hard magnet and when it is low said to be soft magnet. Coercivity is strongly dependent on the condition of the sample, being affected by such factors as heat treatment or deformation. 5
6 All ferro or ferrimagnets when heated to sufficiently high temperatures become paramagnetic. The transition temperature from ferromagnetic to paramagnetic behavior is called the Curie temperature. At this temperat ure the permeability of the material drops to a low value and both coercivity and remanence become zero. The suitability of ferro or ferrimagnetic ma terials for applications is determined principally from characteristics described a bove. As an example, magnetic recording material should have a high remanent and high coercivity. Materials for electromagnets need to have a low remanent and coercivity in order to ensure that the magnetization can easily be reduced to zero as needed (David Jiles). 1. 2. Overview of Research on Fe 3 O 4 Scientists have been continuously working on Fe 3 O 4 to develop a better understanding regarding this material in the sense of structure, tran sport properties, and magnetic properties along with the Verwey transition mechanism and how different factors such as strain, grai n boundaries, and stoichiometry affect for magnetite properties. The substrate effect, thickness effect and eff ect of preparation methods play major roles in determining the properties of Fe 3 O 4 useful for devices. The Verwey transition in magnetite (Fe 3 O 4 ) has attracted extensive research interest since its discovery more than sixt y years ago. As menti oned earlier, though the exact mechanism is still unclear, scientists ha ve contributed bits a nd pieces to the puzzle of Fe 3 O 4 ever since. Here we use some of these references and their experimental results to establish background knowledge.
7 It is an accepted fact that the stoichio metry and the film thickness play a major role in the Verwey transition. The sharpness of the Verwey transition is often considered to represent the high quality of material. Du e to a small lattice mismatch (0. 344%), epitaxial growth of Fe 3 O 4 (a=8. 397 ) on MgO (a=4. 213) has been actively pursued during the recent past (S. K. Arora, 2005). Here we focus on some results from the reference of Gong et al where they have grown film on MgO substrates using the pulsedlaser deposition technique utilizing a fre quency tripled Nd:YAG laser (355 nm). The Figure 1. 3 shows the graph extracted from the Gong el al reference which shows the resistivity and magnetization ch anges as a function of temper ature for two different film thicknesses1500 and 6600 The Verwey transition temperature (T v ) is determined to be about 120 K for both samples. Also the magnetization measurement yields a T v of 123 K for the thick film. Both films exhi bit a broadened transition, compared to measurements made on bulk samples. In particul ar, the thin films transition is broader than that for the thick films. Gong et al ascribe this as possibly due to the residual strain in the films resulting from the lattice mismat ch with the substrate. Also the film they grew with a thickness of 670 has produced a transition temperature of 116. 5 K, suggesting that T v decreases with film thickness (G. Q. Gong 1997). This is also consistent with the results reported by Margulies (1996).
Figure 1. 3. The resistivity as a function of temperature for 1500 and 6600 thick Fe 3 O 4 films in the range of 60350 K. The magnetization as a function of temperature is also shown for the 6600 film measured in a field of 300 Oe (produced from the reference of G. Q. Gong). There has not been much investigation done with Fe 3 O 4 films on Si or SiO 2 substrates in terms of transport or magnetic properties. Especially the reference of S. Jain and A. O. Adeyeye, is the only reference that could be found for Fe 3 O 4 on a SiO 2 substrate. Figure 1. 4 shows corresponding RT curves for Fe 3 O 4 films deposited on four different substrates Si, Ta, Ti and SiO 2 buffer layers. For 1500 of Fe 3 O 4 film deposited on a Ti buffer layer, however, T v is deduced to be 123. 5 K, which is very close to the Verwey transition temperature. The increase in resistance from high temperatures to low temperatures does not show a sharp transition, but a gradual one. Surprisingly, for Fe 3 O 4 films deposited on SiO 2 buffer layers, T v increases to 155. 5 K. The resistance shows a sharp jump at the transition temperature for the SiO 2 buffer layer (S. Jain and A. O. Adeyeye, 2005). It should be pointed out that the graph shown here is in the linear scale whereas most references use logarithmic scale in order to show the transition. We will 8
show in our work presented in this thesis that our films are of much higher quality as we observe a sharper Verwey transition in our samples. Figure 1. 4. Resistance vs. temperature curves for 1500 of Fe 3 O 4 deposited on a Si substrate and on Ta, Ti and SiO 2 buffer layer (Adopted from S. Jain, 2005). The details above are the substrate dependence of the Verwey transition. Next the focus will be on the film thickness dependence of the Verwey transition. Figure 1. 5 displays the resistance (R) as a function of temperature for Fe 3 O 4 films grown on MgO (100) substrates with different thicknesses. Overall, the films exhibit a broadened Verwey transition as compared to measurements reported for bulk samples. The transition is the sharpest for the 6600 thick film, with a transition temperature (T v ) of approximately 121 K. 9
Figure 1. 5. Resistance as a function of temperature for Fe 3 O 4 films with different thicknesses. The Verwey transition temperature (T v ) determined from Arrhenius plots as a function of film thickness is shown in the inset (adopted from X. W. Li, 1998). The resistance change at the transition decreases in magnitude and the transition gets broader with decreasing film thickness, and is not noticeable for the thinnest films. The transition temperature, as determined from the Arrhenius plot, is plotted as a function of film thickness and shown as an inset in Figure 1. 5. T v increases quite rapidly with the initial increase in film thickness and then gradually levels off at a value close to the bulk (X. W. Li, 1998). It is also known that magnetite has a small negative magneto resistance (MR). Though mostly refer MR as (R H -R 0 ) /R 0 different authors has adopted different way of expression to calculate MR. G. Q. Gong at all have observed MR values as high as 32% for a 6600--thick film at 60 K under a 4-T field. The MR ratio defined according to (R o 10
R H )/R H where R o is the zero-field resistance and R H is the resistance in the applied field H. X. W. Li et al have also studied the magneto-resistance in Fe 3 O 4 films. They have briefly discussed the low-field magneto-resistance (MR) properties of epitaxial Fe 3 O 4 films grown on MgO and the polycrystalline films grown on polycrystalline SrTiO 3 substrate. Figure 1. 6. Magneto-resistance at H=1500 Oe as a function of temperature for a polycrystalline (850 thick) and an epitaxial (1500 thick) Fe 3 O 4 film. The resistance hysteresis as a function of magnetic field for the polycrystalline film at 123 K is shown in the inset (Adapted from the reference of X. W. Li). Figure 1. 6 shows the MR ratio at 1500 Oe as a function of temperature for the two films. The resistance hysteresis loop for the polycrystalline film at 123 K is shown as an inset. The MR ratio is defined according to (Rp-R 1500 )/R 1500 where Rp is the peak resistance which occurs at the coercive field, and R 1500 is the resistance at 1500 Oe. The MR behavior for the two films is quite similar, with a peak occurring close to T v as has 11
also been previously observed by G. Q. Gong et al. Consistent with the transport and magnetic data, the MR peak occurs at a slightly higher temperature and is somewhat broader for the polycrystalline film on SrTiO 3 than for the epitaxial film on MgO. The MR magnitude has been found to be quite small (2%% at 1500 Oe) for both the films, with very little contribution resulting from grain boundary transport (X. W. Li, 1998). Figure 1. 7. Magnetization hysteresis loops at 90K and 300 K for the 6600film with field scans up to 5. 5 T. The insets show the expanded hysteresis behavior at low fields. (reproduced from reference G. Q. Gong). Figure 1. 7. displays the in-plane hysteresis loops measured at two different temperatures for the 6600 film. The saturation moment (M s ) is 458 emu/cc for the thick film and 427 emu/cc for the thin film at 90 K, while at 300 K the M s for the 6600film is 415 emu/cc, less than the reported bulk value of 471 emu/cc. The even lower M s value for the 12
13 thinner film suggests that strain may play a role in the reduction of magnetization. It has also been suggested that the reduction may be caused by a thin layer of -Fe 2 O 3 present on the surface (C. A. Kleint 1995). The inse ts show the expanded low-field hysteresis behavior at the two temperatures. Also as seen in the inset, the coercivity increases from 200 to 470 Oe, going from 300 to 90 K. Antiphase boundaries (APB) are formed as a natural growth defect in the Fe 3 O 4 /MgO hetero-epitaxy. This is due to the fact that the spinel crystal structure of Fe 3 O 4 (Fd3m) is lower in symmetry than MgO (Fm3m) and its unit cell size is twice the size of MgO. Density of the APBs is str ongly influenced by the growth conditions and substrate microstructure. Presence of APBs has a deleterious effect on the magnetic properties and electr onic structure of Fe 3 O 4 films For example, the magnetization does not saturate in high magnetic fields and at smaller thickness, films exhibit superparamagnetic behavior (S. K. Arora, 2005). There has only been a little investiga tion on the current-voltage dependence ( I-V ) of Fe 3 O 4 films. In fact, only one paper could be found which has th is characterization. The conductivity of magnetite film is known to be due to the exchange of electrons between the ferrous and ferric ions in the oc tahedral lattice sites, and thus the random distribution of the ions above the transition leads to an isotropic conductivity. Below the transition, however, the conductivity should be anisotropic. The energy required for the exchange of ferrous and ferric ions for conductiv ity to take place is called the activation energy.
14 1. 3. Conclusion Magnetite has application potential for various electronic devices. In order to integrate with current electronic industry it is important for Fe 3 O 4 to be grown on semiconductor substrates. Then there w ould be good potential for the use of Fe 3 O 4 in magneto-electronic devices. Our motivation for th is research to grow stoichiometric films on four different substrates such as Si with SiO 2 buffer layer, MgO, Si substrates with different resistivities. Systematic studies ar e then conducted on the structural, transport and magnetic characteristics.
15 CHAPTER TWO GROWTH OF Fe 3 O 4 FILMS BY PULSED LASER DEPOSITION The pulsed laser deposition (PLD) technique was used to fabricate all the thin films investigated in this thesis. All the PL D growth of films was done in the thin films laboratory of University of South Florida under the supervision of Prof. Witanachchi and with help from his group members. PLD is a method for deposition of a wide variety of materials in thin film form, collected from a plume of vaporized material which was created by high power laser pulses directed at the desired target ma terial. This deposition technique often generate s films with the same stoichiometry as the target material, an important advantage of this method. Maintain ing the same ratio of elements in the resulting film as the initial target com ponents simplifies the target selection and fabrication process. Matching stoichiometry is made possibl e by the rapid heating rate provided by the pulsed laser bei ng faster than the individual low and high vapor pressure components of the target elements, and mi cro-particulates ej ected into the high temperature of the plasma are further decom posed into a stoichiometric vapor. PLD does not require an atmosphere gas as in sputteri ng, however, reactive gases can be added to the vacuum system to vary the desired stoi chiometry of the deposition. In general, the useful range of laser wavelengths for materi al ablation to grow thin films by PLD lies
16 between 200 nm and 400 nm. The absorption coe fficient of materials tends to increase toward the short wavelength end of this range and the penetration depth of the target materials is also reduced, but has characteristi cs which aid in reducti on of particulate size and quantity. Further advantages of PLD are the simplicity and flexibility of the technique, small target size as compared to sputtering targets, and low substrate temperatures can be utilized due to the high energy of the vaporized species in the plume, typically 5 eV to greater than 100 eV. There are two major disadvantages in th e PLD method; particulate generation and lack of large area uniformit y. Particulates in the form of molten droplets and/or irregularly shaped fragments can range in size from sub-micrometer to several micrometers. The negative effects of partic ulates on thin films are reduced smoothness of the micro-structure, non-uniformity of the film thickness, and point defects in devices. Large area non-uniformity of film thickness is due to the narrow a ngular distribution of the plume. Uniform coverage area can be improved by rotation and translation of the substrate, and rastering of the laser irradiated area of the target. Ro tation, translation, and rastering were not available in the current deposition syst em; however, the disadvantage of large area non-uniformity of the thickness was minimized by selecting deposition areas near to the on-axis positions of the plume. A schematic diagram of the basic PLD system is shown in Figure 2.1.
17 Vacuum Chamber Target Figure 2.1. Schematic diagram of pulsed laser deposition system. 2.1. Overview of the Deposition Setup The deposition system employed consists of a few key components including an excimer laser, rotating target, heated substrate holder, and vacuum system, as shown in Figure 2.1. The beam of the UV wavelength (248nm) excimer laser is directed into the vacuum chamber through the use of two UV grade mirrors and is focused onto the Fe 3 O 4 target using a single UV-AR lens. The number of mirrors, lens, and windows are minimized to reduce losses at each surface, while obtaining desired laser spot size and position. The lens is mounted on an X-Y translation stage to facilitate the positioning and size of the incident laser spot on the target. The target is rotated using a variable speed motor externally attached to the chamber. The target is rotated at a predetermined rate so the incident laser pulses will evenly ablate the surface of the target without pitting. Possible effects of target pitting are undesired particulates and change of the direction of Substrate Plume Excimer Laser Lens Mirrors
the plume. The substrate is fixed to a stainless steel heater block using conductive paint to provide good thermal contact. The heater block is positioned such that the face of the substrate is parallel to the target, and the center of the deposition area is on-axis with the plume. The heat source is provided by a 0-120 VAC, 600 Watt, halogen bulb inserted in to the heater block. The temperature of the heater block is monitored through the use of a K-type thermo-couple attached closely to the substrate. The vacuum system consists of a chamber, roughing pump, turbo-molecular pump, and a cryogenic pump. This vacuum system allows the pressure inside the chamber to be maintained as low as 10 -7 Torr. 2.1.1. Excimer Laser The laser system utilized a Lambda Physik Compex 102 KrF excimer laser. The light is in the UV range with a wavelength of 248 nm, a pulse width of ~ 20 ns, and the maximum energy per pulse is 280 mJ leading to maximum power per pulse of: MWsJtEPsJp14140000001020280.09 [2.1] The frequency range of the laser pulse is 1-10 Hz (1-10 pulses per second). The laser was operated by a LabView interface program allowing control of the pulse repetition rate, total number of pulses, and monitoring of the energy per pulse output at the laser. 2.1.2. Laser Target Interaction Electronic sputtering is considered to be the principal interaction mechanism of a laser pulse with the target. The mechanism is not a single process but rather a group of 18
19 processes, all of which have the common feat ure of involving some form of excitations and ionizations. The incident photons strike the target, producing electron-hole pairs and electronic excitations in a femt o second timescale. After a few picoseconds, the energy is transferred to the crystal lattice, and during the laser pul se, within a few nanoseconds, a thermal equilibrium between the electrons a nd the lattice is reached. This leads to a strong heating of the lattice and, with conti nued irradiation, to a massive emission of material from the surface (R. Kelly, 1994). 2.1.3. Plume The laser beam focused on the target ablates the target material during laser pulsing. This results in the appearance of a luminous plasma like plume composed of the ablated material and oriented in direction nearly perpendicular to the surface of the target, regardless of the angle of in cidence of the laser beam on the target. (Saenger, 1992). Typical plasma temperatures measured by emission spectroscopy during initial expansion are on the order of 10,000 K. This is well a bove the boiling points of most materials (generally less than 3000 K) (Chrisey). The laser fluence is a measure of energy density of a laser beam and is defined by, Fluence = Laser Energy Laser s pot Area at the Target [2.2] Fluence is typically expressed as Joules per square centimeter (J/cm 2 ).
20 2.2. Film Preparation A commercially acquired Fe 3 O 4 (99.9%) target was used for the deposition process. The XRD and EDS characterizati on was done on the bulk target and it was verified that the target had the stoichiometry of Fe 3 O 4 and the crystalline structure Fd3m, face centered cubic of Fe 3+ [Fe 2+ Fe 3+ ] O 4 magnetite. Film deposition was carried out on four different substrates, namely MgO, Si with a 5000 thick SiO 2 layer (later referred to as SiO 2 substrate), high resistive Si ( >50 cm) and Boron doped low resistive Si ( <0.005 cm). All the substrates us ed had (100) or ientation. Our goal was to deposit films on all the substrates under the same condition such as pressure, fluence, and substrate temperature with the same thickness in order to compare the substrate effect on the film. The major concern here is maximizing the area coverage. The substrates were cut into 1c m x 1cm squares and were mounted close to each other in a square shape, (as show n in Figure 2.2). The mounting block was positioned in a way such that the plume center axis pointed at the center of the configuration. Samples deposited in this configuration have the advantage of being deposited under same conditions such as pre ssure, fluence, and substrate temperature. After some preliminary investig ations, the system pressure was found to be critical for film stoichiometry. The deposition was perf ormed in the vacuum system described in section 2.1 and the operation pressure was maintained at 10 -7 Torr. The laser fluence was set at 3.5 J cm -2 and maintained throughout the entir e sample making procedure. The repetition rate of the laser pulse was set to 6 Hz and the target was rotated while laser pulses struck the target. The distance from the target to the substrate plays a major role in terms of area coverage, uniformity and quality of film. The substrate to target distance
was maintained at 6 cm from the target surface in our deposition procedure. The surface of the target was ablated for 15 minutes before the substrate was exposed to the deposition. This was done in order to remove any surface contamination and oxidation of the target surface. During this initial ablation, a shielding plate was placed between the target and the substrate. This plate shields the substrates from the plume while the target is ablated, preventing any contaminated material from reaching the substrate. The temperature of the substrate was maintained at 350 C. A LabView program was used to control the process by setting the number of pulses desired, pulse rate and the laser energy. After the deposition, the samples where cooled at the rate of 2C/minute to minimize film fractures caused by mechanical stresses in rapidly cooling films. Under the conditions described above a 50 /min (~0 .14 per pulse) film growth rate was obtained. Substrate Holder (metal block) 21 Figure 2.2. Substrate mounting configuration. Figure 2.3. Laser spot sample. SiO 2 Si_L Substrates Si MgO
2.3. Process Flow The process flow chart which shows the film deposition procedure is shown in Figure 2.4. followed by detailed description for each step. Substrate Cleaning Vacuum system activation Laser adjustment Substrate Installation Substrate Temperature Adjustment Cooling of the substrate Vent system pressure Laser Deposition Remove samples Figure 2.4. Film deposition process flow. 22
23 1. Sample substrates were cut into 1cm x 1cm squares. Substrate surface preparation included an ultrasonic bath in acetone, meth anol, and de-ionized wa ter respectively for 15 minutes each. Subsequently, the substrates were dried using compressed dry nitrogen. The substrates were then mounted on to th e substrate holder. To assure good thermal contact with the heating elem ent, a layer of silver paste was applied in between the substrate and holder. The use of this layer is to avoid any temperature gradient across the substrate and to ensure a good therma l contact with the heating block. 2. The appropriate lens and mirror adjustments were carried out in order to focus the laser beam on the target. The laser energy and lase r spot size were adjusted according to the desired fluence (energy/area). In order to measure the spot size, the developed black photographic paper was used as a temporary target. The temporary target was then exposed to focused laser beam, while holding it at the surface of th e target. The ablated area of the temporary target is of different color as seen in Figure 2.3. The laser spot size is determined by measuring the area of the ablated material. 3. The substrate holder/heater block was placed inside the chamber 6 cm away from the target. The temperature measuring thermocouple junction is attached to the block in close location to the substrate. 4. The pressure system consists of three pumps: roughing pump, turbo-molecular pump, and cryogenic pump. Roughing pump is used to remove the gasses inside of the chamber until pressure drop below 200 mTorr. Then the turbo-molecular pump is enabled which
24 allows for further reduction of the pressure (down to 10 -6 Torr). When the pressure of the system reaches 10 -6 Torr the Turbo-molecular and r oughing pumps valves are closed and cryogenic pump valve is opened. The cryogenic pump allows for the system pressure to be maintained at the level of 10 -7 Torr. A stable pressure of around 5x 10 -7 Torr was maintained during film depositions. 5. The temperature of the s ubstrate holder is set at 350 C by adjusting the voltage across the halogen bulb (located in the substrate holder). 6. After shielding the substrate, by using the stainless steel shield that can be adjusted from outside the chamber, the target was ab lated for 15 minutes to condition the target. The laser pulse frequency, energy, and total pulses desired were programmed using the excimer laser.vi laser control program. Fo llowing the targets 15 minute ablation, the substrate shield was rotated out of th e way, and the main deposition commenced. 7. After the deposition, the temperature wa s slowly lowered by decreasing the voltage applied across the bulb. 8. The cryogenic pump valve was closed. 9. The vacuum was released by opening the v acuum release valve. The substrates were taken out from the chamber for future testing and characterization.
25 2.4. Conclusion Although great care was taken in aligning th e plume to the subs trates center, the plume has been shifted towards right of the substrate configuration shown in Figure 2.2. This is one disadvantage of PLD system that the adjustments can not be made while the deposition chamber atmosphere is evacuated. This can be lead to thickness variation between substrates. Uniform area coverage is a known issue in PLD growth, and many PLD systems including the set up used in th is research, are not equipped for uniform thickness or large area depositions. Uniform cove rage could be achieved, to some extent, if the system allow for translational and/or rotational movements of the substrate.
26 CHAPTER THREE STRUCTURAL CHARACTERIZATION This chapter describes the micros tructure analysis and results of the Fe 3 O 4 films using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) characterization of the Fe 3 O 4 films grown on various substrates including SiO 2 MgO, high resistive Si (> 50 -cm), and low resistive Si (< 0.005 -cm). The XRD characterization of the target was performed prior to deposition to verify the Fe 3 O 4 composition. The XRD analysis of th e thin films shows the crystalline nature, orientations and evidence for Fe 3 O 4 In addition, images of films were taken using SEM in order to compare the film surface quality. SEM images show the epitaxy of the films on the MgO substrate. In addition, films on other substrates show a pol ycrystalline nature. The EDS data was taken for the target as well as the films to get the composition ratio of the chemical elements. Thickness measurements were done by usi ng a Dektak 3030ST auto surface texture profiler and results are shown at the end of the chapter.
27 3.1. X-Ray Diffraction (XRD) A Bruker D8 Focus powder diffractometer with wave length of 1.504 from a Cu source was used to perform the measurem ents. X-ray diffraction (XRD) is a standard technique for structural charac terization of materials. In XR D, a collimated beam of xrays impinges on a sample and the intensity of the reflected beam is measured. The x-rays are scattered at the crystallogr aphic lattice planes of the sa mple. The scattering intensity has a sharp maximum when the waves are s cattered specularly from different planes interfering constructively. This is the case wh en the relationship be tween the scattering angle and the inter-planar spacing fulfills the Braggs condition. We now present characteristic XRD scans for the target as well as films grown on different substrates. The matching database scan lines are inserted with the film scan to show the peak match. Also, the angles and th e (h k l) planes are shown for appropriate peaks. The XRD scan for the target is shown in Figure 3.1. Here we can see the peaks with multiple orientations. The most prominent peak is observable at 2 = 35.5 for the X-ray scattered from the (311) plane.
Fe3O4 Target New 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 10 20 30 40 50 60 70 80 90 1 0 Counts ( arb. ) 18.3 [1,1,1] 30.1 [2,2,0] 35.5 [3,1,1] 37.1 [2,2,2] 43.1 [4,0,0] 53.5 [4,2,2] 57.0 [5,1,1] 62.6 [4,4,0] 75.0 [6,2,2] 79.0 [4,4,4] 89.7 [7,3,1] 94.5 [8,0,0] 2 Figure 3.1. X-Ray diffraction scans for Fe3O4 target (black). Standard spectra of Fe3O4, JCP reference pattern 065-3107 (red). 28
29 3.1.1. X-Ray Diffraction of Fe 3 O 4 on SiO 2 Buffer Layered Si Substrate Figure 3.2 shows the XRD data of the films grown on Si/ SiO 2 It clearly shows that the film on SiO 2 buffer layer has the preferred orientation along (111) plane. The peaks are well matched with the referen ce pattern (PDF No. 065-3107) of magnetite. Also it is immediately noticeable that the mo st intense peak is (111), appeared at 18.3 whereas the most intense peak of the target appeared at 35.5 (311). This shows the good crystallization growth of the film. As clearly seen in the figure, the intensity of (111), (222), (333) and (444) peaks ar e in descending order as expected. In addition, a Si peak can be seen due to the Si ( 100) substrate underneath the 5000 thick SiO 2 layer. The key distinction we obtained from this XR D scan is the impurity peak at 44.8 for the film made on SiO 2 substrate. That peak matched with Fe 2 Si reference (PDF No. 083-1259) the peaks in database. This suggests that Fe 3 O 4 may have chemically reacted with the SiO 2 layer. However, it is interesting to note that the impurity peak is not seen in other films including the ones grown on Si. This may be due to the fact that SiO 2 is amorphous and does not have a stable stru cture as other substrates. So that it is easy for Fe 3 O 4 to react with SiO 2 To study the interface, it would be very useful to obtain a cross sectional image of the film including the substrate us ing transmission electron microscopy (TEM). In order to do this we have to deposit films on a special grid and in the future we intend to investigate more about this impurity pea k. However, we do not explore this impurity peak further in this thesis.
Fe3O4 /SiO2 0 1000 2000 3 000 4000 5000 6000 15 20 30 40 50 60 70 80 90 44.8 Fe-Si 33.0 Si 692 Si30 37.1 [2,2,2] 57.0 [3,3,3] 18.3 [1,1,1] 79.0 [4,4,4] Counts ( arb. ) 2 Figure 3.2. X-Ray diffraction scan for the Fe3O4 on SiO2 buffer layered Si substrate. Impurity peak in addition to Si substrate peaks can be seen with Fe3O4 peaks at 44.8
31 3.1.2. X-Ray Diffraction of Fe 3 O 4 on MgO Substrate Two peaks around 43 and 94 are visible for the 2 scan from 15 to 100 No other orientation of Fe 3 O 4 peaks can be seen other than (100). This shows the high crystallinity of the film. Fe 3 O 4 film peaks were hard to di fferentiate from those of MgO due to the fact that the MgO peaks and Fe 3 O 4 peaks overlap. The reason for this is that the lattice parameter of Fe 3 O 4 (8.397 ) is nearly double that of MgO (4.213 ), and this leads to an epitaxial film. The Fe 3 O 4 peaks seen in contrast with the MgO substrate peaks are even difficult to resolve as our MgO subs trate shows split peaks. It is known that any distortion of the lattice result ing in lower symmetry such as poor orientation and uneven sample surface will lead to sp lit peaks. However, we were able to resolve the peaks corresponding to the film, by comparing magnifi ed data of the substrate alone and data from substrate with film. The Figures 3.3 and 3.4 show the zoomed X-ray diffraction peak around 43 and 94 respectively to illustrate this point.
0 100000 200000 300000 400000 500000 600000 700000 42 43 0 100000 200000 300000 400000 500000 600000 700000 800000 900000 1000000 42 43 Counts ( arb. ) Counts ( arb. ) 2 Figure 3.3. Zoomed X-ray diffraction scans around 43 for the MgO substrate (top), Fe 3 O 4 on MgO substrate (bottom). 32
33 2 Figure 3.4. Zoomed X-Ray diffraction scan around angle 94 for the MgO substrate (top), Fe 3 O 4 on MgO substrate (bottom). 0 10000 20000 30000 40000 50000 60000 92 93 94 95 0 10000 20000 30000 40000 50 000 92 93 94 95 9 Counts ( arb. ) Counts ( arb. )
34 3.1.3. X-Ray Diffraction of Fe 3 O 4 on Si (>50 -cm) Substrate As seen in Figure 3.5, the Fe 3 O 4 film on Si (>50 cm) was found to be oriented in the (111) direction. The XRD pattern does not show any phase of iron oxide other than Fe 3 O 4 There are two peaks associated with th e Si substrate that can be seen at 33 and 69.2 The Fe 2 Si peak of 44.8 as discussed in section 3.1.1, does not appear in the film deposited on Si. 3.1.4. X-Ray Diffraction of Fe 3 O 4 on Si (<.005 -cm) Substrate The orientation of the film on a high conduc ting Si substrate presented in this section is the same as that in section 3.1.3. The lower count is due to the fact that a smaller piece was used. The location of the observed peaks in Figure 3.6 and 3.5 are identical.
011306_Si 0 100 200 300 400 500 600 700 800 900 1000 16 20 30 40 50 60 70 80 90 18.3 [1,1,1]37.1 [2,2,2]57.0 [3,3,3]79.0 [4,4,4] 33.0 Si69.2 Si Counts ( arb. ) 2 Figure 3.5. X-Ray diffraction scan for the Fe 3 O 4 on Si substrate. Si (> 50 cm) substrate peaks can be seen at 33 and 69. 35
Fe3O4 on Si low resistive sub 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 15 20 30 40 50 60 70 80 90 36 18.3 [1,1,1]37.1 [2,2,2]57.0 [3,3,3]79.0 [4,4,4] 33.1 Si69.2 Si Counts ( arb. ) 2 Figure 3.6. X-Ray diffraction scan for Fe 3 O 4 film on Si (<.005 -cm) substrate. Si substrate peaks can be seen addition to Fe 3 O 4 peaks.
37 3.2. Scanning Electron Microscopy (SEM) Scanning electron microscopy (SEM) is a method for high-resolution imaging of surfaces. A Hitachi S-800 scanning microscope was used to capture these images. The SEM uses electrons T for imaging, much as a light microscope uses visible light. The advantages of SEM over light microscopy include greater magnification (up to 300,000X) and much greater depth of field. An incident electron beam is raster-scanned across the sample's surface, and the result ing electrons emitted from the sample are collected to form an image of the surf ace. Imaging is typically obtained using secondary electrons for the best resolution of fine surf ace topographical features. Alternatively, imaging with backscattered electrons gives contrast based on atomic number to resolve microscopic composition variations, as well as, topographical information. Even though these films were grown in s itu, there are signific ant differences in the film surface from substrate to substrate. Figure 3.7 (a) shows the topology of film on SiO 2 under 30 000X magnification. We can see th e crystallites under 0.1 m distributed on the film on a SiO 2 substrate. The grain boundaries are also visible in this image. The image with 3000 magnification included an in set in Figure 3.6 (a) (i) has shown the uniformly distributed crystallites and, th e image of 15 0000 magnification has shown the texture, i.e. distribution of crystallographic orientation. In contrast, the MgO film has a very clean surface that is free of particulates or crystallites. This is convincing evidence of the epitaxial formation of the Fe 3 O 4 on MgO substrates. Films on other substrates show the polycrystalline nature.
(ii) (i) (a) (ii) (i) (b) Figure 3.7. SEM images for Fe 3 O 4 film on SiO 2 substrate (a) and Fe 3 O 4 film on MgO substrate (b). The insets (i) and (ii) are the magnifications of 3000 and 150000 respectively. 38
The film made on Si (> 50 -cm) also appears to be particle-free when compared with SiO2 substrate. Grain boundaries are also smaller in Si substrate than that on SiO2 substrate, as shown in Figure 3.8. (i) (ii) Figure 3.8. SEM images for Fe3O4 film on high resistive Si (>50 -cm) substrate. The insets (i) and (ii) are the magnifi cations of 3000 and 150000 respectively. 3.3. Energy Dispersive Spectroscopy (EDS) Energy dispersive x-ray spectroscopy (EDS) is a chemical microanalysis technique performed in conjunction with a scanning electron microscope (SEM) A EDAX model Hitachi S800 132-2.5 EDS detecti ng unit was utilized to perform these measurements. The technique utilizes x-rays that are emitted from the sample during bombardment by the electron beam to characterize the elemental composition of the analyzed volume. The EDS x-ray detector measures the number of emitted x-rays from the sample surface versus their energy. The ener gy of the x-ray is ch aracteristic of the 39
element from which the x-ray was emitted. A relative counts versus energy of the detected x-rays is obtained and evaluated for qualitative and quantitative determinations of the elements present in the sampled volume. 050100150200250300012345Energy (keV)Intensity (arb. ) FeO C Figure 3.9. EDS graph for Fe 3 O 4 target. This shows the chemical composition is Fe, O and C. The carbon (C) peak is due to the instrument error resulting from scattering from the rough target surface. Table 3.1. Elements, weight and Atomic weight percentage for the target. Element Wt % At % Atomic Ratio C K 7.39 16.71 1.24 O K 31.58 53.61 4 Fe K 61.03 29.68 2.21 Total 100 100 40
41 3.4. Thickness Measurements The profilometer, well known and rather uncomplicated, thickness measurement was used to measure all the film thicknesses. The profilometer uses a diamond tip to scan a surface to get information about surface t opography. The tip scans across the surface of the sample, and an inductive sensor register s the vertical motion of the tip. The signal generated by the motion of the tip is used to create a two-dimens ional profile of the surface. The thicknesses obtained from thes e measurements are shown in Table 3.2. There were five measurements taken in each of four different places which were used to obtain an average thickness that were rounded up to the nearest 100 Table 3.2. Average thickness measurements for the Fe 3 O 4 films. Substrate Average Thickness SiO 2 1800 MgO 1800 Si (>50 cm) 1100 Si (<0.005 cm) 1100
42 CHAPTER FOUR ELECTRICAL AND MAGNETIC CHARACTERIZATION The electrical and magnetic characterization was mainly done using a model 6700 Physical Property Measurement System (PPMS) available in our Functional Material Laboratory at University of South Florida (USF). In additi on, I-V characteristics were investigated using the standard four-point probe configura tion which is in the LAMSAT laboratory at USF. The PPMS provides a flexible, automated wo rk station that can perform precise thermo control of various experiments su ch as magnetic, electro-transport, or thermoelectric measurements from temper atures ranging from 350 K to 2 K. Liquid helium is the cryogen and computerized temp erature control is used by the PPMS for accessing a wide range in temperature. The PPMS also is equipped with a superconducting magnet that can be changed to produce fields up to 7T. We used the PPMS system to measure resistance as function of temperature, R (T), both in the case of presence and the absence of applied magnetic fields. Also, magnetization as a function of applied magnetic field M (H), and as a function of temperature M (T) were measured. This chapter consists of eight sub sec tions which describe seven experimental procedures and the results. S ection 4.1 is dedicated the overv iew of the PPMS to give the
43 reader a brief understanding. Sections 4.2 and 4.3 cover the resistance measurements and magneto-resistance measurements for the magnetite films on SiO 2 MgO, Si substrates. Resistivity variation as a function of temperature is also presented and compared with other literature. Section 4.4 and 4.5 pr esent the magnetization measurements. Magnetization as a function of applied field (M-H) in section 4.4 a nd magnetization as a function of temperature (M-T) in section 4.5 are presented along with the analysis. Section 4.6 presents the I-V characteristics for the Fe 3 O 4 films on SiO 2 substrate. 4.1. Overview of the PPMS System The PPMS system is incorporated with a temperature controller, gas-flow controller, magnetic-field cont roller, and helium-level measuring subsystems. It is a commercial system from Quantum Design and is a very popular platform used by many groups for conducting electrical and magnetic ch aracterization of mate rials. A schematic diagram of the PPMS instrument is shown in Figure 4.1. (a).
(a) 44 (b) Figure 4.1. (a) Schematic of Physical Property Measurement System (PPMS), (b) the different probes for different measurements. (Extracted from PPMS m a nual)
45 The dewars outer jacket has super insu lation to help minimize He consumption. This outer layer is evacuated through an evacuation valve. The cooling annulus is the active region of temperature control. Th e continuously pumping vacuum pump draws helium from the dewar through the impedance tu be and into the cooling annulus. There are two different thermometers in this syst em. A platinum resistance thermometer reads temperature above 80 K and a negative temperature coefficient thermometer accurately reads temperatures below 100 K. There are seve ral probes, as shown in Figure 4.1.(b) that are available to study the sample with PPMS in order to carry out different measurements under different conditions. Computer controlled multi Vu software is used to operate and collect data from PPMS. In a ddition, other LabView modules ar e also available in our lab for user experiments using the PPMS. 4.2. Electrical Resistance Measurements This section presents the sample preparat ion, installation, and results for electrical resistance measurements for the films grow n in this project. All the samples were prepared with 0.3 cm x 1 cm area for the electrical measurements. Two samples were mounted onto the resistivity puc k in four point measurement configuration in a single run. A layer of silicone vacuum grease was applied in between the sample mount and substrate in order to achieve good thermal contact. The puck was mounted into the PPMS after checking the lead conn ections. A quick resistance measurement was performed to confirm the electrical contact leads attached using silver paint were robust and at the same time to acquire an approximation of the sample resistance.
46 The bridge set up which is a part of the PPMS control was completed by choosing the channel, current, drive mode, etc. The re sistance data was collected in the cooling cycle for all the samples pr esented in this section. The experiment was performed for the Fe 3 O 4 films on different substrates such as MgO, SiO 2 Si (low resistive and high resistiv e) to see substrate dependence on the transport. The following sub sections pr esent resistance raw data and the calculated resistivity data for the different substrates. 4.2.1. Temperature Dependence of the Resistance in Fe 3 O 4 Films Grown on SiO 2 Figure 4.2 shows the resistance variation as a function of temperature. The resistance increases as temperature is decr eased and there are th ree distinct regions marked by the shape of resistance change. The resistance increases slowly by one order of magnitude as the temperature is decrea sed from 300 K to 124 K. At 124 K there is a sharp increase in resistance. At the transi tion, the resistance increa ses with higher slope than that of the first zone. The sharp resistance change in the second zone is called the Verwey transition ( T v ) and it occurred at 124 K. As e xplained in the introduction, this sharp transition is associated with the struct ural transition from a cubic high temperature to a monoclinic low temperature phase. The Verwey transition temperature observed here matches well with the reported values. In gene ral the transition observed in these films is broader in comparison to the sing le crystal data possibly due to the residual strain in the films resulting from the lattice mismatch with the substrate. The strain effects also result in shifting of the Verwey transition temperature to lower values with film thickness.
501001502002503001x1021x1031x1041x1051x1061x107 CoolingResistance ()T (K) Fe3O4/SiO2 Figure 4.2. Temperature dependence of resistance for Fe 3 O 4 film on SiO 2 substrate. Thickness of the film is ~ 1800 4.2.2. Temperature Dependence of the Resistance in Fe 3 O 4 Films Grown on MgO The Fe 3 O 4 film thickness was approximately 1800 for MgO. The R verses T graph for a film on the MgO substrate is shown in Figure 4.3. As seen in Section 4.2.1, three distinct regions of resistance variation can be identified for the Fe 3 O 4 film grown on MgO. A very sharp transition can be seen at 118 K with higher than one order of magnitude resistance jump. 47
50100150200250300102103104105106107 CoolingResistance ()T (K) Fe3O4/MgO Figure 4.3. Temperature dependence of resistance for Fe 3 O 4 film on MgO substrate. Thickness of the film is ~ 1800 A. The resistance data obtained from four point probe collinear method showed in Figure 4.3. and sample geometrical data were used to calculate the resistivity by using the equation R= l /A. The Figure 4.4 shows the resistivity variation for the Fe 3 O 4 film on MgO for the thickness of 1800 The resistivity of the Fe 3 O 4 film made on MgO substrate was found to be 2.1 m cm at room temperature. Ogale and co-workers have declared in their paper that a typical value of well characterized single crystals is lower than 10 m cm at 300 K. This shows that our experiment results match very well with accepted values. The sharpness and the order of magnitude of the Verwey transition are the characteristics of good quality Fe 3 O 4 In fact, there exist a few papers in literature where sharp transitions are not reported. It is reported that the bulk Fe 3 O 4 (thickness 48
>6000 ) shows a very sharp transition with two orders of magnitude. Figure 4.3 shows the very sharp transition with close to two orders of magnitude in our thin films. This data along with XRD and EDS data show the evidence for good crystalline quality of our films. The transition occurs at 118 K at a lower temperature than the film grown on SiO 2 substrate. This may be due to substrates influence on the films characteristics. 501001502002503005x10-35x10-25x10-15x1005x101 MgO ( cm)T (K) Figure 4.4. Temperature dependence of resistivity for Fe 3 O 4 film on MgO substrate. Thickness of the film is ~ 1800 4.2.3. Temperature Dependence of the Resistance in Fe 3 O 4 Films Grown on High Resistive Si (resistivity > 50 cm) Fe 3 O 4 films grown on a Si substrate had a thickness of approximately 1100 The Verwey transition is measured to be at 118 K. The sharpness of the Verwey transition is not as steep as that of the films we discussed above. This can be due to either 49
the lower thickness of the film compared to the films on SiO 2 and MgO, that have thickness around 1800 or due to the unfavorable growth of an ionic oxide on a covalent substrate. To grow Fe 3 O 4 films on a Si substrate, Jain et.al have introduced buffer layers such as Ta, Ti and SiO 2 as they were unable to see Fe 3 O 4 peaks of the film deposited on bare Si substrate. But in our case, XRD shows the Fe 3 O 4 peaks crystalline in the (111) plane for the films on Si substrate without any buffer layer and the observation of Verwey transition confirms the successful growth of Fe 3 O 4 films. The transition temperature (T v ) as determined by the change in slope of the R-T data is higher in comparison to those of films grown on SiO 2 and MgO substrates. 50100150200250300103104105106 011306/Si(>50cm)Resistance ()T (K) Figure 4.5. Temperature dependence of resistance for Fe 3 O 4 Film on Si substrate (resistivity > 50 cm). Thickness of the film is ~ 1100 50
4.2.4. Temperature Dependence of the Resistance in Fe 3 O 4 Film Grown on Low Resistive Si (resistivity <0.005 cm) The graph shown here is taken from the four point probe method using Keithley meters in LAMSAT laboratory at USF. Here we can see a decrease in resistance around 120 K instead of a sharp increasing expected for Fe 3 O 4 We believe this is due to the high conductivity of the substrate which offers a channel for current flow when the film resistivity shows up in the vicinity of the Verwey transition. This may partly explain the reason why not many groups have been successful in growing Fe 3 O 4 films on Si substrates. As we have shown here, better quality films can be achieved only if the resistivity of the Si substrates considerably high. Resistance vs Temperature1.E-011.E+001.E+01050100150200250300T ( K)Resistance  Figure 4.6. Temperature dependence of resistance for Fe 3 O 4 film on high conducting Si substrate (resistivity <.005 cm). Thickness of the film is ~ 1100 51
4.2.5. Comparisons and Conclusions 50100150200250300103104105106107 Resistance ()T (K) SiO2 MgO Si Figure 4.7. Temperature dependence of the resistance of Fe 3 O 4 film on SiO 2 MgO and high resistive Si (>50 cm) substrates, prepared under same condition at same time. Table 4.1. Verwey transition temperatures for films on SiO 2 MgO and high resistive Si (> 50 cm). Substrate T v (K) SiO 2 124 MgO 118 Si (> 50 cm) 118 Figure 4.7 shows the comparison of temperature dependence of resistance for Fe 3 O 4 films grown on different substrates. It can be seen that the film on the MgO 52
substrate has the steepest transition. The Verwey transition for MgO and Si (>50 cm) can be seen at 118 K. The film on the Si substrate has the broadest transition. This can be due to two reasons. One is as others have observed, it may be due to thickness dependence. This is in agreement with the experimental results of X.W. Li et al reference which we showed in Figure 1.6 in Chapter 1. The other reason is there can be substrate dependence. To normalize the effect of different thickness of the films, we calculated the resistivity for the films and displayed all in the same graph as seen in Figure 4.8. 5010015020025030010-310-210-1100101102 ( cm)T (K) SiO2 MgO Si (>50 cm) Figure 4.8. Temperature dependence of the resistance of Fe 3 O 4 film on SiO 2 MgO and high resistive Si (>50 cm) substrates, prepared under the same conditions and at the same time. Resistivity starts at room temperature with the same resistivity range and increases with different rates as the temperature decreases. This is an important comparison because Figure 4.8 shows the role of substrates on Fe 3 O 4 films. The XRD 53
54 results reveal that the film on MgO is ( 100) oriented and other samples are (111) oriented. There is a close match in slope of the resistivity graph which can be seen for films on Si and SiO 2 substrates as the temperatur e decreases from 300 K down to T v Resistivity of Fe 3 O 4 / MgO increases with a lower slope during this temperature range. The film on MgO is oriented in (100) direction and both f ilms on Si and SiO 2 are (111) oriented which is the lowest energy orientation for the magnetite. The sharpness of the transition is affected by the strain aris ing due to the lattice mismatch with the substrate and also oxygen stoichiometry i.e., slight departure from the precise Fe:O stoichiometry. It is also reported that the cha nge in resistivity at the Verwey transition is smaller for (111) than (100) oriented films. Th is is consistent with what was observed in our samples. The higher T v of Fe 3 O 4 /SiO 2 films at room temperature in comparison to the films grown on Si and MgO may be due to the presence of Fe 2 Si phase as seen in XRD analysis. It has been found that iron containing 3% si licon has four times the resistivity of pure iron (C. W. Chen, 1977). There are several models which have been introduced to describe the Verwey transition since its discovery as discussed in Chapter 1. The most resent accepted model is Motts view of the Verwey transition (T < T v ) which describes the transition as due to the variable range hopping of small polarons. The formula is given by = A exp (B/T) 1/4 where is the resistivity and B is the act ivation energy. The conduction above the transition is said to be due to the thermal excitation. The resistance above the T v is given by R = R 0 exp (B/k b T), where k b is the Boltzman constant (Liang Wang, 1999).
4.3. Magneto-Resistance Magneto-resistance is the change in resistance of a material when a magnetic field is applied to the sample versus when no field is applied. The percentage magneto-resistance is then defined as, 1000HHRRRMR [4.1] Where R H is the resistance when a magnetic field is applied, and R 0 is the resistance for zero field. The resistance data of the sample as it is cooled from 300 K to the target temperature of 10 K with a zero magnetic field applied (H= 0 Oe) was described in Section 4.2. The sample was then cooled from 300 K to 10 K, at the same rate, with a magnetic field (FC) of 5 Tesla applied perpendicular to the film surface, and the resistance measurements were recorded. The notation FC is used for sample being field cooled. It is reported that the transport properties of these films also depend on the magnetic history of the sample. We measured the resistance of the sample while heating at a constant rate both in the presence of the field and also in zero field. The magneto-resistance of these films is calculated using equation 4.1. 4.3.1. MR for Fe 3 O 4 on SiO 2 Magnetite is well known for its negative magneto-resistance. The resistance variation in the absence of the field and the 5 T field applied was plotted in the same graph as shown in inset of Figure 4.9 in order to show this effect. As seen in the figure, the resistance of the material in the presence of the magnetic field is lower than that when 55
there is no field applied. The figure shows the percentage ma gneto-resistance variation as a function of temperature. The magneto-resi stance (MR) is increasing monotonically from 300 K to 130 K and close to the Verwey transition MR increases sharply and has a highest value of 17 % at 123 K. Further lowering the temperature, the MR remains almost the same up to the lowest temperatur e of 90 K. The sharp increase in MR close to the first order Verwey transition has been observe d previously by other groups. A sharp magneto-resistance of 16 % in a magnetic field of 7.7 T was observed by Gridin et al for a Fe3O4 single crystal near Verwey transition. 100150200250300 0 10 20 100150200250300 10310 105106 R ()T (K) H 0 T H 5 T (R0-RH)/RH %T (K) Fe3O4/SiO2 Figure 4.9. Temperature dependence of magneto-resistance for Fe3O4/SiO2 film. The inset shows the resistance of the film in the presence of applied field of 5 T and for the 0 T case. 56
4.3.2. MR for Fe3O4 on MgO The magnetoresistance variation with te mperature is plotted in Figure 4.10. The inset shows the resistance of the film meas ured in the presence and absence of 5T magnetic field. As seen in the figure, the re sistance of the film in the presence of magnetic field is significantly lower than that of when there is no field applied. Magnetoresistance shows a small peak close the tran sition temperature. Below the transition the MR first decreases and then shows an upt urn and increases down to the lowest temperature where the compliance voltage is met due to the increased resistance of the film. This is also observed by other groups (Gong et al and Ogale et al). The low temperature increase in MR is in terpreted phenomenologically by Gong et al as due to the field dependence of the activation energy for hopping conduction below the transition temperature. 80100120140 0 10 20 100150200250300 10310 105106 R ()T (K) H = 0T H = 5T (R0-RH)/RH %T (K) Fe3O4/MgO 57 Figure 4.10. Temperature dependence of magneto-resistance for Fe3O4/MgO film. The inset shows the resistance of the film in the presence of applied field of 5 T and absence of field.
4.3.3. Fe 3 O 4 on High Resistive Si (resistivity > 50 cm) The resistance variation in the absence of field and the 5T field applied has been plotted in the same graph as in the above section, and is shown in Figure 4.11. As seen in the figure, there is a small resistance lowering in the presence of magnetic field. 501001502002503001x1031x1041x1051x106 Fe3O4/ Si(>50 cm)R T (K) Cooling (0T) Cooling (5T) Figure 4.11. Resistance dependence on temperature of Fe 3 O 4 film on Si (>50 cm) substrate in the presence of applied field of 5 T and absence of field. Conclusions: We measured MR on Fe 3 O 4 films deposited on different substrates. All the films show an increase in MR at the Verwey transition temperature. The peak in MR matches well with the reported values. The film on SiO 2 shows a sharp peak in MR at T v with no increase at low temperature whereas Fe 3 O 4 on MgO shows a small peak in MR close to T v and MR increase on further lowering the temperature. 58
59 Magneto-resistance has been studied by V.V. Gridin et al (1996) on singlecrystal samples that showed that the MR has a sharp peak around T v with a maximum value of 17% under 7.7 T and decreases very ra pidly as the temperat ure deviates from T v This magneto-resistance has been explained as resulting from the discontinuous change of thermodynamic quantities accompanying the first order Verwey transition. Also G.Q. Gong et. al (1997) has reported magneto-transport pr operties of epitaxial film grown on an MgO substrate and has observe d the similar peak at around T v Additionally; they have observed that the MR increases monotonically below 100 K with decreasing temperature. MR values as high as 32% have been observed for a 6600 thick film at 60 K under a 4 T field. Our results on MgO substrate show a very similar feature. The MR in Fe 3 O 4 thin films is investigated by ma ny groups and it is reported that the transport properties of magnetite below T V is highly dependent on the history of the sample. So MR not only depends on whether th e sample is field cooled or zero field cooled through the transition but also on its prior magnetic state. It is also to be noted that the MR measurements are carried out in our PPMS using the resistivity option in which during the measurement the voltage remains constant and the current changes as the resistance of the sample is changed. In the case of Fe 3 O 4 films the I-V ch aracteristics are non linear at lower temperature. So the change in current during the measurement can influence the observed MR varia tion. In order to see the effect of variation of current and also the magnetic state of the sample on th e transport properties, future experiments would focus on measuring the ac conductivity (in which the curre nt can be kept constant) on Fe 3 O 4 films in a predefined magnetic state.
60 Magnetization Measurements The Quantum Design AC Measurement System (ACMS) which is an option for our PPMS was used for this experiment. DC magnetization measures the samples magnetic moment M in an applied magnetic field H at a specific temperature. In this case the sample moves through a set of copper coil s and the induced signal is analyzed with a digital signal processor to determine th e samples magnetic moment. During a DC magnetization measurement, the PPMS supplies a constant magnetic field to the sample space, and the sample moves through the enti re detection coil set in approximately 0.05 seconds. The ACMS takes 4,096 voltage read ings during the sample translation and creates a voltage profile curve for the translation. The detection coils register a voltage proportional to the rate of change of magnetic flux through them. So the voltage profile is the time derivative of the net flux through the coils. The sample moment M is obtained by integrating the voltage profile. 4.4. Magnetization as a Function of the Applied Magnetic Field (M-H) The same sample used in Section 4.2.1 was used for this experiment. The sample was placed in a gel capsule longitudinally and inserted into a straw. Then the straw was attached to the measurement probe and placed in the dewar. The next step is to center the sample inside the probe co ils. This is done by locating sample option in the ACMS control center and setting a sm all field (500 Oe). Then the M vs. H and M vs. T were measured. Here DC magnetization of ACMS measurement configuration was used to measure magnetization. The magnetization wa s measured at 300 K while changing the applied magnetic field from -6000 Oe to +6000 Oe at a rate of 280 Oe per second, where
10 000 Oe = 1 T. The same procedure was then repeated at 10 K. The M verses T measurement was taken while changing temperature from 300 K to 10 K. 4.4.1. M-H for Fe 3 O 4 on SiO 2 -6-4-20246-600-3000300600 10K -1100 to 990(Oe) 300K -290 to 290 (Oe)Fe3O4/ SiO2 (011306)Magnetization (emu/cc)Magnetic Field (kOe) Figure 4.12. M-H loops for Fe3O4 film on SiO 2 at 10 K and 300 K Magnetization of the film on SiO 2 substrate was measured while varying the applied magnetic field from -6000 Oe to +6000 Oe. This measurement was done at two different temperatures (300 K and 10 K). The hysteresis loops broadens at 10 K with respect to 300 K loop. Also there is a small shift towards the negative side. The coercivity H c at 300 K is about 290 Oe and is almost symmetric. But at 10 K it has two different values for positive 990 Oe and negative 1100 Oe. The saturation moment (M S ) is 440 61
emu/cc at 10 K and 490 emu/cc at 300 K. These values are comparable to the reported bulk saturation moment of 471 emu/cc. The increase in coercivity from 290 Oe to 1000 Oe going from 300K to 10 K also matches very well with the reported values. The low temperature monoclinic phase has higher magnetocrystalline anisotropy in comparison to the high temperature cubic phases resulting in an increase in coercivity at low temperatures. The 300 K loop saturated at about 1.4 k Oe whereas 10 K loop at 2.0 k Oe. 4.4.2. M-H for Fe 3 O 4 on MgO -6-4-20246-600-400-2000200400600 Fe3O4/ Mgo (011306)Magnetization (emu/ cc)Magnetic Field (k Oe) 10K -750 to 580 (Oe) 120k -270 to 250 (Oe) 300K -140 to 140 (Oe) Figure 4.13. M-H loops for film on MgO at 10 K, 120 K and 300 K. The same measurement was performed on the film on the MgO substrate except another measurement of M-H loop at 120 K, the Verwey transition temperature was 62
63 taken. It can be seen that th e loop is broadened as temperat ure is decreased. The loop at 300 K is perfectly symmetric and it has coerci vity of 140 Oe. But as the temperature is lowered, the loops tend to shift toward the nega tive side. At 120 K th e coercivity of the negative side is 270 Oe and that of the positiv e side is 250 Oe. At 10 K we can see a large shift of loop with the coercivity of negativ e side is 750 Oe and positive side is 580 Oe. The large loop shift observed at 10 K in Fe 3 O 4 deposited on MgO substrate is also observed by many groups and this is interprete d as due to the formation of antiphase boundaries (APB) as a natural growth defect in this hetero-epitaxial films. The spinel crystal structure of Fe 3 O 4 films is lower in symmetry than MgO and as explained in Chapter 2 the unit cell size of Fe 3 O 4 is twice the size of MgO. It is reported that the density of APBs is strongly influenced by the growth conditions and the substrate microstructure. The presence of APB which is region coupled by antiferromagnetic interaction with neighboring ferromagnetic domains leads to an exchange coupled system which leads to a shifted hysteresis loop at low temperature. The saturation Magnetization (M S ) of films grown on MgO substrate are 428 emu/cc at 10 K and 370 emu/cc at 300 K. We can clearly see higher magnetization at the 120 K relative to the other temperatures. Bickford studied the temp erature dependence of the ferromagnetic resonance absorption in a synthetic single cr ystal of magnetite and found that above 130 K (which is above T v ) the anisotropy constant is negativ e, indicating that (111) is the easy axis of magnetization, while below 130 K, (100) is the easy axis of magnetization. This might explain the increase in magnetization of our film on MgO at 120 K as the film on
MgO substrate has (100) orientation. Saturation field for 300 K is about 1 kOe and for 120 K, it is about 2.2 kOe. 4.4.3. M-H for Fe 3 O 4 on Si (resistivity > 50 cm) We also measured the M-H loops of films grown on Si (resistivity > 50 cm) substrate. Figure 4.14. shows the M-H loops carried out 10 K, 120 K and 300 K. These M-H loops are as measured without the background correction due to the Si substrate. The downward curvature of these loops at high temperature is due to the diamagnetic contribution from the Si substrate. The saturation magnetization is 330 emu/ cc and is much smaller in comparison to the bulk value. -6000-4000-20000200040006000-455-303-1520152303455 Magnetization (emu/cc)Magnetic Field (k Oe) 300 K 120 K 10 K Figure 4.14. M-H loops for film on high resistive Si substrate (resistivity > 50 cm) at 10 K, 120 K and 300 K. 64
4.4.4. M-H for Fe 3 O 4 on Si (resistivity <0.005 cm) We also measured the M-H loops of films grown on low resistivity Si substrates. Figure 4.15 shows the M-H loops carried out of 10 K and 300 K. These M-H loops are as measured without the background correction due to the Si substrate. The downward curvature of these loops at high temperature again is due to the diamagnetic contribution from the Si substrate. The saturation magnetization is 390 emu/ cc and is smaller in comparison to the bulk value. The shift of the M-H loop at 10 K can be seen in the film on low resistive conducting substrate as seen in for others. 65200400 0 Figure 4.15. M-H loops for film on low resistivity Si substrate at 10 K and 300 K. -6-4-20246-400-200 Fe3O4/ Si_.005 (011306)Magnetizaion (emu/cc)Magnetic Field (kOe) t 10K -1200 to 1000(Oe) 300K -300 to 290(Oe)
66 Conclusions: By comparing the values in Table 4.2., coercivity is remarkably high for the polycrystalline films which have an orie ntation of (111). The epitaxial film on MgO substrate which has an orientati on of (100) has the lowest coer civity at room temperature. This can be explain by the formation of mi crostructure. The Fe stacking sequence of Fe 3 O 4 in the (111) direction is comprised of alternating antiferro magnetically coupled octahedrally and tetrahedrally coordinate d sites. Hence each layer along the (111) direction has a well defined magnetization direct ion in the plane of the sample. So that the (111) direction films show the high coerciv ity. Also it is observable for all the films the coercivity increases as temperature decreases. Large increase in coercivity can be understood by recalling that while cooling through T v the crystal structure changes from cubic to orthorhombic (Sangeeta, 2001). Films on SiO 2 and Si exhibit a higher magnetic remanent compare to film on MgO, which shows the lowest remanent among the substrates used. This might have a direct correlation with the polycrystalline distribution of crystallites. As shown in the SEM images in Chapter 3, SiO 2 had the highest random or ientation and the largest crystallites. Films on MgO have a clear epitaxial nature. Magnetization of films on SiO 2 and Si are higher than the films on MgO. This is due to MgO being oriented in the magnetic hard axis (100) and the others are oriented in to magnetic easy axis (111).
67 Table 4.2. Coercivity, saturation magnetiz ation, remanence values for the Fe 3 O 4 films on SiO 2 MgO, Si (>50 cm) and Si (< 0.005 cm) substrates. H C (Oe) M R (emu/cc) M S (emu/cc) Substrate 10 K 300 K 10 K 300 K 10 K 300 K SiO 2 -1100 1000 -300 300 340 300 440 490 MgO -750 580 -140 140 182 182 428 370 Si (> 50 cm) -900 880 -350 350 274 252 330 330 Si(<0.005 cm) -1100 1000 -300 290 220 260 390 390 4.5. Magnetization Versus Temperature (M-T) The sample was cooled from 300 K to 10 K with zero Magnetic field applied (ZFC), and the magnetization was measured while sweeping the temperature at 2 K per minute. The sample was then warmed up to 300 K to eliminate the history effect. Again the sample was cooled from 300 K to 10 K with 5T magnetic field (FC) with same rate and the magnetization measurements were taken. Magnetization changes with the temperature for both 300 Oe and 5000 Oe are plotted in the same graph. The graphs are displayed in the same scale in order to co mpare the graphs with different substrate.
4.5.1. M-T for Fe 3 O 4 on SiO 2 Magnetization is increased under applied parallel magnetic field as temperature decreases until the transition temperature is reached. At T v one can see the magnetization decrease to a lower value. This sharp transition again verifies the good stoichiometry of the sample. There is an increase in magnetization after the transition which can be seen for the applied field of 300 Oe whereas no such an increment in a field of 5000 Oe after the transition. 050100150200250300185278370463556 Fe3O4/ SiO2 (011306)Magnetization (emu/cc)Temperature (K) 300 Oe 5000 Oe Figure 4.16. M vs.T for the film on SiO 2 under applied field of 300 Oe and 5000 Oe. 68
4.5.2. M-T for Fe 3 O 4 on MgO The magnetization increased under the applied parallel magnetic field as the temperature was decreased until the transition temperature is reached. At the T v can see a magnetization decrease to a lower value. The transition under a low field is sharpest and about 100 emu/cc. This sharp transition again verifies the good stoichiometry of the sample. Strain due to the close lattice match of the substrate and the Fe 3 O 4 may play a role in the reduction of magnetization. The epitaxial nature also plays a major role in the transition. There is an increase in magnetization after the transition which can be seen in both high and low applied fields. 050100150200250300185278370463556 Fe3O4/ Mgo (011306)Magnetization (emu/cc)Temperature (K) 300 Oe 5000 Oe Figure 4 17. M vs.T for the film on MgO Under applied field 300 Oe, 5000 Oe. 69
4.5.3. M-T for Fe 3 O 4 on Si (<0.005 cm) 050100150200250300185277370462555 Fe3O4/ Si.005 (011306)Magnetization (emu/cc)Temperature (K) 300 Oe 5000 Oe Figure 4.18. M vs.T for the film on low resistivity Si substrate under applied field of 300 Oe and 5000 Oe. 4.6. Current -Volatage Characteristics I-V characteristics were investigated using the standard four-point probe configuration which is in the LAMSAT laboratory at University of South Florida. A Keithley model 224 constant current source was used to apply current and a Keithley 182 nanovoltmeter was used to measure the voltage. The sample was current biased. A computer controlled LabView program was used to control the system via applied current and temperature control and extracts data as measured voltage and calculated resistance. A cryogenic He cooling system was used to lower the temperature. The supplied current 70
71 was varied from 1 to 10 A in the both pos itive and negative dire ction. Eight voltage measurements were taken for both directi ons for eight different applied currents. Typically, measurements were performed in the temperature range of 300 K to 55 K. 4.6.1. Current and Voltage Dependence of Fe 3 O 4 on SiO 2 As seen in Figure 4.19, within the bias current range of 0.01uA to 10uA, current and voltage has a linear depe ndence at higher temperatures When the temperature is lower than 90 K, within the same bias curre nt range, a non-linear relationship is seen. Figure 4.20 shows the non-linear current vs. voltage relations hip at temperatures 65 K and 70 K. This trend is consistent with the metal to insulator transition of Fe 3 O 4 We can interpret the result as above the transition as electron transfer within B sites due to the thermal excitation. This should have oh mic behavior and Figure 4.19., shows the agreement with the theoretical model. A ccording to Motts formula, the conduction which occurs due to tunneling and variable range hopping of small polarons occurs below the transition. This is expected to have non linear behavior and our results are in agreement with that expectation.
Current vs Voltage -1.E-05 -6.E-06 -1E-06 4.E-06 9.E-06 -0.04-0.03-0.02-0.01 0 0.010.020.030.04 Voltage [V]Current [A] 300 200 135 125 Figure 4.19. I-V characteristics of the Fe3O4 film on SiO2 for the temperature range of 125 K T 300 K. Current vs Voltage -10 -8 -6 -4 -2 0 2 4 6 8 10 -20 -15 -10 -5 0 5 10 15 20 Voltage [V]Current [A ] 70 65 72 Figure 4.20. I-V characteristics of the Fe3O4 film on SiO2 for temperature 70 K and 65 K.
73 CHAPTER FIVE SUMMARY, CONCLUSIONS AND FUTURE PLANS We were able to successfully grow crystalline Fe 3 O 4 on four different substrates namely, MgO, SiO 2 high resistive Si (resistivity >50 cm) and low resistivity Si (< 0.005 cm) with good iron to oxygen stoichiome try. All the substrates were (100) oriented. A Pulsed laser deposition (PLD) system was used to grow the thin films and proved to be successful. The substrates were kept at 350 C temperature and the deposition process was carried out at a pressure lower than 5 x 10 -7 Torr. Depositions were made at the same time for all four substr ates at a time in order to do a comparative study. The substrate to target distance was ke pt at 6 cm in order to maximize coverage area while maintaining reasonable growth (~0. 1 per laser pulse). Large target-substrate distance provides the ablation plume to be wider when expanding away from the target. Our intention was to deposit films with the same thickness on all th e substrates; however, we were unable to get the same thickness prof ile due to the limitation in coverage area. Films deposited on MgO and SiO 2 had thickness of 1800 while films on Si substrates had average thickness of 1100 This deviation can be caused by two factors. Growth thickness decrease away from the center of the plume is a known fact. The other reason is that the plume axis was pointing slightly off from the center of substrates. The PLD is a
74 known technique for formation of films with stoichiometry be ing the same from source to target. Even with some limitation, PLD is a good technique for making magnetite films as the magnetite properties crucially depend on st oichiometry. The short comings of small area coverage of the film, inherent to PL D can be overcome by introducing a rotational and/or translational motion of the substr ate as we discussed in Chapter 2. X-Ray diffraction (XRD) analysis on all of the films revealed peaks consistent with the crystalline phase of Fe 3 O 4 The target has (311) orie ntation and it can be seen that the highest peak is 35.5 Fe 3 O 4 on MgO substrate had orient ation in (100) plane and the other three films had orie ntation in the (111) plane. These results show that orientation of films is strongly influenced by substrates. Films grown on amorphous SiO 2 showed a strong peak of Fe 2 Si. This suggests a chem ical reaction between Fe 3 O 4 and SiO 2 layers at the interface. One has to ach ieve better control of the interface when considering integration of th e film with semiconductors. Thickness measurements were done usi ng a profilometer and it was found that the films on MgO and SiO 2 have 1800 and the films on Si substrates were ~ 1100 thick. Scanning electron microscopy (SEM) showed evidence for epitaxial structure of the films on MgO substrate. Films on SiO 2 and Si revealed crystallites with a preferred orientation. Physical Property Measurement System ( PPMS) was used to ca rry out all of the transport and magnetic measurements, except for the I-V characteristics. Temperature dependent resistivity serves as a quality factor in determination of quality of Fe 3 O 4 First the resistance measurements were carried ou t on samples and our studies revealed their dependence on substrates. The films on MgO, SiO 2 Si (resistivity >50 cm) showed the Verwey transition at 118 K, 124 K and 118 K respectively. However, the Si (<0.005
75 cm) substrate showed a resistance decrease instead of increasing around the transition temperature. We believe this is due to the substrate itself providing a channel for conduction. This may be utilized in device fabr ication. For example, it may be attractive for devices which need good magnetic proper ties and also low power consumption. We calculated resistivity in order to see whether our results agreed with values reported in literature. The resistivity va lues were found to be ~2 m cm for the films on all the substrates except high conducting Si, and values fall in the expected range. Magneto-resistance was measured for Fe 3 O 4 films deposited on different substrates. A peak was observed in MR at th e Verwey transition temperature. The peak in MR is also consistent with other reports. The film on SiO 2 shows a sharp peak in MR at T V with no increase at low temperature whereas Fe 3 O 4 on MgO shows a small peak in MR close to T v and MR increases upon further lowering of the temperature. The magnetization verses applied magnetic field (M-H) loops were measured on all films. Magnetic parameters such as coer civity, remanent and saturation magnetization values were obtained for all the films and we re in good agreement with literature values. We observed that the coercivity increases with decreasing temp erature. Large increase in coercivity can be due to the crystal struct ure changing from cubic to orthorhombic during cooling through T v (Sangeeta, 2001). J. J. Sen et al have observed that the coercivity and the loop shift both decrease with the increase of the temperature and the latter vanishes at about 200K. It has been ascribed to the decrease of the pinning fiel d with the increasing of the temperature and is consistent with the exchange bias idea (J.J. Shen, 2006). Also the magnetization as a function of temperature (M-T) were measured under high applied field (5000 Oe) and low applied field (300 Oe) the Verwey transition around
76 120 K was observed. Also, films on MgO have higher difference of magnetization at the transition temperature. Both tr ansitions of R-T and M-T are strong indicators of good stoichiometry and high crystall ine phase of the thin film. MgO appears to be the best substrate for obtaining sharp Ve rwey transition. Moreover the I-V curves we obtained for the films on SiO 2 substrate showed linear relationship from room temperature to T v and non liner relationship below T v We believe that our work reports one of the first systematic studies of the substrate influen ce in magnetite films. A manuscript based on the results presented in this thesis will be communicated for publication in the future. 5.1. Future Plans Our immediate future plan will be carrying out magneto-resistance measurements under a controlled environment with extreme care. Another pl an is to grow nano scale films and investigate the grain size influence on the properties. An in teresting plan would be to grow granular films embedded in metallic matrix such as Cu and Au. In this case, we could do spin polarization experiments at low temperature which is currently not possible because of the intervening Verwey tran sition at 120 K. Future plans also include exploring the magnetic anisotropy using RF transverse susceptibility that has being developed in our laboratory.
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