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Optical Properties of Thin-Film High-Temperature Magnetic Ferrites

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Optical Properties of Thin-Film High-Temperature Magnetic Ferrites University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Doctoral Dissertations Graduate School 5-2018 Optical Properties of Thin-Film High-Temperature Magnetic Ferrites Brian Scott Holinsworth University of Tennessee, [email protected] Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss Recommended Citation Holinsworth, Brian Scott, "Optical Properties of Thin-Film High-Temperature Magnetic Ferrites. " PhD diss., University of Tennessee, 2018. https://trace.tennessee.edu/utk_graddiss/4864 This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Doctoral Dissertations by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a dissertation written by Brian Scott Holinsworth entitled "Optical Properties of Thin-Film High-Temperature Magnetic Ferrites." I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Doctor of Philosophy, with a major in Chemistry. Janice Musfeldt, Major Professor We have read this dissertation and recommend its acceptance: Charles S. Feigerle, Veerle Keppens, Ziling Xue Accepted for the Council: Dixie L. Thompson Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) Optical Properties of Thin-Film High-Temperature Magnetic Ferrites A Dissertation Presented for the Doctor of Philosophy Degree The University of Tennessee, Knoxville Brian Scott Holinsworth May 2018 Acknowledgments First and foremost I wish to thank my advisor, Professor Janice L. Musfeldt, for her support, insight, guidance, and encouragement at all times. Thank you for introducing me to the wonderful world of spectroscopy. Your enthusiasm and drive is always an inspiration to me. I am grateful to Professors Jon P. Camden, Robert Compton, Verlee Keppens, Charles Feigerle, and Ziling Xue for serving on my committee, as well as for all of their interest and help. Of course the first two are no longer employed at the University of Tennessee, but I am grateful for their assistance in the early stages of this endeavor. I wish to acknowledge all the members of our research group: Tatiana Brinzari, Qi Sun, Peng Chen, Ken O’Neal, Dipanjan Mazumdar, Michael Yokosuk, Amanda Clune, Laura Casto, Judy Cherian, Amal al’Wahish, Nathan Harms, Kendall Hughey, Kevin Smith, Henok Mikre, Lakenzie Crawford, Brandon Chapman, and Joshua Bleu Wright for their help, encouragement, and collaboration. I am grateful to all the fac- ulty and staff of the Chemistry Department who shared their knowledge with me and helped in variety of ways during my graduate program. ii I would also like to thank Stephen A. McGill, Darrell G. Schlom, Hunter Sims, Arun Gupta, William H. Butler, Sanjoy Sarker, Mehmet K. Yurtisigi, Julia Mundy, Dipanjan Mazumdar, Charles Brooks, Hena Das, and Craig J. Fennie for their col- laboration and inspiring conversations during these years. Without these brilliant scientists giving their input the work would not have been half of what it became. I am also grateful for those near and far that inspired me through their many conver- sations that taught me so many valuable things. One such example of an inspiring conversation came from a neighboring lab; Matthew Bailey and I would start talking in the hall from time to time. We could talk for hours and the topics could cover the wide spectrum of existence. We would even discuss the scientific issues we were running into and make suggestions or give each other support as the conversation necessitated. I thank my parents for always supporting and believing in me, they made me a special kind of crazy. Last but not least, I thank my wife Lakenzie and our children for their support and encouragement throughout my study. My wife has helped me to expand my understanding of life outside of the physical sciences greatly and thus has enriched my passion for life itself. She has also helped me to see the value of what I am doing more deeply. Our children, Elian and Sebastian, have brought a new form of inspiration. They have helped me to realize what is truly important in life, working hard but also enjoying the here and now. iii Abstract Strongly-correlated electron materials reveal rich physics and exotic cross-coupled electronic and magnetic properties, with important fields results e.g. supercon- ductivity and multiferroics. This is because of the competing interaction between charge, structure, and magnetism in the materials. In this dissertation I present a spectroscopic investigation of several model complex iron oxides under external stimuli of magnetic field, electric field, and temperature. The compounds of inter- est include NiFe2O4 [nickel ferrite], CoFe2O4 [cobalt ferrite], h-LuFeO3 [hexagonal lutethium ferrite], and LuFe2O4 [lutethium ferrite]. These materials are attractive systems in the fields of multiferroics and high-temperature magnets for investigating optical band gap tunability, lattice and charge dynamics, spin-charge coupling, and optically-enhanced magnetoresistive effect. In these works, we have combined optical spectroscopy, magnetic circular dichroism (MCD), and (magneto-)photoconductivity, with high-quality thin-film growth, and first-principles calculations to reveal the na- ture of the optical excitations within these strongly correlated iron oxides. NiFe2O4 we found that optical excitations offer the opportunity for producing spin-polarized current. In CoFe2O4 we showed that the band gap is robust with temperatures up iv to 800 K. We found that the direct-gap excitation of LuFe2O4 is highly sensitive the strain induced by epitaxial growth. v Table of Contents 1 Introduction to Ferrites1 Using external stimuli to control material properties ..8 1.1 Overview of Dissertation......................... 11 2 Literature Survey of Magnetic Oxides 21 2.1 High T magnetic oxides, towards spintronics and multiferroicity... 21 2.1.1 High T magnetic and spintronic materials........... 24 2.1.2 Mott-Hubbard transport model for insulators......... 27 2.1.3 Spintronics............................ 29 Introduction to Spintronics ................ 29 2.1.4 Subset of strongly-correlated electron materials: multiferroics. 33 Lone-pair mechanism ................... 40 Geometric driven ferroelectricity ............. 41 Charge ordering ...................... 41 Spin-driven mechanism .................. 42 Composite multiferroics .................. 42 vi 2.2 High Temperature magnetic spinel oxides: NiFe2O4 and CoFe2O4 .. 44 2.2.1 Polder Susceptibility....................... 44 2.2.1.1 Magnetization Dynamics................ 45 2.2.1.2 Equation of Magnetization Motion.......... 45 Solution for magnetization motion equation ....... 46 2.2.1.3 Damping of the Landau-Lifshitz Equation...... 49 2.2.2 Magnetic Properties for Transition Metal Oxides (Goodenough- Kanamori-Anderson Rules)................... 51 2.2.3 Spinel crystal structure...................... 52 2.2.4 Magnetism of NiFe2O4 and CoFe2O4 .............. 53 2.2.5 Electronic Structure from First Principles........... 54 2.2.6 Spin filtering........................... 54 2.2.7 Contemporary discoveries.................... 57 2.3 Multiferroic h-LuFeO3 .......................... 59 2.4 LuFe2O4 .................................. 60 3 Experimental and Theoretical Techniques 77 3.1 Optical Theory.............................. 77 3.1.1 Maxwell’s Equations....................... 80 3.1.2 Polarized light........................... 84 3.1.3 Plane waves in a poorly conducting (rightly insulating) and lossy dielectric medium...................... 90 3.1.4 Beer-Lambert law........................ 92 3.1.5 Glover-Tinkham Analysis.................... 94 vii 3.1.6 Kramers-Kronig Analysis and Sum Rules............ 98 3.2 Transport................................. 101 3.2.1 Transport analysis........................ 106 3.2.2 Magnetotransport......................... 107 3.3 Spectrometers............................... 113 3.3.1 Perkin-Elmer λ-900 Spectrometer................ 113 3.3.2 Reflectance Stages........................ 115 Anisotropic materials and polarizers ........... 115 3.3.3 Magneto-optical properties................... 118 MCD data analysis and interpretation .......... 141 History ........................... 145 Thin-film MCD response ................. 146 3.3.4 Photoconductivity setup..................... 150 3.4 Spectra under Extreme Conditions: Low/High Temperature and High- Field Measurements............................ 154 3.4.1 Low-Temperature Techniques.................. 154 3.4.2 Experimental Set-up at the NHMFL.............. 158 3.4.3 Experimental high voltage field................. 161 3.5 Materials of Interest: Measurements and Calculations......... 162 3.5.1 NiFe2O4 .............................. 162 Thin film growth ..................... 163 Transmittance and reflectance measurements ...... 163 Band gap determination ................. 164 viii Field dependent measurements - magnetic circular dichro- ism ...................... 164 Photoconductivity measurements ............. 165 3.5.2 CoFe2O4 .............................. 166 Thin film growth ..................... 167 Transmittance and reflectance measurements ...... 168 Band gap determination ................. 168 Magnetic field dependent
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