MAGNETIC PROPERTIES OF NANO-COMPOSITE PARTICLES by XIA XU ALAN M. LANE COMMITTEE CHAIR YUPING BAO DAVID E. NIKLES JOHN W. VAN ZEE MARK L. WEAVER TAKAO SUZUKI A DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemical and Biological Engineering in the Graduate School of The University of Alabama TUSCALOOSA, ALABAMA 2015 Copyright Xia Xu 2015 ALL RIGHTS RESERVED ABSTRACT Chemical synthesis routes for hollow spherical BaFe O , hollow mesoporous spherical 12 19 BaFe O , worm-shape BaFe O and FeCo particles were developed. These structured parti- 12 19 12 19 cles have great potentials for the applications including magnetic recording medium, catalyst support, and energy storage. Magnetically exchange coupled hard/soft SrFe O /FeCo and 12 19 MnBi/FeCo composites were synthesized through a newly proposed process of magnetic self- assembly. These exchange coupled composites can be potentially used as rare-earth free perma- nent magnets. Hollow spherical BaFe O particles (shell thickness ~5 nm) were synthesized from eth- 12 19 ylene glycol assisted spray pyrolysis. Hollow mesoporous spherical BaFe O particles (shell 12 19 thickness ~100 nm) were synthesized from ethanol assisted spray pyrolysis, followed by alkaline ethylene glycol etching at 185 oC. An α-Fe O and BaCO nanoparticle mixture was synthesized 2 3 3 with reverse microemulsion, followed by annealing at 900 oC for 2 hours to get worm-shape BaFe O particles, which consisted of 3-7 stacked hexagonal plates. FeCo nanoparticles were 12 19 synthesized by reducing FeCl and CoCl in diphenyl ether with n-butyllithium at 200 oC in an 2 2 inert gas environment. The surfactant of oleic acid was used in the synthesis to make particles well dispersed in nonpolar solvents (such as hexane). SrFe O /FeCo core/shell particles were prepared through a magnetic self-assembly pro- 12 19 cess. The as-synthesized soft FeCo nanoparticles were magnetically attracted by hard SrFe O 12 19 particles, forming a SrFe O /FeCo core/shell structure. The magnetic self-assembly mechanism 12 19 ii was confirmed by applying alternating-current demagnetization to the core/shell particles, which resulted in a separation of SrFe O and FeCo particles. 12 19 MnBi/FeCo composites were synthesized, and the exchange coupling between MnBi and FeCo phases was demonstrated by smooth magnetic hysteresis loop of MnBi/FeCo composites. The thermal stability of MnBi/FeCo composites was investigated by annealing at 250 oC for 2 hours in N environment. The results showed that FeCo nanoparticles were sintered and agglom- 2 erated during the annealing, and exchange coupling between MnBi and FeCo was destroyed. Future work was proposed in three aspects: chemical synthesis of MnBi particles; decreas- ing the particle size of MnBi particles and maintaining their stability; improving the thermal sta- bility of MnBi/FeCo composites. iii DEDICATION This dissertation is dedicated to my family and friends who stood by me through this dissertation journey. iv LIST OF ABBREVIATIONS AND SYMBOLS BaM Barium ferrite EG Ethylene glycol TEM Transmitting electron microscopy SEM Scanning electron microscopy VSM Vibrating sample magnetometry XRD X-ray diffraction Mr Remanent magnetization Ms Saturation magnetization EDX Energy dispersive X-ray spectrum Hc Covercivity ΔM Delta M M Normalized demagnetization remanence d M Normalized isothermal magnetization remanence r H Applied magnetic field (BH) Maximum energy products max SAED Selected area electron diffraction EDTA Ethylene diamine tetra acetic acid PEG Polyethylene glycol RE Rare-earth TM Transition-metal v CTAB Cetyltrimethyl ammonium bromide T Curie temperature c H Intrinsic coercivity c M Magnetization of exchange coupled product ex H Covercivity of exchange coupled product ex K Anisotropy constant of hard phase h K Anisotropy constant of soft phase s f Volume fraction of hard phase h f Volume fraction of soft phase s M Magnetization of soft phase s H Covercivity of hard phase h H Coercivity of soft phase s M Magnetization of hard phase h n-BuLi n-Butyllithium BCC Body centered cubic AC Alternating current Oe Oersted, unit of magnetic field strength in Gaussian units MRI Magnetic resonance imaging LTP Low temperature phase vi ACKNOWLEDGEMENTS First of all, I would like to thank my advisor, Dr. Alan M. Lane, for bringing me to his group. His patient guidance and continuous encouragement make me produce new ideas and broaden my horizons. The numerous discussions with him help me solve research problems and walk to the end of this dissertation. His wisdom also helps me deal with puzzles in life. He is not only an advisor of my research, but also a friend and mentor of my life and future career. I would also like to thank all of my committee members, Dr. Yuping Bao, Dr. David E. Nikles, Dr. John W. Wan Zee, Dr. Mark L. Weaver and Dr. Takao Suzuki, for their inspiring questions and invaluable suggestions for my academic research and dissertation. Thanks extend to all members of Magnetic Materials and Devices Laboratory (MMDL): Dr. Yang-Ki Hong (director), Mr. Jaejin Lee, Mr. Jihoon Park, Mr. Woncheol Lee and Mr. Gat- lin LaRochello, for their help on my research. Especially, I would like to appreciate Dr. Yang-Ki Hong for his tremendous guidance on my research, and Mr. Jihoon Park who stands with me for both happy and tough time in past three years. I thank the members of Central Analytical Facility (CAF) of the University of Alabama: Mr. Johnny Goodwin, Mr. Rich Marten and Mr. Robot Holler, for their assistance on the training and usage of XRD, TEM and SEM. I am also deeply in debt to my family and friends, who give me tremendous love, encouragement and support. The list is endless and I am expressing my appreciation to all of those who helped me in any respect during my research in past four years. vii Lastly, I would like to thank funding agency of Advanced Research Projects Agency- Energy (ARPA-E), and the leadership of Pacific Northwest National Laboratory (PNNL) in this project. viii CONTENTS ABSTRACT .................................................................................................................................... ii DEDICATION ............................................................................................................................... iv LIST OF ABBREVIATIONS AND SYMBOLS ........................................................................... v ACKNOWLEDGEMENTS .......................................................................................................... vii LIST OF TABLES ....................................................................................................................... xiii LIST OF FIGURES ..................................................................................................................... xiv CHAPTER 1 INTRODUCTION .................................................................................................... 1 1.1. Classification of Magnetism .................................................................................................1 1.1.1. Diamagnetism ............................................................................................................2 1.1.2. Paramagnetism ...........................................................................................................2 1.1.3. Ferromagnetism .........................................................................................................3 1.1.4. Antiferromagnetism ...................................................................................................3 1.1.5. Ferrimagnetism ..........................................................................................................3 1.2. Ferromagnetic Materials .......................................................................................................3 1.2.1. Soft magnetic materials..............................................................................................4 1.2.2. Hard magnetic materials ............................................................................................5 1.3. Magnetic Exchange Coupling ...............................................................................................7 1.3.1. Theory of magnetic exchange coupling .....................................................................8 1.3.2. Experimental development of exchange coupling ...................................................11 1.3.3. Synthesis challenges of exchange coupled magnet .................................................14 1.4. References ...........................................................................................................................19 ix