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Fabrication and Characterization of Chemically Modified Multiferroic Bismuth Ferrite Thin Films

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Fabrication and Characterization of Chemically Modified Multiferroic Bismuth Ferrite Thin Films FABRICATION AND CHARACTERIZATION OF CHEMICALLY MODIFIED MULTIFERROIC BISMUTH FERRITE THIN FILMS YAN FENG (M. Sc) A THESIS SUBMITED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 Acknowledgements I would like to express my sincere gratitude to my supervisors, Prof. Lu Li, and Prof. Lai Man On from Department of Mechanical Engineering, National University of Singapore (NUS), and Prof. Zhu Tiejun from Department of Materials Science and Engineering, Zhejiang University (ZJU) for giving me the opportunity to work on this exciting project. I would especially like to thank all of them for their guidance and support throughout my PhD study at NUS. I benefited from their guidance in every aspect during my Ph.D research, such as the discussions we held and intellectual suggestions they made regarding my work. I am grateful for the insights and advice Prof. Zeng Kaiyang shared with me. His suggestions and guidance were a great help in performing piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM) on the microscopic ferroelectric properties as related to the experiments in Chapter 6. I would like to thank Prof. Rüdiger-A. Eichel from Institut für Physikalische Chemie, Albert-Ludwigs-Universität Freiburg (German) for his kind assistance and advice on the understanding of the magnetic phase transition and magnetoelectric coupling effect via electron paramagnetic response (EPR). It is a great honor for me to have opportunity to work in his group for four months. Also, I would like to thank Dr. Emre Erdem and Dr. Peter Jakes in this group for their kind advice and suggestions. I would like to thank Prof. I. M.Reaney from Department of Engineering Materials, University of Sheffield for his kind assistance the understanding of the microstructure of the investigated thin films via HRTEM. I I would also like to thank Dr. Zhang Zhen as a collaborator. All the ideas we shared and discussed have proven very useful. This project would not have gone so well without his contribution at the forefront of this research. I specially thank Dr. Wang Shijie for his help and advice at the beginning of my studies. I would like to express my gratitude for the help of my colleagues and collaborators: Dr. Xiahui, Mr. Wang Hailong, Mr. Ye Shukai, Mr. Xiao Pengfei, Miss. Zhu Jing, Mr. Song Bohang and other members in Prof. Lu’s research group. In addition, I would like to give my special thanks to the staff of Materials Science Laboratory, Department of Mechanical Engineering, and National University of Singapore. Finally, I want to thank my family for their understanding, encouragement, and endless love throughout my life. II Table of Contents Acknowledgements .......................................................................................................... I Table of Contents .......................................................................................................... III Abstract ........................................................................................................................ VII List of Tables ............................................................................................................. VIII List of Figures ............................................................................................................... IX List of Symbols and Abbreviations .......................................................................... XVII List of Publications .................................................................................................. XVIII Chapter 1 Introduction .................................................................................................... 1 1.1 Overview & Motivations. ...................................................................................... 1 1.2 Outline ................................................................................................................... 3 Chapter 2 Literature Review ........................................................................................... 4 2.1 Magnetoelectric effect and multiferroic materials ................................................ 4 2.1.1 Magnetoelectric effect[14] ............................................................................. 4 2.1.2 Multiferroics ................................................................................................... 6 2.1.3 Single phase multiferroics [3] ........................................................................ 8 2.1.4 Multiferroic composites ................................................................................ 10 2.2 Multiferroic BiFeO3 ............................................................................................ 10 2.2.1 Structure ....................................................................................................... 10 2.2.2 Ferroelectricity.............................................................................................. 12 2.2.3 Dielectric Properties ..................................................................................... 13 III 2.2.4 Magnetism .................................................................................................... 14 2.2.5 Magnetoelectric Coupling ............................................................................ 15 2.2.6 Domains and domain walls[46] .................................................................... 16 2.3 Device application ............................................................................................... 18 2.3.1 Data storage [1] ............................................................................................ 18 2.3.2 Optoelectronic devices ................................................................................. 19 2.4 Thin film deposition and characterization ........................................................... 21 2.4.1 Pulsed Laser Deposition (PLD) .................................................................... 21 2.4.2 X-Ray diffraction (XRD).............................................................................. 22 2.4.3 Atomic force microscopy (AFM) ................................................................. 22 2.4.4 Macroscopic Ferroelectric measurement ...................................................... 23 2.4.5 Dielectric measurement ................................................................................ 24 2.4.6 Piezoresponse force microscopy (PFM) ....................................................... 25 2.4.7 Switching Spectroscopy PFM (SS-PFM) ..................................................... 26 2.4.8 Kelvin probe force microscopy (KPFM) ...................................................... 27 2.4.9 Vibration sample magnetometer (VSM) ...................................................... 28 2.4.10 Magnetic force microscopy (MFM) ........................................................... 28 Chapter 3 Experimental Procedures.............................................................................. 30 3.1 Fabrication of Targets ......................................................................................... 30 3.1.1 Pure BiFeO3 target ........................................................................................ 30 3.1.2 Chemically Modified BiFeO3 targets ........................................................... 30 3.2 Thin film growth ................................................................................................. 31 IV 3.2.1. Substrate and target cleaning ....................................................................... 31 3.2.2 Film deposition ............................................................................................. 31 3.3 Thin film characterization ................................................................................... 32 Chapter 4 Pure BFO thin films ..................................................................................... 34 4.1 Introduction ......................................................................................................... 34 4.2 Epitaxial BiFeO3 thin films ................................................................................. 34 4.2.1 Structures of (001), (011) and (111) oriented BiFeO3 thin films ................. 34 4.2.2 Surface morphology ..................................................................................... 37 4.2.3 Macroscopic electrical properties of (001), (011) and (111) oriented BiFeO3 thin films ................................................................................................................ 37 4.2.4 Ferroelectric domain structure using PFM ................................................... 40 4.2.5 Magnetoelectric coupling of BiFeO3 thin films via PFM and MFM ........... 46 4.3 Polycrystalline BiFeO3 thin films ....................................................................... 48 4.3.1 BFO on Pt/TiO2/SiO2/Si substrate ................................................................ 48 4.3.2 Size effect on the piezoelectric response of BFO on Pt/TiO2/SiO2/Si substrate ................................................................................................................. 56 4.4 Effects of bottom electrode on switching behavior at nanoscale ........................ 61 4.4.1 Structure and multiferroic properties of BFO on LaNiO3/Si and SrRuO3/Si substrates ..............................................................................................................
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