Optical Characterization of Ferromagnetic and Multiferroic Thin- Film Heterostructures
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W&M ScholarWorks Dissertations, Theses, and Masters Projects Theses, Dissertations, & Master Projects 2015 Optical characterization of ferromagnetic and multiferroic thin- film heterostructures Xin Ma College of William & Mary - Arts & Sciences Follow this and additional works at: https://scholarworks.wm.edu/etd Part of the Artificial Intelligence and Robotics Commons Recommended Citation Ma, Xin, "Optical characterization of ferromagnetic and multiferroic thin-film heterostructures" (2015). Dissertations, Theses, and Masters Projects. Paper 1539623372. https://dx.doi.org/doi:10.21220/s2-66yg-6612 This Dissertation is brought to you for free and open access by the Theses, Dissertations, & Master Projects at W&M ScholarWorks. It has been accepted for inclusion in Dissertations, Theses, and Masters Projects by an authorized administrator of W&M ScholarWorks. For more information, please contact [email protected]. Optical Characterization of Ferromagnetic and Multiferroic Thin-Film Heterostructures Xin Ma Lingbi, Suzhou, Anhui, P. R. China Bachelor of Science, the University of Science and Technology of China, 2009 A Dissertation presented to the Graduate Faculty of the College of William and Mary in Candidacy for the Degree of Doctor of Philosophy Department of Applied Science The College of William and Mary January, 2015 APPROVAL PAGE This Dissertation is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy 10 I ' I o- Xin Ma Approved by the Committee, November, 2014 Committee Cnair Professor Gunter Luepke, Applied Science The College of William and Mary Professor Michael Kelley, Applied Science The College of William and Mary Professor Mark Hinders, Applied Science The College of William and Mary Professor Mumtaz Qazilbash, Physics The College of William and Mary ABSTRACT This thesis presents optical characterization of the static and dynamic magnetic interactions in ferromagnetic and multiferroic heterostructures with time-resolved and interface-specific optical techniques. The focus of the thesis is on elucidating the underlying physics of key physical parameters and novel approaches, crucial to the performance of magnetic recording and spintronic devices. First, time-resolved magneto-optical Kerr effect (TRMOKE) is applied to investigate the spin dynamics in L1o ordered FePt thin films, where perpendicular magnetic anisotropy Ku and intrinsic Gilbert damping ar 0 are determined. Furthermore, the quadratic dependence of Ku and a0 on spin- orbit coupling strength f is demonstrated, where f is continuously controlled through chemical substitution of Pt with Pd element. In addition, a linear correlation between cro and electron scattering rate 1 /re is experimentally observed through modulating the anti-site disorderc in the L1 0 ordered structure. The results elucidate the basic physics of magnetic anisotropy and Gilbert damping, and facilitate the design and fabrication of new magnetic alloys with large perpendicular magnetic anisotropy and tailored damping properties. Second, ultrafast excitation of coherent spin precession is demonstrated in Fe/CoO heterostructures and Lao.6 7 Cao.3 3 Mn0 3 thin films using TRMOKE technique. In the Fe/CoO thin films, Instant non-thermal ferromagnet (FM) - antiferromagnet (AFM) exchange torque on Fe magnetization through ultrafast photo-excited charge transfer possesses in the CoO layer is experimentally demonstrated at room temperature. The efficiency of spin precession excitation is significantly higher and the recovery is notably faster than the demagnetization procedure. In the Lao.67Cao.33Mn03 thin films, pronounced spin precessions are observed in a geometry with negligible canting of the magnetization, indicating that the transient exchange field is generated by the emergent AFM interactions due to charge transfer and modification of the kinetic energy of eg electrons under optical excitation. The results will help promoting the development of novel device concepts for ultrafast spin manipulation. Last, the interfacial spin state of the multiferroic heterostructure PbZro.5 2 Tio.4 8 O 3/Lao.6 7 Sro.3 3 MnO3 and its dependence on ferroelectric polarization is investigated with interface specific magnetization induced second harmonic generation (MSHG). The spin alignment of Mn ions in the first unit cell layer at the heterointerface can be tuned from FM to AFM exchange coupled, while the bulk magnetization remains unchanged as probed with MOKE. The discovery provides new insights into the basic physics of interfacial magneto-electric (ME) coupling. TABLE OF CONTENTS Acknowledgements iii Dedication iv List of Tables v List of Figures vi Chapter 1. Introduction 1.1 Magnetic recording and spintronics 1 1.2 Magnetization switching and dynamics 3 1.3 Interface magnetization 6 1.4 Scope of dissertation 9 Chapter 2. Experimental techniques 2.1 Magneto-optical Kerr effect 12 2.2 Time-resolved Magneto-optical Kerr effect 17 2.3 Magnetization induced second harmonic generation 26 Chapter 3. Time resolved studies of spin dynamics in FePt thin flims 3.1 Introduction 34 3.2 Samples and characterizations 36 3.3 Magnetic anisotropy and damping in FePtxPd(i-x) thin films 43 3.4 Magnetic anisotropy and damping in FePtthin films with different chemical order 55 Chapter 4. Time resolved spin precession in Fe/CoO heterostructure 4.1 Introduction 67 i 4.2 Samples and characterizations 69 4.3 Magnetic anisotropy 72 4.4 Ultrafast spin exchange torque via photo excited charge transfer processes 82 Chapter 5. Spin precession excitation in manganite thin films 5.1 Introduction 100 5.2 Samples and experiments 102 5.3 Coherent Spin Precession via Photoinduced Anti-ferromagnetic Interactions in Lao. 6 7 Cao.3 3 Mn0 3 103 Chapter 6 . Magneto-electric coupling at PbZr 0 5 2 Ti 0 .4 8 O3 / Lao 6 7 Sr0.3 3 MnO3 interface 6.1 Introduction 117 6.2 Samples and experiments 119 6.3 Charge control of antiferromagnetism at PbZr0 52Ti 0 4 8 O 3/ La0.67Sro.33Mn03 interface 124 Chapter 7. Conclusions 136 Bibliography 139 Vita 156 ii ACKNOWLEDGEMENTS I am truly grateful to everyone who has directly or indirectly supported and helped me during the completion of this dissertation. First and foremost, I would like to thank my advisor, Professor Gunter Luepke, for his guidance, encouragement and mentoring throughout my doctoral study at the College of William and Mary. He has always provided enthusiasm and perspective for the research process. This dissertation would not have been possible without his support and guidance. I am particularly grateful to Professor Haibin Zhao for guiding my experiment in the lab and fruitful scientific discussions. I truly appreciate his sharing of academic experience with me as a friend. I owe special thank to Professor Shiming Zhou and his group members - Pan He and Li Ma at Tongji University in Shanghai, China. I am indebted to them for providing us state-of-art FePt thin films, structure characterizations and inspired visions. I truly appreciate Professor Yizheng Wu and his group members- Qian Li and Jie Zhu at Fudan University in Shanghai for their support and providing us high-quality Fe/CoO thin films. I am similarly grateful to Professor Ram Katiyar and Dr. Ashok Kumar for providing us high-quality multiferroic heterostructures and characterization results. I also appreciate Professor James Scott for helpful suggestions. I am indebted to Professor Qi Li from Penn State University, for her generosity in providing us high-quality manganite films and sharing academic experience. My sincere gratefulness goes to my lab mates: Fan Fang, Wei Zheng, Haowei Zhai. I appreciate all their friendships and their collective encouragement to finish this dissertation. I have enjoyed every moment that we have worked together. I would also like to express my appreciation to my doctoral dissertation committee members: Professor Michael Kelley and Professor Mark Hinders of Applied Science, Professor Mumtaz Qazilbash of Physics for their comments and suggestions during the course of completing this dissertation. Finally, I wish to express my sincere gratitude to my parents - Renmi Ma and Ling Huang, and my wife - Yufei Ding. Their constant love, encouragement and support have always been the source of my strength and motivation. Anything I have been able to accomplish is a tribute to them. This dissertation is dedicated to my family. LIST OF TABLES 2.1 Fresnel coefficients, where = cosO, 6 the light incident angle, a2 = J l - sin20 /n 2, QP,QT,QLthe polar, transverse, and longitudinal projections of the Voigt vector Q, respectively. 2.2 Nonzero tensor elements of the nonlinear susceptibility tensor * (2) for the (0 0 1) cubic surface, the magnetization parallel to the x-axis (longitudinal configuration), y-axis (transversal configuration) and parallel to the z-axis (polar configuration). * (+) and denote the even and odd elements with magnetization. Other non-zero elements with mixed polarization of s and p are not included. For simplicity of notation the tensor components are indicated by their indices only 3.1 Landau g factors and Ku derived from TRMOKE measurements for different doping level x at 20K and 200K. LIST OF FIGURES 1.1 Schematics of magnetic recording arrays with traditional writing mechanism. 2 1.2 Schematic of precessing magnetization. 3 1.3 Relative orientations of the atomic moments in the AFM and FM are shown schematically, illustrating the lateral offset in the magnetization curve. 7 2.1 Schematics of longitudinal, transverse and polar MOKE probing geometry, where POS is the plane of sample (x-y plane), and POI is the plane of incident (y-z plane). 15 2.2 Geometry of TRMOKE measurements. 17 2.3 Stages of the excitation process: (a) M aligns along the equilibrium direction H eff \ (b) new equilibrium direction H'eff when pump pulse “heat up” the sample; (c) M precesses around H'eff (d) heat diffused and M processes around H eff. 19 2.4 TRMOKE result from L10 FePt thin film with growth temperature 550 °C at magnetic field 3.5T. 20 2.5 Schematic of TRMOKE measurement geometry for L1 o FePt thin films.