Theoretical Study on Spin and Orbital Transport in Magnetic Systems

다국어 초록 (Multilingual Abstract)

This thesis deals with spin and orbital transport in magnetic systems and magnetic properties originating from the spin-orbit coupling. Since the spin and orbital transport have unique features depending on the magnetic systems, there are arising interest in field of spintronics such as the anomalous and spin Hall effects in various magnetic materials, i.e., ferromagnets [1-3], antiferromagnets [4-7], nonmagnets [8-10], and the orbital Hall effect which is the orbital analogue of spin Hall effect [11-13]. Moreover, the spin transport and related spin-transfer torque in magnetic textures such as domain walls [14-15] and magnetic skyrmions [16] also have been studied. At the interface of magnetic heterostructures, there is Rashba spin-orbit coupled transport such as interfacial spin current [18, 19], spin and orbital Rashba-Edelstein effects [19,20], and intrinsic spin swapping effect [21]. In this thesis, we focus on the spin transport, orbital transport, and interfacial magnetic properties in magnetic systems.
In chapter 1, we introduce the generation of spin and orbital currents such as spin Hall, orbital Hall, and spin swapping effects (section 1.2). In addition, we briefly introduce the spin-transfer torque and the spin-orbit torque (section 1.3). Finally, we show the numerical methods to calculate the spin and orbital related quantities (section 1.4).
In chapter 2, we theoretically demonstrate the spin swapping effect of band structure origin in centrosymmetric ferromagnets. We show that the magnitude of intrinsic spin swapping conductivity is large at the band anticrossing which is a crossing point of different spin and orbital character bands. Also, we confirm that the intrinsic spin swapping conductivity is comparable to spin Hall conductivity in transition metal ferromagnets using density functional theory.
In chapter 3, we numerically compute the spin-transfer torques for antiferromagnetic domain walls (DWs). We show that the spin-mistracking phenomenon, which results in a nonadiabatic torque, is remarkable for antiferromagnetic DWs. Furthermore, unlike for ferromagnetic DWs, we theoretically and numerically confirm that the dynamics of antiferromagnetic DWs is determined by the nondiabetic torque only.
In chapter 4, we study the Rashba spin-orbit coupled spin properties. These properties include the both equilibrium and nonequilibrium properties such as surface anisotropy, Dzyaloshinskii-Moriya interaction (DMI), and Rashba-Edelstein effect. We confirm that the equilibrium spin properties can be manipulated by the electron populations, whereas nonequilibrium spin density can be induced by the lateral inversion symmetry breaking.
We note that the most of results in this thesis, including the figures and schematics, have already been published in scientific journals. Results of chapter 2 and chapter 3 are reproduced from the [22, 23] with the permission of American Physical Society. Chapter 4 is reproduced from the [24, 25] with the permission of American Physical Society and [26] which are open access articles.

목차 (Table of Contents)

• 1. Theoretical background 1
• 1.1 Introduction 1
• 1.2 Spin and orbital current originated from the spin-orbit coupling 2
• 1.2.1 Spin and orbital Hall effects 2
• 1.2.2 Spin swapping effect 3
• 1.3 Spin-orbit torque and spin-transfer torque 5
• 1.4 Density functional theory and linear response theory 7
• 2. Spin swapping effect of band-structure origin in centrosymmetric ferromagnets 9
• 2.1 Introduction 10
• 2.2 Spin swapping effect due to orbital textures 12
• 2.3 Tight binding model calculation 15
• 2.4 First-principles calculations for Co, Fe, and Ni 17
• 2.5 Orbital swapping effect in ferromagnets 20
• 2.6 Discussions 21
• 3. Numerical computation of spin-transfer torques for antiferromagnetic domain walls 23
• 3.1 Introduction 24
• 3.2 Microscopic approach to compute spin-transfer torques 26
• 3.3 Effective spin-transfer torques acting on an antiferromagnetic domain wall 31
• 3.4 Results and discussions 36
• 3.5 Conclusion 42
• 4. Rashba spin-orbit coupled spin properties 43
• 4.1 Introduction 44
• 4.2 Correlation between the magnetic anisotropy energy and the Rashba spin-orbit coupling 45
• 4.3 Slater-Pauling behavior of interfacial magnetic properties 48
• 4.4 Theoretical and numerical demonstration of electric-field induced z-SOT 52
• Conclusion 55
• Supplementary materials 56
• Bibliography 66
• Curriculum Vitae 77