Spin-Orbit Torques and Magnetic Damping in Magnetic Multilayers

다국어 초록 (Multilingual Abstract)

An electric field induces spin-torque via spin-orbit coupling(SOC), which is spin-orbit torque(SOT), has been focus of research interest. Because of efficient manipulation of magnetization, it has great potential for advantage in device application including non-volatile magnetic memory and logic devices. Because of strong SOC, the heavy metal elements have large spin Hall effect(SHE) in which longitudinal charge current generates transverse spin current. Also SOC enhance magnetic damping via spin pumping effect (SP). In heavy metal(HM)/ferromagnet(FM)/Oxide structure, researches has been focus on HM and HM/FM interface and has been shown that the generated spin current and magnetic damping are dominated by HM and HM/FM interface. However, theoretical and experimental researches suggest that the FM/Oxide interface also has potential to play major role in determining magnetic anisotropy, magnetic damping and SOTs. we investigate both interfacial effects on magnetic properties. To clarify influences at the interfaces, we utilized ferromagnetic multilayers with insertion of additional ferromagnetic layers and different capping materials by using spin-torque ferromagnetic resonance(ST-FMR). In this dissertation, we experimentally investigate magnetic properties of magnet multilayers in presence of interfacial modification. Furthermore, theoretical researches suggest the existence of the orbital angular momentum flux from orbital texture, which has potential to transfer to spin flux. In last chapter of dissertation, we experimentally demonstrate theoretical prediction of the orbital Hall effect (OHE).

목차 (Table of Contents)

• Introduction 1
• 1 Background 4
• 1.1 Magnetization dynamics 4
• 1.2 Magnetic susceptibility 6
• 1.3 Magnetic damping 9
• 1.3.1 Intrinsic damping: breathing Fermi model 9
• 1.3.2 Spin pumping 10
• 1.4 Spin-orbit torque 11
• 1.4.1 Rashba effect 11
• 1.4.2 Spin Hall effect 12
• 1.4.3 Orbital Hall effect 12
• 1.5 Spin-torque ferromagnetic resonance 16
• 1.5.1 Ratio analysis 18
• 1.5.2 Voltage line-shape analysis 19
• 1.5.3 DC modulation analysis 21
• 1.6 X-ray ferromagnetic resonance 22
• 2 Effects of Gd-Oxide interdiffusion on ferrimagnetic properties 26
• 2.1 Introduction 27
• 2.2 Samples and methods 29
• 2.3 Results and discussion 34
• 2.4 Summary 42
• 3 Independent manipulation of magnetic damping and spin-orbit torques with interfacial modification 43
• 3.1 Introduction 44
• 3.2 Samples and methods 47
• 3.3 Results and discussion 52
• 3.3.1 Effect of FM stacking order 52
• 3.3.2 Contribution of FM
• MgO interface on magnetic damping 58
• 3.4 Summary 63
• 4 Magnetic damping and SOTs in presence of anti-ferromagnetic oxide interface 64
• 4.1 Introduction 65
• 4.2 Samples and methods 68
• 4.3 Results and discussion 71
• 4.3.1 Interfacial magnetic oxide formation 71
• 4.3.2 Magnetic damping and surface magnetic anisotropy energy 74
• 4.3.3 Spin-Orbit torques 79
• 4.4 Summary 84
• 5 Experimental observation of orbital Hall effect 85
• 5.1 Introduction 86
• 5.2 Detection scheme of the orbital Hall effect 89
• 5.3 Experimental test of orbital torque 93
• 5.4 Other possible mechanisms of the abnormal DLT sign 98
• 6 Discussion 101