Title Page
ABSTRACT
국문 초록
Contents
CHAPTER 1. Introduction 27
CHAPTER 2. Theoretical Background 30
2.1. III-V Diluted Magnetic Semiconductors 30
2.2. Magnetotransport Phenomena 34
2.2.1. Magnetoresistance 34
2.2.2. Hall Effect 36
2.3. Spin Phenomena 39
2.3.1. Spin-orbit Coupling (SOC) 39
2.3.2. Spin-orbit Field (SOF) 43
2.3.3. Spin-orbit Torque (SOT) 45
CHAPTER 3. Sample Preparation and Experimental Setup 47
3.1. Sample Preparation 47
3.1.1. Molecular Beam Epitaxy (MBE) 47
3.2. Experimental Installation and Setup 56
3.2.1. Electrical Measurement 57
3.2.2. Closed-cycle Refrigerator (CCR) Systems 59
CHAPTER 4. Current-induced Magnetization Switching in GaMnAsP Single Layer 61
4.1. Sample Preparation and Experimental Configuration 61
4.1.1. Sample Preparation 61
4.1.2. Experimental Configurations 62
4.1.3. Magnetotransport Properties of GaMnAsP Film 62
4.2. SOT Magnetization Switching 66
4.2.1. SOT Switching with Current along the [1-10] & [110] Crystalline Directions 69
4.2.2. SOT Switching with Current along the [100] & [010] Crystalline Directions 75
4.3. Quantifying SOFs by Angle Scan 88
4.3.1. [1-10] & [110] Crystalline Directions 88
4.3.2. [100] & [010] Crystalline Directions 104
4.4. Summary 117
CHAPTER 5. SOFs in GaMnAsP Single Layers with Different Thicknesses 119
5.1. Experiments 119
5.1.1. Sample Preparation 119
5.1.2. Crystal Structure Characterization 119
5.1.3. Magnetotransport Properties of GaMnAsP Films 120
5.2. SOT Magnetization Switching 125
5.2.1. Current Scan along the 〈110〉 Crystal Directions 125
5.2.2. Critical Current Density 127
5.3. Quantifying SOFs 133
5.3.1. Angle Scan Measurement 133
5.4. Film Thickness Dependence of SOT Efficiency 136
5.5. Summary 139
CHAPTER 6. Field-free SOT Magnetization Switching 140
6.1. Bias Field Dependence of SOT Magnetization Switching in the Single Layer 140
6.2. Possible Origin of Internal Field 147
6.2.1. Exchange Coupling to MnO Antiferromagnet at the Surface 147
6.2.2. Lateral Symmetry Breaking by the Incorporation of P 151
6.3. Summary 158
CHAPTER 7. Conclusions 159
REFERENCES 160
Table 3.1. Photolithography for etching process with recipe. 52
Table 3.2. Photolithography for electrode deposition process with recipe. 55
Table 4.1. Experimental configurations for SOT measurements. 67
Table 5.1. Film thickness dependence of the effective field and SOT efficiency. 138
Figure 2.1. Magnetic anisotropy of GaMnAsP system induced by strain. (a) Compressive strain. (b) Zero strain. (c) Tensile strain. 33
Figure 2.2. Schematic illustrations of (a) Ordinary Hall effect and (b) anomalous Hall effect. 37
Figure 2.3. Schematic diagram of the configuration of the magnetization direction and Hall resistance. 38
Figure 2.4. Schematic illustrations of the spin-orbit coupling. (a) The rest frame of the nucleus. (b) The rest frame of the electron. 40
Figure 2.5. Inversion asymmetry of the GaMnAsP structure. (a) Bulk inversion asymmetry (BIA) of the zinc-blende crystal structure. (b) Structural inversion... 42
Figure 2.6. (a) The Dresselhaus-type and (b) Rashba-type spin-orbit fields. The black arrows indicate the crystal directions. The red and blue arrows indicate the... 44
Figure 2.7. Schematic diagram of torques exerting on the magnetization. 46
Figure 3.1. Schematic diagram of the molecular beam epitaxy (MBE) including reflection high-energy electron diffraction (RHEED) system. 48
Figure 3.2. Flow chart of Hall Device fabrication. 50
Figure 3.3. Etch rate of GaAs films by Ar ion beam milling with 30° incident angles. 54
Figure 3.4. Optical microscope images of Hall device fabrication. (a) After development. (b) After dry etching. (c) After photoresist cleaning. (d) After electrode... 56
Figure 3.5. Schematic diagram of experimental setup for magnetotransport measurement consisting of helium cryostat and electromagnet system. 57
Figure 3.6. Schematic diagram of pulsed current followed by DC measurement. 59
Figure 3.7. Images of (a) the He cryostat and (b) the sample holder inside the cryostat. 60
Figure 4.1. (a) Optical images of Hall devices with a width of 10 um along the 〈110〉 and 〈100〉 crystal dictions. (b) Experimental configurations for the... 63
Figure 4.2. Magnetotransport properties of the 12.5 nm GaMnAsP film. (a) Temperature dependence of resistance (upper panel) and HR (lower panel)... 65
Figure 4.3. Experimental configurations corresponding to the Table 4.1. Each configuration referred to as configuration #1 through #8. 68
Figure 4.4. The SOT magnetization switching obtained by applying a current along the [1-10] (represented by green symbols) and [110] (represented by... 70
Figure 4.5. (a)-(e) Process of SOT magnetization switching achieved by applying a current along the [1-10] crystal direction under a positive in-plane bias field Hext...[이미지참조] 74
Figure 4.6. The SOT magnetization switching obtained by applying a current along the [100] (represented by blue symbols) and the [010] (represented by... 77
Figure 4.7. (a)-(e) Process of SOT magnetization switching achieved by applying a current along the [100] crystal direction under a positive in-plane bias field Hext...[이미지참조] 79
Figure 4.8. The SOT magnetization switching obtained by applying a current along the [100] (open blue symbols) and [010] (open orange symbols) crystal... 81
Figure 4.9. (a)-(e) Process of SOT magnetization switching achieved by applying a current along the [100] crystal direction under a positive in-plane bias field 500... 84
Figure 4.10. Plot of SOT critical current density (Jc) as a function of the external bias field (Hext). (a) Jc values obtained from SOT magnetization switching...[이미지참조] 87
Figure 4.11. Angle scan results obtained current along the [1-10] in configuration #1. Hysteresis shifts depend on current polarities. 89
Figure 4.12. Schematic diagrams of angle scan experiments with a configuration of SOTs for J∥[1-10], Hext ⊥+y (Configuration #1). Diagrams (a)-(d) represent...[이미지참조] 91
Figure 4.13. Angle scan results obtained with current along the [1-10] in configuration #1. With increasing the magnitude of the external field, the amount... 94
Figure 4.14. Hysteresis shift of the angle scan for an applied current of 2.5 mA along the [1-10] crystal direction during rotation of the external field in the plane... 95
Figure 4.15. Angle scan results obtained current along the [110] in configuration #3. Hysteresis shift depending on current polarities. 98
Figure 4.16. Schematic diagrams of angle scan experiments with a configuration of SOTs for J∥[110], Hext ⊥+y (Configuration #3). Diagrams (a)-(d) represent...[이미지참조] 99
Figure 4.17. Angle scan results obtained current along the [110] in configuration #3. With increasing the magnitude of the external field, the amount of hysteresis... 101
Figure 4.18. Hysteresis shift of the angle scan for an applied current of 2.5 mA along the [110] crystal direction during rotation of the external field in the plane... 103
Figure 4.19. Angle scan results obtained current along the [100] in configuration #5. Hysteresis shifts depend on current polarities. 105
Figure 4.20. Schematic diagrams of angle scan experiments with a configuration of SOTs for J∥[100], Hext ⊥+y (Configuration #5). Diagrams (a)-(d) represent...[이미지참조] 107
Figure 4.21. Angle scan results obtained current along the [100] in configuration #5. With increasing the magnitude of the external field, the amount of hysteresis... 109
Figure 4.22. The fitting results (solid line) of the hysteresis shift of the angle scan are shown for an applied current of 2.5 mA along the [100] crystal direction, while the... 110
Figure 4.23. Angle scan results obtained current along the [100] in configuration #6. Hysteresis shifts depends on current polarities. 112
Figure 4.24. Schematic diagrams of angle scan experiments with a configuration of SOTs for J∥[100], Hext ⊥+x (Configuration #6). Diagrams (a)-(d) represent...[이미지참조] 113
Figure 4.25. Angle scan results obtained current along the [100] in configuration #6. With increasing the magnitude of the external field, the amount of hysteresis... 115
Figure 4.26. The fitting results (solid line) of the hysteresis shift of the angle scan are shown for an applied current of 2.5 mA along the [100] crystal direction, while... 116
Figure 5.1. (Left panel) The crystalline quality of the GaMnAsP films examined by using HAADF imaging in TEM. The thicknesses of the films (a) 7 nm, (c) 12.5... 121
Figure 5.2. Temperature dependence of the resistance and the Hall resistance (HR) for GaMnAsP films with thicknesses of (a) 7 nm (Tc=33 K), (b) 12.5 nm...[이미지참조] 122
Figure 5.3. (a) Field scan hysteresis obtained at 2.5 K representing magnetization transitions for GaMnAsP films. (b) Dependence of coercive field on the film thickness. 124
Figure 5.4. SOT magnetization switching obtained under the bias field of Heff=+300 Oe along x-axis and with the current along the [1-10] (green symbols) and...[이미지참조] 126
Figure 5.5. Dependence of critical current density (Jc) on Hext for varying film thicknesses of (a) 7 nm, (b) 12.5 nm, (c) 18 nm, and (d) 25 nm, with green and...[이미지참조] 128
Figure 5.6. EDS line profile results. Sky blue, magenta, light green lines represent the Mn, P, and O elements, respectively. Oxide and GaMnAs layers are shaded... 131
Figure 5.7. SOT magnetization switching process along the (a) [1-10] and (b) [110] crystal directions, considering the Oersted field. Oersted torque assisting or... 132
Figure 5.8. Angle scan of HR for sample with four different thickness. The left and right columns represent data obtained with current along the [1-10] and [110]... 134
Figure 5.9. Fitting results of hysteresis loop shifts for GaMnAsP films of varying thicknesses; (a) 7 nm, (b) 12.5 nm, (c) 18 nm, and (d) 25 nm. 135
Figure 5.10. Dependence of SOT efficiency on the film thickness. (a) SOT efficiency for the [1-10] and [110] crystal directions decreases with increasing... 137
Figure 6.1. SOT-induced magnetization switching in a single layer GaMnAsP film along the [110] crystal direction with up (a) and down (b) magnetization... 141
Figure 6.2. Percentage of HR amplitude change as a function of the in-plane bias field for the [1-10] crystal direction for up (a) and down (b) magnetization... 143
Figure 6.3. Current scan along the [1-10] crystal direction with up (a) and down (b) magnetization initializations. For +100 Oe bias field (black symbols), the SOT... 145
Figure 6.4. Percentage of HR amplitude change as a function of the in-plane bias field for the [1-10] crystal direction. For large positive and negative in-plane bias... 146
Figure 6.5. TEM and EDS images of 12.5 nm GaMnAsP film. (a) Thickness of MnO layer is marked with white arrow. Color mapping of EDS results shows the... 148
Figure 6.6. Schematic diagram of antiferromagnetic MnO layer couples to the magnetization. 149
Figure 6.7. Zinc-blend crystal structure of GaAs (a) and GaMnAsP (b) systems. (a) Mirror symmetry in GaAs with respect to 〈110〉 crystal directions (black... 152
Figure 6.8. Field-free magnetization switching with current applied along [110] crystal direction. (a) Current scan results with solid and open magenta symbols... 155
Figure 6.9. Field-free magnetization switching with current applied along [1-10] crystal direction. (a) Current scan results with solid and open green symbols for... 157