검색 상세

Electrical manipulation of the magnetization in ferromagnetic semiconductors

  • 이상엽 (대학원 물리학과 고체물리학전공)

초록/요약

This dissertation investigated the manipulation of magnetization by electrical current, which generates Joule heat as well as the effective magnetic field in ferromagnetic GaMnAs layer. This current manipulation of magnetization was utilized to obtain magnetic memory and logic functionality in the GaMnAs Hall device. First, we studied current generated thermal effects that modify magnetic anisotropy of the GaMnAs layer. In order to monitor the direction of magnetization, we used to planar Hall effect (PHE) measurements. The magnetic anisotropy of the GaMnAs shows systematic change with increasing current density in the same way as increasing bath temperature of the sample. Furthermore, Planar Hall resistance (PHR), which is due to PHE, in a GaMnAs film with two in-plane easy axes shows well-defined maxima and minima state that can serve as two binary logic states. We demonstrated that the switching of magnetization between the easy axes in a GaMnAs film depends strongly on the magnitude of the current flowing through the film due to the thermal effect that modifies its magnetic anisotropy. This effect was further utilized to achieve magnetic logic functions by choosing appropriate magnitude of the applied current as an input signal and detecting the PHR value as an output signal in a GaMnAs Hall device. Specifically, logic functionalities obtained in such a device can be reconfigured into AND, OR, NAND, and NOR gates by selecting appropriate initial conditions. These results devices hold promise for realizing programmable logic elements in magnetic electronics. Second, we have investigated the effect of spin orbit induced (SOI) magnetic fields on magnetization reorientation in GaMnAs films. Such SOI fields are generated by the electric current when it passes the material with a noncentrosymmetric crystal structure, and its direction is determined by the direction of the current. This current dependence of SOI field provides an opportunity for electrical manipulation of magnetization in the ferromagnetic GaMnAs film. The presence of the SOI field generated by the electric current was confirmed by the observation of hysteresis between two opposite polarity currents during the magnetization reorientation process in the GaMnAs film. The magnitude of this SOI field was calculated from the width of hysteresis, which increases with current density. We further show that the magnetization switching between two magnetic easy axes by switching the polarity of the applied current. Such switching of magnetization provides a means of electrical control for the value of resistance in the GaMnAs film (in this case the PHR), a process that can be exploited in spintronic devices. Finally, we have investigated the selective manipulation of the magnetization alignment in hybrid magnetic system of Fe/GaAs/GaMnAs structure by SOI magnetic fields. A key feature for such selective manipulation is that the SOI field depends strongly on the crystal structure of the ferromagnetic film. Therefore, even if the same current is simultaneously applied to the Fe layer and the GaMnAs layer of such a hybrid structure, each of the magnetic layers experiences different strength of the SOI field. This difference in the SOI field between two magnetic layers provides a unique opportunity to control the magnetization in one layer (in the presence case in GaMnAs) by the current while the magnetization in the Fe layer remains fixed. Owing to such ability to control the magnetization in the GaMnAs layer selectively, we can manipulate the relative spin configurations in Fe/GaAs/GaMnAs structure between collinear and non-collinear alignments simply by switching current polarity even in the absence of an external field.

more

목차

ABSTRACT
CONTENTS
TABLES
FIGURES
CHAPTER 1. INTRODUCTION 1
CHAPTER 2. THEORETICAL BACKGROUND 7
2.1 Magnetic anisotropy 7
2.1.1 Uniaxial anisotropy 8
2.1.2 Cubic anisotropy 10
2.2 Resistances in magnetic film 13
2.2.1 Anisotropic magneto-resistance (AMR) 13
2.2.2 Hall resistance in magnetic materials 14
2.3 Spin orbit interaction 16
2.3.1 Spin-orbit interaction in semiconductor 20
2.3.2 Spin-orbit interaction due to inversion asymmetry 22
2.4 Joule heating effect by electrical current 27
CHAPTER 3. SAMPLE PREPARATION AND EXPERIMENTAL SETUP 28
3.1 Sample grown on GaAs substrate 28
3.2 Sample fabrication 30
3.2.1 Photolithography process 30
3.2.2 Device fabrication 33
3.3 Transport measurement setup 35
3.4 Hall measurement 37
CHAPTER 4. NON-VOLATILE LOGIC GATES BASED ON PLANAR HALL EFFECT IN MAGNETIC FILMS 39
4.1 Magnetic anisotropy in GaMnAs film 39
4.2 The Joule heating effect in GaMnAs film 43
4.3 The preparation for logic device with GaMnAs film 47
4.3.1 The concept of the logic gate in Hall device 47
4.3.2 The process of magnetization reversal at different currents 49
4.3.3 Transition of magnetization by the current pulse 53
4.4 Realization of logic function using Hall device 56
4.4.1 The AND logic function 56
4.4.2 The OR logic function 61
4.4.3 The NAND logic function 64
4.4.4 The NOR logic function 67
CHAPTER 5. MANIPULATION OF MAGNETIZATION BY SOI-MAGNETIC FIELD IN MAGNETIC SINGLE LAYER 70
5.1 The effect of SOI magnetic field on magnetization switching 70
5.1.1 The Identify the presence of SOI fields by field sweeping 71
5.1.2 The Identify the effect of SOI fields by the angular dependent 80
5.2 The Calculation of the magnitude of SOI magnetic field 83
5.3 The manipulation of magnetization by SOI field 92
CHAPTER 6. MANIPULATION OF MAGNETIZATION IN A FE/GAAS/GAMNAS HYBRID STRUCTURE 98
6.1 The preparation for selectively manipulating the magnetization 98
6.2 Joule heating effect on magnetic anisotropy 99
6.3 The PHR measurement in hybrid structure 107
6.3.1 The behavior of PHR in small magnetic field 108
6.3.2 The behavior of PHR in large magnetic field 112
6.4 The effect of SOI field on the reorientation of magnetization in the hybrid structure 115
6.5 The interlayer exchange coupling between the GaMnAs and Fe layers 118
6.6 Selective manipulation of magnetization in hybrid structure 121
6.7 Magnetization switching only using current without magnetic field 124
CHAPTER 7. SUMMARY 136
REFERENCES 139

more
반출 Meta View 목록