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Title Page

한글 초록:

Abstract:

Contents

Chapter 1. Introduction 11

Chapter 2. Theoretical Background 14

2.1. Anomalous Hall Effect 14

2.2. Perpendicular Magnetic Anisotropy 15

2.3. Voltage-Controlled Magnetic Anisotropy 16

2.4. Magnetization switching induced by Spin Orbit Torque 18

2.5. Atomic Layer Deposition 19

Chapter 3. Experiment 26

3.1. Sample fabrication 26

3.2. ALD setup 27

3.3. Measurement 28

3.3.1. Anomalous Hall Resistance Measurement 28

3.3.2. Spin orbit torque induced magnetization switching 29

3.3.3. Material Analysis of gate oxide 30

Chapter 4. VCMA effect on TiO₂ gate oxide 37

4.1. Non-volatile & Time dependent property of VCMA effect on TiO₂ gate oxide 37

4.2. VCMA effect on TiO₂ gate oxide with different deposition temperature 38

4.2.1. Modulation of He and RAHE by Electric field[이미지참조] 38

4.2.2. Modulation of magnetic anisotropy field by Electric field 39

4.2.3. Modulation of SOT switching current by Electric field 41

Chapter 5. Material analysis of TiO₂ 48

5.1. X-Ray Diffraction Analysis 48

5.2. X-ray Photoelectron Spectroscopy Analysis 48

5.3. I-V Measurement 49

5.4. Relation between efficiency of VCMA effect and material properties of TiO₂ 49

Chapter 6. Conclusions 53

Bibliography 55

Curriculum Vitae 61

List of Figures

Figure 1.1. Comparison of device performance with memory type 13

Figure 1.2. Schematic of in-plane MTJ and perpendicular MTJ 13

Figure 2.1. A simple picture showing the origin of PMA. When the hybridization of out-of plane orbitals of ferromagnet and O 2pz orbital occurs, it leads to an uncompensated occupation in FM in-plane orbitals and results...[이미지참조] 20

Figure 2.2. Schematic of the effect of the electric field on electron filling of the 3d orbitals in the ultrathin Fe layer. Application of a negative voltage, for example, may suppress the number of electrons in the mz ＝ 0 states...[이미지참조] 21

Figure 2.3. Calculated minority-spin band structures and MCA energy contribution, EMCA(k) along high-symmetry directions for (a) Fe/MgO and (b) Fe/FeO/MgO structures in external electric fields of -1 V/Å (dashed... 22

Figure 2.4. Magnetic Anisotropy field of Pt/Co/AlOx samples, measured by AHE, as a function of the oxidation time of Al. 22

Figure 2.5. Schematic view of gate-electrode structure. g-k Polar MOKE hysteresis loops measured at room temperature showing the device in its virgin state (g), after applying Vg＝-4 V at 100 ℃ for 1 s (h), 150 s (i) and... 23

Figure 2.6. Normalized XAS spectra (a) and XMCD spectra (b) at the Co L₃ edge showing the EF-controlled oxidation state and magnetization of ultrathin Co films. The curves have been vertically shifted for clarity. 23

Figure 2.7. Two origins of spin orbit torque (a) Rashba effect, Conduction electrons in ferromagnet moving in the x direction experience an electric field along the z-direction due to structural inversion asymmetry at the interface.... 24

Figure 2.8. Schemes of full ALD process, (a) Precursor feeding process, (b) Purge process, (c) Reactant exposure process, (d) Purge process. 25

Figure 3.1. Schematic of HM/FM/Oxide structure. The device consists of HM line (Ta) and island (CoFeB/MgO/AlOx). TiO₂ gate oxide is deposited on top of island structure by ALD. 32

Figure 3.2. Chemical properties of TDMATi. 32

Figure 3.3. Optimizing ALD process with varying condition. (a) Optimization of precursor feeding process, purge time and oxidation time is fixed to 10s and 4s (t₁/10s/4s/10s). (b) Optimization of oxidation process, precursor... 33

Figure 3.4. ALD window of TDMATi, from 110 ℃ to 175 ℃, GPC shows constant value about 0.7A/s. At temperature above 200 ℃ GPC increases because of precursor decomposition. 34

Figure 3.5. Schematic of measurement technique for anomalous hall voltage. When current flows in the x-direction, the anomalous hall voltage is generated in the y-direction. 34

Figure 3.6. R-H curve of Ta(5)/CoFeB(1.2)/MgO(1.6)/AlOx(1.4)/TiO₂(35) structure with different direction of magnetic field. (a) R-H curve with magnetic field in the direction perpendicular to the film plane (easy axis). (b)... 35

Figure 3.7. Schematic of Bragg's law. When incident x-ray with wavelength of λ satisfies the Bragg's law, scattered x-rays have strong intensity due to constructive interference which is called diffraction. 35

Figure 3.8. Schematic of principle of XPS. Electrons are emitted by incident X-ray with energy of hv and its kinetic energy is KE＝hv - BE - Φ. Therefore, we can extract the binding energy (BE) and analyze the chemical... 36

Figure 4.1. Anomalous hall measurement of Ta/CoFeB/MgO/AlOx/TiO₂ structure. After electric field application, AHE measurement was performed. TiO₂ is deposited at 150 ℃. (a) Non-volatile property of VCMA effect. The... 43

Figure 4.2. Anomalous hall measurement of Ta/CoFeB/MgO/AlOx/TiO₂ structure with different deposition temperature of TiO₂. (a) When negative bias is applied, all of samples showed decrease of coercivity and... 44

Figure 4.3. Changes in (a) anomalous hall resistance and (b) coercivity as a function of polarity of the electric field. 45

Figure 4.4. Changes in magnetic anisotropy field depending on the polarity of the electric field. 45

Figure 4.5. Relation between deposition temperature of TiO₂ and rate of change of magnetic anisotropy field. all four samples showed the change of magnetic anisotropy field when electric field is applied with non-volatile property 46

Figure 4.6. Changes in SOT induced switching current depending on the polarity of the electric field. 46

Figure 4.7. Relation between deposition temperature of TiO₂ and rate of change of switching current. all four samples showed the change of Jc when electric field is applied with non-volatile property[이미지참조] 47

Figure 5.1. X-Ray Diffraction of TiO₂ 35㎚ depending on the deposition temperature. 51

Figure 5.2. XPS spectra of (a) Survey, (b) O 1s, (c) Ti 2p. (d) O 1s spectra depending on the deposition temperature. (e) Relation between NLO contents and atomic ratio of O / Ti. 51

Figure 5.3. (a) I-V measurement of TiO₂ 35㎚ depending on the deposition temperature. (b) Relation between deposition temperature of TiO₂ and Leakage current. (c) Relation between Leakage current and rate of change in... 52

Figure 5.4. Plots showing the relation between NLO contents and the rate of change in (a) Anomalous hall resistance, (b) Coercivity, (c) Magnetic anisotropy field, (d) SOT switching current. 52

초록보기

 강자성/산화물 계면에서 나타나는 수직 자기 이방성은 오비탈 혼성화에 의한 것으로 알려져 있으며 전계 인가를 통해 계면의 상태를 변화시켜 강자성층의 자기적 특성을 제어할 수 있음이 보고된 바 있다. 특히 전계에 의한 산화물 내 산소 이온의 이동을 통해 계면의 산화 상태를 변화시킬 경우, 자기적 특성을 효율적으로 제어할 수 있으며 변화된 자기적 특성이 유지되는 비휘발성을 가지고 있어 활발히 연구가 진행되고 있다. 본 논문에서는 전계에 의한 자기 이방성 제어 효과에 영향을 미치는 산화물의 재료 인자에 관한 연구에 대해 보고하고자 한다. 연구를 위해 TiO₂를 채택하였으며 전계에 의해 보자력, 수직 자기 이방성, 스핀궤도토크 스위칭 전류 등의 자기적 특성을 제어할 수 있음을 확인하였다. 또한 산화물의 증착 온도에 따라 전계에 의한 자기적 특성의 변화량이 달라짐을 관찰할 수 있었다. 산화물의 재료 분석을 진행하여 전계에 의한 자기 이방성 제어 효과에 영향을 주는 재료 인자를 확인하고 실험 결과를 해석하였다.

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