Kaiyou Wang is Director of State Key Laboratory for Superlattices & Microstructure, Institute of Semiconductors, Chinese Academy of Sciences

Meiyin Yang is Professor at the Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics of Chinese Academy of Sciences

Jun Luo is Professor at the Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics of Chinese Academy of Sciences

List of Contributors xi

Series Preface xiii

Preface xv

1 Introduction 1
Kaiyou Wang

2 Giant Magnetoresistance (GMR) Materials and Devices for Biomedical and Industrial Applications 3
Kai Wu, Diqing Su, Renata Saha, and Jian-Ping Wang

2.1 Introduction 3

2.2 Giant Magnetoresistance (GMR) Effect 4

2.3 Different Types of GMR Sensors 7

2.3.1 Rigid GMR Sensors 7

2.3.1.1 Long-strip GMR Sensors 7

2.3.1.2 Large-area GMR Sensors 8

2.3.2 Flexible GMR Sensors 9

2.3.3 Printable GMR Sensors 11

2.3.4 Granular GMR Sensors (Thin Film- and Solution-based) 11

2.4 GMR Sensors: Surface Modification and Auxiliary Tools 12

2.4.1 GMR Sensor Surface Modification for Biomedical Applications 12

2.4.2 Integration of a Magnetic Flux Concentrator (MFC) 14

2.4.2.1 Superconducting MFC 14

2.4.2.2 Soft-ferromagnetic Material-based MFC 14

2.4.3 Integration of Microfluidic Channels 16

2.5 GMR-based Biomedical Applications 16

2.5.1 GMR-based Immunoassays 16

2.5.1.1 Wash-free and Non-wash-free Immunoassays 17

2.5.1.2 Different Immunoassay Methods 17

2.5.1.3 GMR for Disease Diagnosis 19

2.5.1.4 GMR-based Point-of-Care (POC) Devices 24

2.5.2 GMR-based Genotyping 25

2.5.3 GMR-based Bio-magnetic Field Recording 28

2.5.4 GMR-based Food and Drug Safety Supervision 32

2.6 GMR-based Industrial Applications 34

2.6.1 GMR for Position Sensing 34

2.6.2 GMR for Current Sensing 35

2.6.3 GMR for Material Defect Inspection 37

2.7 Conclusions and Outlook 39

References 40

3 Tunneling Magnetoresistance (TMR) Materials and Devices for Magnetic Sensors 51
Zitong Zhou, Kun Zhang, and Qunwen Leng

3.1 Principle of Tunneling Magnetoresistance Effect 52

3.1.1 Tunneling Process 52

3.1.2 Spin-dependent Tunneling Process 53

3.1.3 The Julliére Model 54

3.1.4 Typical Structure of the Magnetic Sensing Unit 56

3.2 Material and Process 56

3.2.1 TMR Barrier Materials 56

3.2.2 Ferromagnetic Layers in TMR 59

3.2.3 TMR Film Stack 61

3.2.4 Perpendicular Magnetic Anisotropy (PMA) in TMR 65

3.2.5 Material Fabrication and Pattern Process 65

3.2.5.1 Magnetron Sputtering 66

3.2.5.2 Ion Beam Deposition (IBD) 67

3.2.5.3 Evaporation 67

3.2.5.4 Chemical Vapor Deposition (CVD) 67

3.2.5.5 Photolithography 69

3.2.5.6 Etching 69

3.3 The Noise of TMR Sensors 70

3.3.1 The Source of Noise from TMR Sensors 70

3.3.2 Methods to Suppress the Noise 72

3.3.2.1 Increase the Number of MTJs in TMR Device 72

3.3.2.2 Optimize Free Layer Volume 73

3.3.2.3 Flux Concentrator 73

3.3.2.4 Applying a Bias Magnetic Field 74

3.4 TMR Sensors and Applications 75

3.4.1 TMR Read Heads 75

3.4.2 The TMR Angle Sensors 76

3.4.3 Geomagnetic Measurement 79

3.4.4 Spin-MEMS Combined Application 80

3.4.5 Nondestructive Testing (NDT) 82

3.4.6 Ultra-low Magnetic Field Detection: Biosensor 83

3.5 Conclusion 85

References 86

4 Spin-Transfer Torque Materials and Devices for Magnetic Random-Access Memory (STT-MRAM) 93
Yan Cui and Jun Luo

4.1 The Background and Mechanism of STT-MRAM 93

4.1.1 The Background of STT-MRAM 93

4.1.2 The Mechanism of STT-MRAM 93

4.1.2.1 LLGS Equation 93

4.1.2.2 The Write Mechanism of STT-MRAM 94

4.1.2.3 The Magnetism of STT-MTJ 97

4.1.2.4 The Switching Properties of STT-MTJ 99

4.2 The Integrated Process of STT-MRAM 102

4.2.1 CMP Technology 102

4.2.2 Magnetic Film Deposition Technology 103

4.2.3 Photolithography Technology 103

4.2.4 Etching Technology 103

4.2.5 Dielectric Isolation Technology 104

4.2.6 Contact Technology 104

4.2.7 Passivation Deposition 104

4.3 Testing of the STT-MTJ Device 105

4.4 The Development Status of STT-MRAM 105

References 107

5 Spin-Orbit Torque (SOT) Materials and Devices 113
Yucai Li, Kevin William Edmonds, and Kaiyou Wang

5.1 Spin-Orbit Coupling in Materials 113

5.2 Manipulation of Magnetic Materials by SOT 116

5.2.1 The Mechanism of SOT in Ferromagnets 116

5.2.2 Measurement Techniques of SOT 117

5.2.3 Field-Free SOT Magnetization Switching in Ferromagnets 119

5.2.4 Domain Wall and Skyrmion Motion Driven by SOT 121

5.2.5 Manipulation of Antiferromagnets by SOT 122

5.3 SOT Materials 123

5.3.1 Traditional Materials 123

5.3.2 Interfacial Engineering 124

5.3.3 Oxide Heterostructures 125

5.3.4 The van der Waals Materials and Topological Materials 125

5.4 Devices and Application 128

5.4.1 SOT-MTJ and SOT-MRAM 128

5.4.2 In-memory Computing 129

5.4.3 SOT Artificial Intelligence Device 130

5.4.4 Internet of Things 131

5.5 Conclusion 131

References 132

6 Spin Oscillators 139
Huayao Tu and Zhongming Zeng 

6.1 Introduction 139

6.2 Fundamental Physics 140

6.2.1 Spin Transfer Torque and Magnetization Dynamics 140

6.2.2 Spin Hall Effect (SHE) and Spin-Orbit Torque (SOT) 141

6.2.3 Operation Principle of SO 142

6.3 Device Classification 143

6.3.1 Geometries 143

6.3.2 Magnetic Equilibrium States 145

6.3.3 Material Structures 145

6.3.3.1 Spin Valves 145

6.3.3.2 Magnetic Tunnel Junctions 146

6.3.3.3 Bilayer 146

6.3.3.4 Single Layer 147

6.4 Emerging Spin-torque Oscillators Based on Magnetic Solitons 148

6.4.1 Vortex 148

6.4.2 Skyrmion 149

6.5 Functional Properties 150

6.5.1 Frequency 150

6.5.1.1 Modulation Properties 152

6.5.2 Output Power 152

6.5.3 Linewidth 155

6.5.4 Phase-locking and Synchronization 157

6.6 Applications 159

6.6.1 Microwave Source 159

6.6.2 Spin Wave Emitter 160

6.6.3 Microwave Detector and Energy Harvester 160

6.6.4 Magnetic Field Detector 163

6.6.5 Neuromorphic Computing 164

6.7 Summary and Outlook 166

References 167

7 Magnetic Tunnel Junctions for Artificial Neural Network 179
Meiyin Yang, Tengzhi Yang, and Jun Luo

7.1 Introduction of Neural Computing 179

7.2 Hardware Requirements for an Artificial Intelligence Neural Network 182

7.3 Introduction to Magnetic Tunnel Junction Devices 183

7.4 Magnetic Tunnel Junction for Neuron Hardware 185

7.4.1 Introduction of STT-MTJ and SOT-MTJ 185

7.4.2 Different MTJ-Based Neuron Hardware 186

7.4.2.1 Step Function 187

7.4.2.2 Nonlinear Activation Function 188

7.4.2.3 Spike or Probability Based Neuron 189

7.5 Magnetic Tunnel Junctions for Synaptic Devices 192

7.6 Learning Methods Suitable for MTJs 194

7.7 Summary and Outlook 195

References 195

8 Three-Dimensional Magnetic Structures of B20 Chiral Magnets 203
Kejing Ran, Dongsheng Song, Weiwei Wang, Haifeng Du, and Shilei Zhang

8.1 Theoretical Development 203

8.2 Observation Technique 206

8.2.1 Electron Holography 206

8.2.1.1 Historical Survey 206

8.2.1.2 Experimental Setup 207

8.2.2 Resonant Elastic X-ray Scattering 209

8.2.2.1 Historical Survey 209

8.2.2.2 Theoretical Treatment 210

8.2.2.3 Experimental Setup 212

8.3 Experimental Results 214

8.3.1 Magnetic Bobbers 214

8.3.2 Surface Twists 216

References 217

9 Multiferroelectric Materials  221
Xiaobin Guo and Li Xi

9.1 Electric Field-driven Magnetization Switching 222

9.2 Electric Field-driven Exchange Bias Reversal and Antiferromagnetic
Domain Wall Motion 229

9.3 Electric Field-driven Antiferromagnetic Vector Switching 237

Acknowledgements 239

References 240

10 Robust Manipulation of Magnetic Properties in (Ga,Mn)As 243
Hailong Wang and  Jianhua Zhao

10.1 Background and Introduction 243

10.2 Electric Field Effects on the Magnetic Properties of (Ga,Mn)As 245

10.3 Manipulation of the Magnetism in (Ga,Mn)As by Light and Strain 256

10.4 Giant Modulation of Magnetism via Organic Molecules 257

10.5 Conclusion and Outlook 260

Acknowledgements 262

References 262

11 Antiferromagnetic Materials and Their Manipulations  271
Xionghua Liu and Kaiyou Wang

11.1 Introduction 271

11.2 Antiferromagnetic Materials 272

11.2.1 Metallic Antiferromagnets 272

11.2.2 Insulating Antiferromagnets 273

11.2.3 Semiconducting and Semimetallic Antiferromagnets 274

11.3 Manipulations of Antiferromagnetic States 275

11.3.1 Magnetic Control of Antiferromagnets 275

11.3.2 Strain Control of Antiferromagnets 277

11.3.3 Optical Control of Antiferromagnets 279

11.3.4 Electrical Control of Antiferromagnets 281

11.4 Topological Antiferromagnetic Spintronics 283

11.5 Summaries and Prospects 286

References 286

12 Prospects 295
Meiyin Yang and Kaiyou Wang

Index 299

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