Magnetic Sensors for Biomedical Applications

Hadi Heidari, PhD, is an Assistant Professor (Lecturer) in the School of Engineering and lead of the Microelectronics Lab (meLAB) at the University of Glasgow, UK. He is a senior member of the IEEE, and is a Fellow of Higher Education Academy (FHEA). Dr Heidari has authored/co-authored over 100 peer-reviewed publications in top-tier journals or conference proceedings. He has been the recipient of a number of awards including the 2019 IEEE Sensors Council Young Professional Award. Vahid Nabaei, PhD, is a Postdoctoral Research Assistant in the Microelectronics Lab (meLAB) at the School of Engineering, University of Glasgow, UK. Before this he was an assistant professor at the Department of Electrical Engineering, Islamic Azad University, Hidaj Branch, Iran. He has worked as an author/co-author of top-tier journals in modeling and simulation of magnetic sensors for different applications.

Preface xiii

1 Introduction 1

1.1 Overview 1

1.2 History of Magnetism Studies and of Its Use in Magnetic Sensors 2

1.3 Natural and Technical Magnetic Fields and Their Order of Magnitude 3

1.3.1 Natural Magnetic Fields 3

1.3.1.1 The Earth’s Magnetic Field 3

1.3.1.2 Magnetic Fields in Outer Space 3

1.3.1.3 Biomagnetic Fields 3

1.3.2 Technical Magnetic Fields 5

1.3.2.1 Magnetic Fields in the Vicinity of Transformers and Electric Motors 5

1.3.2.2 Fields of Permanent Magnets 5

1.4 Magnetic Terms and Units 6

1.5 Magnetic (Micro) Sensors 7

1.5.1 Definition of Magnetic Sensors 7

1.5.2 Soft and Hard Magnetic Materials for Sensors 8

1.5.2.1 Shape of the Hysteresis Loop 8

1.5.2.2 Saturation Polarization Js and Coercivity Hc 10

1.5.2.3 Initial Permeability μi 10

1.5.2.4 Specific Electrical Resistivity ρ 11

1.5.3 Mechanical Properties of Magnetic Materials 12

1.5.4 Relations Between Sensing Techniques and Sensor Applications 12

1.5.5 Classification of Magnetic Sensors 14

1.6 Characteristics of Magnetic Sensors 15

1.6.1 Characteristics Related to OUT(B)C 15

1.6.1.1 Magnetosensitivity 15

1.6.1.2 Nonlinearity 16

1.6.1.3 Calibration 17

1.6.1.4 Sensor Excitation 17

1.6.1.5 Frequency Response 17

1.6.1.6 Resolution 17

1.6.1.7 Error 17

1.6.1.8 Accuracy 18

1.6.1.9 Hysteresis 18

1.6.1.10 Repeatability 18

1.6.2 Characteristics Related to OUT(C)B 18

1.6.2.1 Noise 18

1.6.2.2 Offset 18

1.6.2.3 Cross-Sensitivity and Temperature Error 19

1.6.2.4 Drift and Creep 19

1.6.2.5 Response Time 19

1.6.3 Characteristics Related to the System Description 19

1.6.3.1 Electrical Excitation 19

1.6.3.2 Input and Output Impedance 20

1.6.3.3 Environmental Conditions 20

1.7 Magnetic Noise 20

1.7.1 Noise Formalism 20

1.7.1.1 Fluctuations, Average and Distribution 20

1.7.1.2 Correlations 22

1.7.1.3 Frequency Space and Spectral Density 22

1.7.2 Sensitivity, Signal-to-Noise Ratio, and Detectivity 24

1.7.3 Different Sources of Noise 25

1.7.3.1 Separation of Magnetic and Nonmagnetic Noise 25

1.7.3.2 Frequency-Independent Noise (Thermal or Johnson–Nyquist Noise), Shot Noise 25

1.7.4 Low Frequency Noise 26

1.7.4.1 1/f Noise 26

1.7.4.2 Random Telegraph Noise 28

1.7.5 High Frequency Noise and Ferromagnetic Resonance 28

1.7.6 External Noise 29

1.7.7 Electronics and Noise Measurements 29

1.7.7.1 Electronics Design 29

1.7.7.2 Connections Noise 30

1.7.7.3 Correlation for Preamplification Noise Suppression 30

References 30

2 Magnetic Sensors Based on Hall Effect 33

2.1 Overview 33

2.2 Devices Based on Hall Effect 34

2.2.1 Geometry 34

2.2.2 Material 35

2.3 Horizontal Versus Vertical CMOS Hall Devices 36

2.4 Current-Mode Versus Voltage-Mode Technique 37

2.5 Magnetic Sensor Characteristics 39

2.5.1 Sensitivity 39

2.5.2 Offset 43

2.5.2.1 Current Spinning Technique 44

2.5.3 Noise 46

2.5.4 Nonlinearity 46

2.6 State-of-the-art in CMOS Hall Magnetic Sensors 47

2.6.1 Sensitivity Improvement 47

2.6.2 Offset Reduction 48

2.7 Applications of Hall Magnetic Sensors 49

2.7.1 Biosensors 49

2.7.2 Contactless Current Sensors 50

2.7.3 Contactless Angular, Linear, and Joystick Position Sensors 50

2.7.4 Electronic Compass 51

2.7.5 Speed and Timing Sensors 52

2.7.6 Specific Sensors 52

References 53

3 Magnetoresistive Sensors 57

3.1 Introduction 57

3.2 Materials and Principles of AMR, GMR, and TMR 58

3.2.1 Anisotropic Magnetoresistance 58

3.2.1.1 Anisotropic Magnetoresistance Effect and Principles 58

3.2.1.2 AMR Device Material 61

3.2.2 Giant Magnetoresistance 62

3.2.2.1 Giant Magnetoresistance Effect and Principles 62

3.2.2.2 Mechanism of GMR Effect 64

3.2.2.3 GMR Effect in Multilayers 66

3.2.3 Magnetic Tunnel Junctions 67

3.2.3.1 TMR Structures 69

3.3 Classes of Magnetoresistive Sensors 71

3.3.1 General Purpose Magnetometers 71

3.3.2 MR Sensors in Harsh Environments 73

3.3.3 Electrical Current Sensing 74

3.3.3.1 Industrial Electronics Applications (Large to Medium Currents) 75

3.3.3.2 Differential Currents 76

3.3.3.3 Switching Regulators 76

3.3.3.4 Wattmeter 77

3.3.3.5 IC Current Monitoring 78

3.3.4 Automotive Applications 78

3.3.4.1 BLDC Rotor Position Measurement 78

3.3.4.2 Steering Angle Application 79

3.3.4.3 Crankshaft Speed and Position Measurement 79

3.3.4.4 Wheel Speed Measurement for ABS and ESC Systems 80

3.3.5 Magnetoresistive Elements in Data Storage Applications 80

3.3.6 Space 81

3.4 Modeling and Simulations 81

3.4.1 Finite Element Modeling and Methodology 81

3.4.2 Finite Element Method 82

3.4.3 Finite Difference Method 82

3.4.4 The Boundary Element Method 82

3.4.5 MR Sensors Simulation and Modeling 83

3.5 Design and Fabrication Technologies 87

3.5.1 GMR Devices 87

3.5.1.1 Deposition Techniques 87

3.5.1.2 Patterning 90

3.6 Biomedical Magnetoresistive Sensing Applications 94

3.6.1 Detection of Bioanalytes 95

3.6.2 Monitoring of Magnetic Fluids 95

3.6.3 Biomolecular Recognition Experiments 95

3.6.4 Ultrasensitive Magnetic Array for Recording of Neuronal Activity (UMANA) 98

References 100

4 Resonance Magnetometers 113

4.1 Introduction 113

4.2 Nuclear Magnetic Resonance 115

4.2.1 Classical Model 116

4.2.1.1 Rotating Frame of Reference 117

4.2.1.2 Strength of RF Pulses 117

4.2.2 Basic Design of a NMR Spectrometer 118

4.2.3 Nuclear Magnetic Resonance in Molecular and Atomic Beams 120

4.2.4 The Sources of Magnetic Fields 123

4.2.5 NMR Spins Used in Life Science 124

4.2.6 NMR Relaxation 125

4.2.6.1 Relaxation Rates 125

4.2.6.2 Molecular Mechanisms Leading to Relaxation 126

4.2.7 NMR and Biological Structures 127

4.2.8 Difficulties in Studying Biological System by NMR 128

4.2.8.1 Sensitivity 128

4.2.8.2 Resolution 129

4.2.8.3 Water Signal 129

4.2.8.4 Line Widths 130

4.2.8.5 Quantification 130

4.3 Magnetic Resonance Imaging 130

4.3.1 Introduction 130

4.3.2 The Obtaining of Spin Images from NMR Induction Signals in Inhomogeneous Field 131

4.3.3 MRI Instrumentation 132

4.3.3.1 Magnets and Designs 135

4.3.3.2 Resistive Electromagnets 135

4.3.3.3 Permanent Magnets 136

4.3.3.4 Superconducting 136

4.3.3.5 Stability, Homogeneity, and Fringe Field 137

4.3.3.6 Gradient Coils 137

4.3.3.7 RF Coils 138

4.3.3.8 RF Decoupling 138

4.3.4 MRI of Flow 139

4.3.4.1 Time-of-Flight Techniques 139

4.4 Electron Spin Resonance 143

4.4.1 The ESR Experiment 145

4.4.1.1 Sensitivity 146

4.4.1.2 Saturation 147

4.4.2 Operation of an ESR Spectrometer 147

4.4.3 Optimization of Operating Parameters 150

4.4.3.1 Microwave Frequency 150

4.4.3.2 Center Field, Sweep Width, and Field Offset 151

4.4.3.3 Sweep Time 151

4.4.3.4 Modulation Frequency 151

4.4.3.5 Second Harmonic Detection 151

4.4.3.6 Modulation Amplitude 152

4.4.3.7 Modulation Phase 152

4.4.3.8 Signal Gain 152

4.4.4 Biological Application of the ESR 152

4.4.4.1 ESR Oximetry 153

4.4.4.2 Direct Detection of Paramagnetic Species 154

4.4.4.3 EPR Revealed the Nitrite Reductase Activity of Myoglobin 155

4.4.4.4 Mitochondrial Dysfunction in Severe Sepsis 155

4.4.4.5 Spin Trapping ESR 155

References 157

5 SQUID Sensors 163

5.1 Introduction 163

5.1.1 History 163

5.2 SQUID Fundamentals 164

5.2.1 Josephson Junctions 164

5.2.2 DC SQUIDs 166

5.2.2.1 Practical Devices 171

5.2.3 rf SQUID 174

5.2.4 Cryogenics and Systems 177

5.2.5 SQUID Electronics 178

5.2.5.1 Flux Locked Loop 178

5.3 SQUID Fabrication 180

5.4 Lithography and Thin-Film Techniques 180

5.4.1 Junction Fabrication 182

5.5 SQUID Applications in Biomagnetism 183

5.5.1 Biomagnetism 183

5.5.2 History of SQUID Applications in Biomagnetism 184

5.5.3 Biomagnetic Fields 185

5.5.3.1 Gradiometers 186

5.5.4 Magnetoencephalography 188

5.5.4.1 MEG Signals 189

5.5.4.2 Sensor Types for MEG 191

5.5.5 Magnetocardiography 195

5.5.5.1 Cardiomagnetic Instrumentation 196

5.5.6 Magnetoneurography 197

5.5.6.1 History of Measuring Signal Propagation in Nerves 197

5.5.6.2 Measurement Technique and Signal Processing 199

5.5.6.3 Source Modeling for Magnetoneurography 200

5.5.6.4 Clinical Perspective 200

References 201

6 Conclusion 213

6.1 Outlook 213

6.2 A Conclusion on Galvanomagnetic Sensors 214

6.2.1 Hall Elements: Hall Voltage Mode Versus Hall Current Mode and Magnetoresistance Mode of Operation 215

6.2.2 Hall Sensors Versus Ferromagnetic Magnetoresistors 215

6.2.3 Performance of Integrated Hall Magnetic Sensors 216

6.2.4 Performance of Ferromagnetic Magnetoresistors 216

6.2.5 Integrated Hall Sensors Versus AMRs and GMRs 218

6.3 A Conclusion on NMR and ESR Spectroscopy 218

6.3.1 Differences Between NMR and ESR 219

6.3.1.1 Resonant Frequency 219

6.3.1.2 Relaxation Times 219

6.3.1.3 Differences Between ESR and NMR Imaging 220

6.3.1.4 ESR Applications 220

6.4 Superconductive Quantum Interference Devices 221

6.4.1 SQUID Fabrication Trend 221

6.4.2 Trends in SQUID Electronics 222

6.4.3 Trends in SQUIDs for Nondestructive Evaluation of Materials 222

References 223

Index 225