Magnetic Phenomena in Thin Layers and at Their Interfaces Open Access

Abstract

Magnetism in thin layers and at their interfaces exhibits behaviors profoundly different from bulk systems due to quantum confinement, enhanced electron correlations, and strong spin-orbit interactions. This dissertation investigates these effects across a range of ferromagnetic and ferromagnet–antiferromagnet heterostructures, employing both experimental techniques and theoretical models to uncover novel magnetic phenomena relevant for spintronic and neuromorphic applications.

We begin by examining magnetization dynamics in ferromagnet–antiferromagnet bilayers, where interfacial frustration suppresses coherent spin precession, giving rise to a viscous magnetic regime characterized by overdamped, memory-retaining behavior. This regime enables the realization of nearly ideal memristors, resistive devices whose output depends on the integral of the input signal. We demonstrate such memristive functionality in both field-driven and spin-torque-driven systems, showing that it emerges from interfacial exchange frustration and can be tuned via proximity to magnetic glass transitions. These findings suggest a possible pathway toward synaptic elements for neuromorphic computing.

To further explore the microscopic origins of such behaviors, we develop a two-orbital Hubbard model for ultrathin (111)-oriented films of late transition metals. Our analysis reveals ferromagnetic orbital correlations stabilized by Mott–Hund’s interactions, while orbital ordering is frustrated by the mismatch between crystal and orbital symmetries. This results in an orbital liquid state that can manifest as perpendicular magnetic anisotropy (PMA) in the presence of spin-orbit coupling. The model suggests a plausible explanation for the enhanced PMA in ultrathin films and indicates a possible route to tune anisotropy via correlation effects.

In a complementary experimental study, we investigate ultrathin ferromagnet–heavy metal bilayers near the Curie temperature and observe an anomalous magnetic behavior that deviates from conventional Weiss magnetism. Through anomalous Hall effect measurements, magneto-optical Kerr effect imaging, and Brillouin light scattering, we identify Rashba-influenced magnetic behavior at the interface arising from the interplay between interfacial spin-orbit coupling and magnetism. This phase exhibits unusual temperature and bias dependence and suggests a mechanism for dynamically tunable magnetoelectronic behavior driven by spin-orbit effects.

Together, these results advance our understanding of magnetism in reduced dimensions, emphasizing the key roles of interfacial coupling, electron correlations, and spin-orbit interactions. These findings also provide a basis for developing future spintronic and neuromorphic devices that leverage the interfacial magnetic effects demonstrated here.

Table of Contents

Acknowledgments

1 Introduction to Magnetism in Low-Dimensional Systems 1

1.1 Historical Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Types of Magnetic Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Theoretical Framework for Magnetism . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3.1 Models and Mechanisms of Magnetic Ordering . . . . . . . . . . . . . . . 8

1.3.2 Magnetic Ground States in Two Dimensions . . . . . . . . . . . . . . . . . 17

1.3.3 Crystal Field and Orbital Quenching . . . . . . . . . . . . . . . . . . . . . 22

1.3.4 Magnetic Domains and Domain Structures . . . . . . . . . . . . . . . . . . 25

1.3.5 Vortex States in Thin Magnetic Films . . . . . . . . . . . . . . . . . . . . 28

1.4 Experimental Methods to Probe Magnetization . . . . . . . . . . . . . . . . . . . 31

1.4.1 Anomalous Hall Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

1.4.2 Spin–Orbit Torques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

1.4.3 Magneto-Optic Kerr Effect . . . . . . . . . . . . . . . . . . . . . . . . . . 41

1.4.4 Brillouin Light Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

1.4.5 Ferromagnetic Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

1.4.6 Direct Magnetometry Techniques: VSM and SQUID . . . . . . . . . . . . 55

1.4.7 X-ray Magnetic Circular Dichroism . . . . . . . . . . . . . . . . . . . . . 57

1.5 Motivation and Scope of This Work . . . . . . . . . . . . . . . . . . . . . . . . . 59

2 Memristive Behavior in Ferromagnet/Antiferromagnet Heterostructures Arising from Viscous Magnetization Dynamics 64

2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

2.2 Memristors: Types and State of the Art . . . . . . . . . . . . . . . . . . . . . . . 67

2.3 Viscous Magnetization in Frustrated Systems . . . . . . . . . . . . . . . . . . . . 73

2.3.1 Frustration at FM/AFM Interfaces . . . . . . . . . . . . . . . . . . . . . . 73

2.3.2 Analogy with Memristive Behavior . . . . . . . . . . . . . . . . . . . . . 78

2.4 Viscous Model for FM/AFM Bilayers . . . . . . . . . . . . . . . . . . . . . . . . 79

2.4.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

2.4.2 Predicted Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

2.5 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

2.5.1 Sample Fabrication and Structure . . . . . . . . . . . . . . . . . . . . . . 84

2.5.2 Measurement Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

2.6 Experimental Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 90

2.6.1 Coercivity and Exchange Bias across the Samples . . . . . . . . . . . . . . 90

2.6.2 AMR Hysteresis: Viscous Magnetic Response . . . . . . . . . . . . . . . . 97

2.6.3 Memristive Field-Sweep Hysteresis Loops . . . . . . . . . . . . . . . . . . 99

2.6.4 Frequency Dependence and Deviations from Ideality . . . . . . . . . . . . 100

2.6.5 Implications for Neuromorphic Applications . . . . . . . . . . . . . . . . . 103

2.7 Prospects for Vortex States in FM/AFM Bilayers . . . . . . . . . . . . . . . . . . . 104

2.8 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

3 Rashba-Like Magnetic Phases in Ultrathin Ferromagnet/Normal Metal Bilayers 114

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

3.2 Overview of Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . 116

3.3 Materials and Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

3.3.1 Platinum versus Palladium . . . . . . . . . . . . . . . . . . . . . . . . . . 120

3.3.2 Platinum versus Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

3.3.3 Buffer Layers and Interface Engineering . . . . . . . . . . . . . . . . . . . 128

3.3.4 Thickness Dependence . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

3.3.5 Other Ferromagnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

3.4 MOKE: Transverse AC Susceptibility . . . . . . . . . . . . . . . . . . . . . . . . 138

3.5 Effect of Interface Engineering on Magnetic Anisotropy . . . . . . . . . . . . . . . 144

3.6 Effects of Current in Microstructures . . . . . . . . . . . . . . . . . . . . . . . . . 148

3.6.1 Effect of Direct Current on AHE . . . . . . . . . . . . . . . . . . . . . . . 149

3.6.2 Brillouin Light Scattering . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

3.6.3 Spin-Torque Ferromagnetic Resonance . . . . . . . . . . . . . . . . . . . . 162

3.6.4 Harmonic Measurements, Field-like and Damping-like torques . . . . . . . 166

3.7 Supplementary Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . 171

3.7.1 X-ray Magnetic Circular Dichroism Results and Analysis . . . . . . . . . . 171

3.7.2 Magnetometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

3.8 Rashba Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

3.9 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

4 Orbital Correlations and Magnetic Anisotropy in Ultrathin Transition Metal Films 180

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

4.2 Orbital Hubbard Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

4.3 Two-Site Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

4.4 Three-Site Model for Ferromagnetic Ordering . . . . . . . . . . . . . . . . . . . . 189

4.5 Extension to Larger Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

4.6 Effects of Spin-Orbit Coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

4.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

5 Tailoring Magnetic States in Artificial Ferrimagnets of Transition and Rare-Earth Metals 205

5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

5.2 Micromagnetic Simulations of Disordered Vortex States . . . . . . . . . . . . . . . 207

5.3 Magnetic Properties of FeGd Films . . . . . . . . . . . . . . . . . . . . . . . . . . 209

5.3.1 FeGd Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211

5.3.2 FeGd Multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213

5.4 Magnetic Behavior of CoTb Multilayers . . . . . . . . . . . . . . . . . . . . . . . 221

5.4.1 DMI-enhanced synthetic ferrimagnet: Pt- and Al-Capped . . . . . . . . . . 223

5.4.2 Low-DMI: Pt-free, oxide-capped Co/Tb . . . . . . . . . . . . . . . . . . . 223

5.4.3 Pt-spaced, Ta-capped Co/Tb, no Al . . . . . . . . . . . . . . . . . . . . . . 224

5.4.4 Deposition Order Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . 226

5.4.5 Domain Directionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

5.4.6 Temperature-Dependent Exchange Bias . . . . . . . . . . . . . . . . . . . 229

5.4.7 Current-Induced Switching . . . . . . . . . . . . . . . . . . . . . . . . . . 232

5.5 Conclusion and Future Plans . . . . . . . . . . . . . . . . . . . . . . . . . 238

6 Design of Custom Instrumentation 240

6.1 Compact Halbach Magnet Array . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

6.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241

6.1.2 Design and Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

6.1.3 Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

6.1.4 Testing and Experimental Verification . . . . . . . . . . . . . . . . . . . . 247

6.1.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250

6.2 Magneto-Optical Kerr Effect Microscope . . . . . . . . . . . . . . . . . . . . . . . 250

6.3 Rotating Stage for Film Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . 253

6.3.1 Motivation for Rotating Deposition . . . . . . . . . . . . . . . . . . . . . . 253

6.3.2 Comparison of Film Properties . . . . . . . . . . . . . . . . . . . . . . . . 254

6.3.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257

Conclusion 259

References 268

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School	Laney Graduate School
Department	Physics
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Physics, Condensed Matter Nanotechnology Engineering, Materials Science
Keyword	Brillouin light scattering Magnetic heterostructures Artificial ferrimagnets Ferromagnet/antiferromagnet bilayers Exchange bias Spintronics Dzyaloshinskii-Moriya interaction English Neuromorphic computing Magnetic memristor Spin-orbit torque Magneto-optical Kerr effect Interfacial spin-orbit coupling Micromagnetics Perpendicular magnetic anisotropy Rashba effect Fe-Gd multilayers X-ray magnetic circular dichroism Custom instrumentation Co-Tb multilayers Ultrathin magnetic films Viscous magnetization dynamics Random anisotropy Anomalous Hall effect Magnetic vortex states Spin-torque ferromagnetic resonance Magnetic Proximity Effect
Committee Chair / Thesis Advisor	Urazhdin, Sergei, Emory University
Committee Members	Santos, Luiz H., Emory University Harutyunyan, Hayk, Emory University Burton, Justin C., Emory University Evangelista, Francesco, Emory University

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	Memristor Hysteresis at Two Frequencies (I(t), V(I), and R(I) loops compared for low vs high drive)	2025-10-13 23:11:58 -0400	Press to Select an action Download
	Viscous Oscillator Response (F(t) drive and x(F) parametric plot showing phase lag from viscosity)	2025-10-13 23:12:11 -0400	Press to Select an action Download
	Simulated Magnetic Memristor Loop (Hysteresis from magnetization-dependent resistance in a toy model)	2025-10-13 23:12:25 -0400	Press to Select an action Download
	BLS Spectroscopy Schematics (Geometry and signals for I<0, I=0, and I>0 configurations)	2025-10-13 23:12:42 -0400	Press to Select an action Download
	AC-MOKE Frequency Dependence (Kerr amplitude/phase versus drive frequency across a broad range)	2025-10-13 23:12:57 -0400	Press to Select an action Download
	Magnetic Domains: Si/Ta(5)Pt(5)[Pt(3)Co(1.3)Tb(0.5)Al(3)]5Pt(2) at RT (Room-temperature domain texture in the multilayer stack)	2025-10-13 23:13:13 -0400	Press to Select an action Download
	Magnetic Domains: Si/Ta(5)Pt(5)[Pt(3)Co(1.3)Tb(0.5)Al(3)]5Pt(2) at 350K (Domain contrast at 350K for temperature comparison)	2025-10-13 23:13:28 -0400	Press to Select an action Download
	Magnetic Domains: Si/Ta(3)Pt(2)[Pt(1)Co(1.2)Tb(1)]5Ta(3) at RT (Room-temperature domain texture in the Co/Tb multilayer)	2025-10-13 23:13:46 -0400	Press to Select an action Download
	Magnetic Domains: Si/Ta(2)[Pt(1)Co(0.25)Tb(0.25)]20Al2O3(4) at RT (RT domain pattern in the 20-repeat ultrathin stack)	2025-10-13 23:14:05 -0400	Press to Select an action Download
	Magnetic Domains: Si/Ta(2)[Pt(1)Co(0.25)Tb(0.25)]20Al2O3(4) at 350K (Domain contrast at 350K for the same stack)	2025-10-13 23:14:23 -0400	Press to Select an action Download
	Magnetic Domains: Si/Ta(3)Pt(2)[Pt(1)Co(0.8)Tb(1)]5Ta(3) at RT (Room-temperature domain texture in the 5-repeat multilayer)	2025-10-13 23:14:42 -0400	Press to Select an action Download
	Magnetic Domains: Si/Ta(3)Pt(2)[Pt(1)Co(0.8)Tb(1)]5Ta(3) at 350K (Domain contrast at 350K for temperature comparison)	2025-10-13 23:14:55 -0400	Press to Select an action Download