Electronic, Optical, and Microwave Studies of Nanomagnetic Systems Público
Freeman, Ryan (Spring 2020)
Published
Abstract
This dissertation includes a variety of research projects focused on understanding and manipulating magnetic
phenomena at nanoscale. These projects were experimentally driven, but also involve simulations and
calculations in mostly a supplementary role. Most of my work is related in some way to the optical, electronic,
or microwave generation of spin waves, the dynamical eigenmodes of spin precession that are present in
magnetic materials. Future technological applications of spin waves would require the ability to efficiently
generate and transport them in nanostructures. This long term technological goal underpins much of the
research presented in this thesis.
One of the most common ways to generate spin waves in nanostructures is via the injection of spin
angular momentum into a magnetic material, via 'spin currents' generated by various spin-orbit effects. The
second chapter of my thesis includes research into the nanoscopic length scales over which spin currents are
generated and dissipated. I was able to identify evidence for a previously unrecognized physical effect that
contributes to the dissipation of spin currents in materials with large spin-orbit interaction. There is evidence
that this spin relaxation mechanism could be the reason why Pt, a material with strong magnetic properties,
never reaches a fully magnetic state. The third chapter includes research into the generation of spin waves
excited by spin-polarized electrical currents, showing that when electrons scatter in magnetic materials, spin
waves are generated in a quantum mechanical process that mirrors the process of photon generation in
lasers. The fourth chapter shows the implementation of an anti-reflective coating that drastically increases
the signal to noise ratio in common magneto-optical experiments used to study magnetic films excited in the
'ultra-fast' terahertz frequency range. The fifth chapter shows how nanoscale optical antenna can be used
to spectroscopically probe high frequency, high wavevector spin waves that are inaccessible by free-space
light. The sixth chapter shows experimental results on a recently discovered magneto-electronic effect that is
the consequence of both spin-dependent electronic scattering and spin-orbit effects that are modified by the
presence of spin waves. The seventh and final chapter includes research on the generation of spin waves in
the nonlinear regime, where the damping of spin waves is fully compensated by the effects of spin currents.
Table of Contents
Introduction: Crash Course in Magnetism 2
1.1 Magnetism in atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Magnetism in solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Magnetic ordering: anisotropy and domains . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Spin Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5 Suggested Textbooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2 Spin Relaxation in Pt - Evidence for Undiscovered Spin Relaxation Mechanism 15
2.1 Motivation and background on spin-orbit effects . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Interfacial spin-orbit effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.3 Measurement technique: Giant Magnetoresistance . . . . . . . . . . . . . . . . . . . . . . . . 17
2.4 Nanofabrication details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.5 Electronic measurement details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2.6 Experimental and calculated results and analysis of spin diffusion . . . . . . . . . . . . . . . . 22
2.7 Characterization of the electronic properties of the studied Pt films . . . . . . . . . . . . . . . 26
2.8 Analysis of mechanisms of spin relaxation in Pt . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.9 Interplay between Spin-Orbit Effects and Magnetism . . . . . . . . . . . . . . . . . . . . . . . 29
2.10 Possible effects of magnetic fluctuations on spin relaxation . . . . . . . . . . . . . . . . . . . . 31
3 Stimulated and Spontaneous Emission of Magnons Driven By Electrical Current 38
3.1 Background on spin transfer: semi-classical and quantum models . . . . . . . . . . . . . . . . 38
3.2 Magnons generated by current excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.3 Relation between magnon population and magnetoresistance of nanomagnetic spin-valve . . . 42
3.4 Magnetoelectronic measurements and calculations of spontaneous and stimulated emission of
magnons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
iii
4 Anti-Reflective Coating for THz Nano-antennae 50
4.1 Introduction and Background on THz light-matter interaction . . . . . . . . . . . . . . . . . . 50
4.2 THz Meta-Material (Nano-antenna) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.3 Anti-Reflective Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.4 Simulated Near-Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
4.5 Magneto-optical Measurements on Magnetic Nanowire using Anti-Reflective Coating . . . . . 57
4.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
5 Brillouin Light Spectroscopy from Spin Waves Inaccessible with Free-Space Light 60
5.1 Background of Brillouin Light Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.2 Mathematical formulation of BLS spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
5.3 Spectral properties of spin waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
5.4 Context of this project: Nanostructure Assisted Optical Techniques . . . . . . . . . . . . . . . 66
5.5 Characterization of YIG Film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
5.6 Ag and Al Deposition and Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
5.7 Choosing Antenna Widths: Simulations of Antennae in Contact With YIG . . . . . . . . . . 71
5.8 Effect of antenna geometry on local electric field . . . . . . . . . . . . . . . . . . . . . . . . . 72
5.9 Experimental BLS spectra from Al/YIG sample . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5.10 Simulated BLS spectra and Spatial Harmonics Generated by Al Nanoantenna . . . . . . . . . 78
5.11 Results from Ag/YIG Device: Broad BLS Sensitivity . . . . . . . . . . . . . . . . . . . . . . . 80
5.12 Results from Devices with Conductive Magnetic Film: Permalloy . . . . . . . . . . . . . . . . 81
5.13 Results from Ag/Py Device: Fourier Peaks and Broad BLS Sensitivity . . . . . . . . . . . . . 82
5.14 Results from Al/Py Device: Fourier Peaks and Broad BLS Sensitivity . . . . . . . . . . . . . 84
5.15 Results from Al/Py/Al Device: Broad BLS Sensitivity . . . . . . . . . . . . . . . . . . . . . . 86
6 Unidirectional Magnetoresistance 92
6.1 Background and Mechanisms of Unidirectional Magnetoresistance . . . . . . . . . . . . . . . . 92
6.2 Current dependence of UMR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
6.3 Effect of Suppressed Magnon Population on UMR . . . . . . . . . . . . . . . . . . . . . . . . 96
7 Coherent Spin Waves Generated by Spin Current in the Nonlinear Regime 101
7.1 Background on 'auto-oscillation' spin wave modes . . . . . . . . . . . . . . . . . . . . . . . . . 101
7.2 Controllable excitation of quasi-linear and bullet modes in a spin-Hall nano-oscillator . . . . . 104
iv
7.3 Nanomagnetic device capable of nonlinear generation and propagation of coherent spin wave
modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
8 Conclusion 111
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