Date of Award

Summer 8-23-2025

Document Type

Dissertation

Degree Name

Ph.D. in Physics and Astronomy

Organizational Unit

College of Natural Science and Mathematics, Physics and Astronomy

First Advisor

Xin Fan

Copyright Statement / License for Reuse

All Rights Reserved
All Rights Reserved.

Keywords

Self spin-orbit torque, Spin-orbit torque, Spintronics, Ferromagnetic

Abstract

Spin–Orbit torques (SOTs) provide an energy-efficient pathway to control magnetization in nanoscale heterostructures. Recently, spin-orbit coupling within ferromagnets has been shown to generate self spin-orbit torques (SSOTs) on the magnetization itself. Previous work on self spin-orbit torques relies on thick ferromagnets with complicated film structures with chemical disorder or compositional gradients to generate inversion symmetry breaking. This dissertation advances experimental methodology and presents data on the quantification of self spin-orbit torques in magnetic thin films due to the spin-orbit coupling of standard 3d ferromagnets.

First, I develop an oblique-incidence magneto-optical Kerr effect (MOKE) technique that exploits a mirror-assisted double reflection and a selectable quarter-wave plate to separate polar and longitudinal Kerr responses in a single geometry. Applied to Pt/Co/Pt and Pt/Co/Cu/NiFe films, the method suppresses the unwanted polar contribution by a factor of six and, after accounting for mirror birefringence, by more than 160 times compared to conventional MOKE methods.

Next, I investigate how depth-dependent magnetization perturbations affect SOT quantification when ferromagnetic layer thicknesses approach the dynamic exchange coupling length. Using numerical models, three widely used SOT measurement techniques: MOKE, second-harmonic Hall, and spin-torque ferromagnetic resonance (ST-FMR) are compared, revealing that the first two are significantly impacted by SOT induced nonuniform magnetization tilting, whereas ST-FMR remains relatively robust. This work provides guidance for choosing appropriate ferromagnetic layer film ii thickness and measurement methods in studies of spin-orbit torques which otherwise result in significant overestimation of SOTs.

Next I present my work on quantifying the self spin-orbit torque in a symmetric spin valve ferromagnetic system consisting of NiFe/Cu/NiFe arising from the intrinsic spin–orbit coupling of the most prototypical ferromagnet NiFe. SOT-MOKE measurements on the spin valve yield an effective SSOT conductivity of 290 ± 20 Ω−1cm−1 in NiFe, comparable to that of a NiFe/Cu/Ta heavy-metal system. This finding not only challenges the current understanding of the spin-orbit torques but also illustrates the important role self spin-orbit torques may play in efficiently generating spin that has thus far been neglected in spintronics research.

Finally, I report preliminary results on my efforts to enhance the SSOT in ferromagnetic bilayers through targeted doping. In Pt-doped NiFe/CoPt, no SSOT was observed, which is interpreted as strong exchange coupling out competing the spin absorption due to spin-orbit coupling. In contrast, in Cu-doped NiFe/NiCu bilayers, the undoped system exhibited a significant SSOT with a spin Hall conductivity of 80 ± 5 Ω−1cm−1, which peaks at 125 ± 5 Ω−1cm−1 at 20% Cu doping and decreases with increasing Cu concentration consistent with the strong spin-orbit coupling of Ni. These opposing behaviors highlight the delicate balance between exchange coupling interactions and spin-orbit coupling in governing self spin-orbit torques in ferromagnetic systems and motivate further systematic studies to optimize self-torque enhancement in ferromagnetic films for more energy efficient computing.

Copyright Date

8-2025

Publication Statement

Copyright is held by the author. User is responsible for all copyright compliance.

Rights Holder

Ryan Greening

Provenance

Received from ProQuest

File Format

application/pdf

Language

English (eng)

Extent

172 pgs

File Size

17.2 MB

Download



Share

COinS