Date of Award

Summer 8-22-2026

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

Second Advisor

Barry Zink

Third Advisor

Pavel Salev

Fourth Advisor

Alex Huffman

Copyright Statement / License for Reuse

All Rights Reserved
All Rights Reserved.

Keywords

Field-like torque, Harmonic detection, Self-spin-orbit torque, Spintronics

Abstract

Spin-orbit torques (SOTs) have been realized as an energy efficient mechanism to manipulate and drive magnetization dynamics in magnetic multilayer films. Their damping-like and field-like components provide distinct pathways for controlling the magnetization, enabling switching, sustaining magneto-oscillations, and driving domain-wall motion with potential applications in nonvolatile memory, spin-based logic, and microwave signal generation. However, the accurate quantification and interpretation of these torques remains complicated by experimental sensitivity, the co-symmetry of the Oersted field and the effective field due to the field-like torque, and assumptions regarding the material layers from which the torques originate. The body of work set forth in this dissertation addresses these limitations to provide a more complete framework for quantifying spin-orbit torques and give insight into their physical origins.

In this dissertation, I begin by introducing an alternative device, a square Wheatstone bridge, which can be used to measure the field-like spin-orbit torque (FLSOT) via second harmonic anisotropic magnetoresistance (AMR) measurements. The square Wheatstone bridge has a geometric aspect ratio enhancement over the signal measured in the conventional Hall bar, suggesting that this structure can be used as a more sensitive technique to quantify the FL-SOT in systems with inherently smaller signals or heterostructures that suffer from current shunting. Additionally, the square Wheatstone bridge geometry enables in-situ calibration allowing for characterization of the torques in magnetic systems with non-negligible anisotropies.

I then address the issue that has plagued direct quantification of the effective field due to the field-like torque – its shared symmetry with the Oersted field. A perpendicularly magnetized layer (PML) is used to generate an unconventional spin-rotated spin current that gives rise to an orthogonal torque equivalent to a fieldlike effective field perpendicular to the current-generated Oersted field. This field is quantified through spin-rotated harmonic Hall measurements. Free-layer ferromagnet thickness-dependence studies of the effective fields due to the FL-SOT and the damping-like spin-orbit torque (DL-SOT) are performed and a general linear trend with 1/tFM is observed. Additional measurements of the DL- and FL-SOTs via spin rotation (SR) are conducted with modified PMLs to discern whether the origins of the spin torques are due to the direct or indirect mechanisms. Preliminary results suggest the direct mechanism to be the dominant mechanism at hand.

Finally, I present work that provides evidence of the existence of the self-spin orbit torque (SSOT). Historically, charge-to-spin current conversion in ferromagnet/heavy-metal bilayers has been attributed primarily to the spin Hall effect within the heavy metal while the ferromagnet was the spin current absorber and recipient of the spin-generated torques. More modern works theorized the ability of ferromagnets to generate spin current due to their own spin-orbit coupling, but in order to realize the self-generated torques, interfacial symmetry must be broken and spins from the FM must be dissipated. To showcase the sizable magnitude of the SSOT, we use a platinum/tungsten (Pt/W) alloy to break symmetry. This alloy has sizable spin-orbit coupling such that it can efficiently dissipate spins. However, at a specific atomic ratio, it also has a suppressed spin Hall angle, θSH, such that it mitigates the contribution of conventional spin Hall-generated torques to the overall torque on the magnetization. To determine the relative sign and magnitude of θSH, we employ the spin pumping technique and find the alloy with approximately zero spin Hall angle to be comprised of 54% platinum and %46 tungsten, Pt54W46. We then deposit an adjacent Co40Fe40B20 layer and perform ST-FMR measurements to ascertain the magnitude of the self-torque. The extrapolated SSOT is then compared to the total torque possible via measurements of the anomalous spin-orbit torque via MOKE. We find that the SSOT corresponds to roughly 30% of the internal spin–orbit torque generated within the ferromagnet. These results suggest that self-torques must be considered for a holistic understanding of the spin-orbit torques in spintronic devices.

Copyright Date

8-2026

Publication Statement

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

Rights Holder

Kyle Jeffrey Peterson

Provenance

Received from ProQuest

File Format

application/pdf

Language

English (eng)

Extent

131 pgs

File Size

51.1 MB

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