Spin-torque-driven magnetization dynamics for neuromorphic functionality

Klause, Robin

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https://hdl.handle.net/2142/129895 Copy

Description

Title

Spin-torque-driven magnetization dynamics for neuromorphic functionality

Author(s)

Klause, Robin

Issue Date

2025-05-19

Director of Research (if dissertation) or Advisor (if thesis)

Hoffmann, Axel F

Doctoral Committee Chair(s)

Hoffmann, Axel F

Committee Member(s)

• Cahill, David G
• Zuo, Jian-Min
• Rakheja, Shaloo

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Spintronics
• Magnetic This Films
• Spin-Orbit Torques
• Magnetization Dynamics

Abstract

With the ever-increasing demand for computationally demanding tasks and large scale memory, new information processing and storage technology is needed. Spintronics plays a key role in the development of faster, smaller, and more energy efficient technology. In this work, the focus lies on the electrical control of magnetization dynamics through spin-orbit torques and the exploration of their potential for enabling neuromorphic functionality. More specifically, unconventional spin-orbit torques and the coupling of spin-Hall nano-oscillators are explored. First, unconventional spin-orbit torque generation in the non-collinear antiferromagnet and altermagnet Mn$_3$Pt is explored. The thin film growth process is optimized based on the structural characterization, and the electrical transport properties. Spin-torque ferromagnetic-resonance reveals that unconventional spin-orbit torques, resulting from a spin polarization component along the current direction, are generated when current is applied along specific directions with respect to the crystal and magnetic order. Second, unconventional spin-orbit torque generation in CrPt$_3$ is studied. Spin-torque ferromagnetic-resonance and second harmonic Hall measurements of CrPt$_3$/Cu/Ni$_{81}$Fe$_{19}$ heterostructures reveal that unconventional field-like spin-orbit torques are generated in both ferrimagnetic and paramagnetic CrPt$_3$ films indicating that the magnetic ordering does not play a role in generating unconventional torques. Instead, numerical calculations reveal that the unconventional torques stem from indirect non-local spin-orbit torques and symmetry breaking at the CrPt$_3$/Cu interface. Third, micromagnetic simulations show that unconventional spin-orbit torques can be used to generate magnetic droplet solitons in a ferromagnet with perpendicular magnetic anisotropy without applying an in-plane magnetic field. Due to the non-local injection of spin-orbit torques, which differs from the typical spin-valve geometry, multiple magnetic droplets can be generated and annihilated using specific current pulses. Last, Pt/Ni$_{81}$Fe$_{19}$ spin-Hall nano-oscillators are fabricated on Si substrate. Injection-locking of the oscillations to an {\em rf} signal reveal that direct electrical coupling of two oscillators is unfeasible due to the low power generation of a spin-Hall nano-oscillator. However, the frequency behavior of two connected oscillators can be tuned using two voltage sources. In addition, a fabrication process for Pt/Ni$_{81}$Fe$_{19}$ oscillators on top of an Y$_3$Fe$_5$O$_{12}$ film that were grown on Gd$_3$Ga$_5$O$_{12}$ is developed. This is non-trivial as several challenges need to be overcome that are due to the electrically insulating and poor thermally conducting film and substrate.

Graduation Semester

2025-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129895

Copyright and License Information

Copyright 2025 Robin Klause

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