Vehstedt, Erin Kathleen; (2025) Ferromagnetic resonance and spin-to-charge conversion in thin films. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The spintronics community often uses ferromagnetic resonance (FMR) as a tool to characterise magnetic properties. Recently, FMR was used to generate spin currents by inducing resonance in a magnet adjacent to a non-magnetic conductor. This so-called spin pumping can be exploited to study spin transport and spin-to-charge conversion in a variety of materials. Many claims are made by analysing spectral shapes and amplitudes based on phenomenological models for FMR and spin pumping, but differences in experimental setups and processing may influence these outcomes. The first study in this thesis seeks to better understand these experimental methods, how to interpret them, and whether the results can be directly compared. I designed and fabricated an FMR set-up with precise, automated, sample-position control. High-resolution FMR peaks were measured every 5 $\mu m$ to reveal any sample-position dependence of the line shape and/or amplitude. The underlying mechanisms were captured using an analytical model and a comparison of techniques identified critical factors to keep in mind during data processing. The second study focused on electromotive force generated by spin pumping and the inverse spin-Hall effect (ISHE). Spin-pumping experiments typically measure open-circuit voltages across a device; I measured voltages across variable-load resistors connected in series with the sample. The charge currents and associated electric-power dissipation were quantified across a broad parameter space. Finally, I investigated spin-pumping-induced spin-to-charge conversion and symmetry in (Ga,Mn)As, a strongly spin-orbit coupled material with broken inversion symmetry. A unique playground for exploring the ISHE and spin galvanic (SGE) effects. Simultaneous measurements of voltages induced parallel and perpendicular to the injected spins were performed as a function of orientation with respect to the crystallographic axes. The bulk-induced SGE was separated from the ISHE using symmetry analysis. Detailed quantitative analysis has been performed for validity of claims made and further improvements of the experimental set-ups are offered.

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