Disambiguating electrical detection of magnetization dynamics in magnetic insulators

TL;DR

This paper clarifies how to distinguish between spin pumping and spin-torque ferromagnetic resonance signals in electrical measurements of magnetic insulators, aiding accurate interpretation of magnonic experiments.

Contribution

It provides a comprehensive framework for disambiguating spin pumping and ST-FMR contributions in experiments involving microwave excitation and spin-charge conversion.

Findings

Sign and magnitude of signals depend on magnetic configuration and device geometry.

Spin pumping and ST-FMR contributions can be separated based on their distance dependence.

Magnetic damping and spin-wave profiles influence which effect dominates.

Abstract

Electrical detection of magnetization dynamics in magnetic insulators underpins both fundamental studies of magnon transport and the development of low-loss magnonic devices. In heavy-metal/magnetic-insulator heterostructures, spin pumping and spin-torque ferromagnetic resonance (ST-FMR) are widely used for this purpose and are often treated separately in different measurement geometries. In practice, the competition between these two effects gives rise to electrical voltage signals of opposite signs, which can lead to ambiguous interpretations of the underlying physics. Here, we show how to disambiguate their respective contributions and provide a framework for interpreting experiments involving microwave excitation of magnetic insulators and detection of magnetization dynamics via spin-charge conversion in heavy metals. We systematically investigate spin pumping and ST-FMR in nonlocal and local devices using Pt-capped thin films of thulium and bismuth-yttrium iron garnets. We show how spin-wave character, magnetic dissipation, magnetic field orientation and device geometry govern the sign and magnitude of the resulting signals. In several cases, the voltage generated by microwave excitation changes sign between an out-of-plane or in-plane magnetic configuration. We disentangle a contribution due to spin pumping, induced by exponentially decaying propagating spin waves, and a weakly distance-dependent contribution from ST-FMR, remotely induced by inductive coupling. We show that both the spin wave excitation profile across the film thickness and magnetic damping largely determine which of the two contributions dominates. Hence, the sign of the electrical signal cannot be uniquely assigned to the chirality of the magnon modes.