Sagnac Interferometer Measurement of Spin-Orbit-Torque Efficiency Acting on the Insulating Magnet Li_0.5(Al,Fe)_2.5O₄

For insulating magnets, electrically-based techniques to measure spin-orbit-torque efficiency suffer from poor signal-to-noise ratios and unwanted artifacts, while optical measurements based on conventional magneto-optical Kerr effect (MOKE) imaging often lack sufficient sensitivity. Here we show that Sagnac interferometry can enable optical measurements of spin-orbit torque even for thin magnetic insulator films. For spin-orbit torque from 2 nm of platinum acting on lithium aluminate ferrite films, we measure a torque efficiency of ξ_SH = 0.128 ± 0.009, very similar to Pt acting on metallic magnets. For the same samples, we also make comparisons to commonly-used electrical approaches for measuring spin-orbit torque efficiency, and we illustrate some of the pitfalls of those approaches when applied to magnetic insulators.