Spin-orbit torque in magnetic heterostructures: exchange interactions and ultrasensitive Sagnac optical interferometry*

Spin-orbit torques provide the most efficient mechanism known for reliably manipulating the magnetic orientation of nanomagnets, and are therefore both of fundamental scientific interest and of technological interest for next-generation magnetic memories. Improving the metrology of spin-orbit torques is a critical issue, because at present different commonly-used methods often give conflicting results [1].

In this talk, we will present our recent progress in advancing spin-orbit-torque metrology by developing an ultrasensitive fiber-based Sagnac magneto-optic interferometer [2]. We adapt the interferometer design concept originally developed for measuring time-reversal-symmetry breaking in exotic superconductors, and apply it to measure spin-orbit torques. We achieve a DC Kerr sensitivity of < 5μRad/√Hz, which allows quantitative optical measurements of spin-orbit torques for samples with either perpendicular or in-plane magnetic anisotropy. The Sagnac method is especially advantageous for insulating magnets for which conventional transport spin-orbit torque metrology can be disrupted by magneto-thermal artifacts, but it can also be applied widely to broad classes of magnets regardless of conductivity and small net magnetization.

We will also report progress in developing Sagnac interferometry with ultrafast capability and at cryogenic and high magnetic field environments suitable for studying van der Waals (vdW) magnets. We recently showed that exchange bias from the vdW antiferromagnet CrSBr acting on the vdW ferromagnet Fe3GeTe2 induces a spatially non-uniform spin configuration that is not readily achievable with conventional magnetic materials [3]. Cryo-Sagnac and ultrafast Sagnac will provide us with unique perspectives into the spin torque dynamics and non-uniform spin and magnetic orders in vdW magnetic heterostructures.