Magneto-Optic Studies of Spin Orbit Torques in Synthetic Ferrimagnets

Rare-earth/transition-metal synthetic ferrimagnetic structures have been used to enable current-driven operations while exhibiting exciting phenomena, including, but not limited to, self-torques and efficient charge-to-spin conversion. These effects in synthetic ferrimagnets have been extensively studied in the past using electrical transport measurements, such as 2^nd harmonic Hall measurements. Here, we report on high-sensitivity magneto-optical measurement of the current-induced torques in these synthetic ferrimagnetic systems using a Zero-Area Loop Sagnac Interferometer (ZALSI) based on Magneto-Optic Kerr Effect (MOKE). Our ZALSI provides a signal-to-noise ratio that is at least 1000 times greater than that of a conventional MOKE setup. We have achieved a Kerr sensitivity of ~50 nrad, enabling us to probe very small magnetization deflections induced by spin-orbit torques while avoiding effects such as current-mediated rectification and other unidentified artifacts in the estimation of current-induced torques by electrical methods. ZALS-MOKE studies of spin-orbit torques in bilayers of Co/Gd on SiO₂ substrates with different capping layers, such as Al, Pt, and Ta, along with reversal of the bilayer stacking order, will be compared to the results of second-harmonic Hall studies on the same samples.