Sagnac interferometry for high-sensitivity spin-orbit torque measurements with the ferromagnetic insulator bismuth-substituted yttrium iron garnet (Bi:YIG)

We utilize Sagnac interferometry for magneto-optic Kerr effect measurements of spin-orbit torque from heavy metals acting on the ferrimagnetic insulator Bi:YIG. The high sensitivity of Sagnac interferometry permits the first-time optical quantification of spin-orbit torque (SOT) from small-angle magnetic tilting of insulating ferromagnetic samples with perpendicular magnetic anisotropy (PMA). This is a fundamentally different method of measuring spin-orbit torque compared to transport measurements based on the spin Hall magnetoresistance (SMR) effect. We study 10 nm thick films of Bi:YIG grown by pulsed laser deposition from a target with composition BiY₂Fe₅O₁₂ onto a GSGG (Gd₃Sc₂Ga₃O₁₂) substrate. Epitaxial growth yields a film with perpendicular magnetic anisotropy, low coercivity, and low damping, and with a high magneto-optical figure of merit defined as Faraday rotation/optical absorption. We will first elaborate on the mechanism of our Sagnac method, and then discuss questions and insights that arise from comparing the Sagnac SOT readout based on the polar magneto-optical Kerr effect with conventional transport SOT readout based on SMR.