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

We employ Sagnac interferometry to investigate the magneto-optic Kerr effect measurement of spin-orbit torque from heavy metals acting on the ferrimagnetic insulator BiYIG, known for its high Faraday rotation (~2000° cm⁻¹ at 1500 nm [1]) and superior magneto-optical figure of merit compared to other polycrystalline garnets. Epitaxial growth yields a film with perpendicular magnetic anisotropy, low coercivity, and low damping, making it suitable for magnons interconnection. The high sensitivity of Sagnac interferometry enables optical detection of spin-orbit torque (SOT) through small-angle magnetic tilting in thin film samples with perpendicular magnetic anisotropy. Our measurements of damping-like torque efficiencies based on current-induced magnetic deflection using Sagnac interferometer align with conventional transport-based SOT quantification on similar magnetic insulators. This comparison underscores the potential of Sagnac interferometry as a complementary technique for probing spin-orbit torque effects in insulating magnetic materials.