Enhanced Signal-to-noise Ratio in a Magneto-optic Kerr Effect Based Spin-orbit Torque Magnetometer

Current-induced spin-orbit torques have drawn extensive attention in the last few decades, due to their potential application in next-generation spintronic devices. Magneto-optic Kerr effect (MOKE) based magnetometry with balanced detection has been used as a convenient and sensitive tool in spin-orbit torque studies. Although the technique has demonstrated high sensitivity, further improvement of the signal-to-noise ratio is mainly limited by the detector shot noise (0.5√(10×Power/mW) µV/√Hz), and maximum optical power (1 mW) tolerated by the detector. We report a simple “unbalanced” detection method that realizes one order-of-magnitude signal-to-noise ratio improvement, utilizing only one additional neutral-density filter. With its ultrahigh (<100 nrad) sensitivity and enhanced signal-to-noise ratio, we demonstrate fast and precise spin-orbit torque vector measurement using polar and quadratic MOKE, on a Permalloy/Pt bilayer sample with no lithography or processing performed. The results show good consistency with balanced detection results. This improved system can be a powerful tool in both fundamental spin-orbit torque studies and application-related characterization.