GB-06: Magneto-optical detection of spin-orbit torque vector with first-order Kerr effects

When spin-polarized currents diffuse into a ferromagnetic (FM) material, they induce spin-orbit torques (SOTs): a powerful mechanism for magnetic order manipulation with applications in magnetic recording, logic devices, and neuro-morphing computing [1]. To date, detection SOTs have relied almost exclusively on measuring electrical voltages arising from the device under excitation. In this line, techniques such as second harmonic generation, spin transfer torque ferromagnetic resonance (STT-FMR), and spin pumping have dominated the scene. In these techniques, signals are often contaminated by unwanted thermo-electric voltages. Also, the sensitivity depends critically on magneto-electric coefficients showing large variations even for typical 3d ferromagnetic materials [2]. This work presents a powerful new method for detecting SOTs based on Magneto-Optical Kerr Effect (MOKE). We can quantify the damping-like and field-like SOT effective fields (hDL and hFL, respectively) of SOTs, employing a simple and low-cost set-up that has already been widely employed in MOKE magnetometry (Fig. 1). We tested our method in NiFe/Pt, NiFe/Pd, and Ta/CoFeB bilayers, obtaining a damping-like SOT efficiency (ζDL) equal to 0.08 ± 0.01, 0.034 ± 0.004 and -0.15 ± 0.01 respectively. On the other side, we obtained hFL\hDL ratios equal to 0.24 ± 0.03 and 0.11 ± 0.03 in the samples with 4 nm thick NiFe and CoFeB, respectively. The results for ζDL are close to the most accepted values reported in the literature for the studied materials. The hFL\hDL ratios fit well inside the diffusive model of spin accumulation in the ferromagnetic layer [3], with a finite spin dephasing length. This effect has often been disregarded in STT-FMR works.References: [1] A. Hiroata et al., J. Magn. Magn. Mater. 509, 166711 (2017). [2] M.-H. Nguyen and C.-F. Pai, APL Materials 9, 030902 (2021). [3] S. Zhang, P. M. Levy, and A. Fert, Phys. Rev. Lett. 88, 236601 (2002).