A sensitive MOKE and optical Hall effect technique at visible wavelengths: insights into the Gilbert damping

It is well known that the anomalous Hall effect displayed in ferromagnets is much stronger than the ordinary Hall effect. Therefore, the optical Hall effect is significantly weaker than the magneto-optical Kerr effect (MOKE) such that it is barely detectable at visible wavelengths. We present a sensitive MOKE technique which is based on large-amplitude modulation of the externally applied magnetic field that is suitable for non-magnetic metals. Using a 440 nm laser, we measure Cu, Au, Al, Ta, and Pt and find partial agreement with the Lorentz-Drude theory implying contributions of the plasma dynamics and interband transitions beyond the approximations of the model. Interestingly, we find that the noise scales with the spin-orbit coupling of the metals. This is manifested by a remarkable correlation between the noise amplitude and the Gilbert damping enhancement associated with these metals. These results suggest that the electromagnetic noise arises from optical interactions with spins that is mediated by the spin-orbit interaction and highlight a possible avenue for measuring the spin-orbit coupling using optical techniques.