Magnetic imaging techniques are widespread critical tools used in fields such as magnetism, spintronics, or even superconductivity. Among them, one of the most versatile methods is the magneto-optical Kerr effect. However, as soon as light is blocked from interacting with the magnetic layer, such as in deeply buried layers, optical techniques become ineffective. In this work, we present a spin accumulation based magneto-optical Kerr effect microscopy technique that enables imaging of a magnetic thin-film covered by thick and opaque metallic layers. The technique is based on the generation and detection of transient spin accumulations that propagate through the thick metallic layer. These spin accumulation signals are directly triggered and detected optically on the same side, lifting any substrate transparency requirements. The spin accumulation signals detected on a Cu layer decay with a characteristic length of 60 nm, much longer than the 12 nm optical penetration depth, allowing for the detection of magnetic contrast with Cu capping layers up to hundreds of nm. This method should enable magnetic imaging in a wide range of experiments where the surface of interest is covered by electrodes.
E. et al. (Mon,) studied this question.