  Here, we report a novel and conceptually straightforward technique to detect
the spin-polarization directly, with laser spot spatial resolution, at buried
ferromagnet/insulator interfaces in ambient settings. This has been
accomplished by monitoring the voltage-induced change of the longitudinal MOKE
signal as a function of the applied magnetic field and applying an AC voltage
across the interface. For the case where the spin polarization enters the VMOKE
signal, a simple quantitative model is proposed. A distinct positive majority
spin polarization has been found for Fe and Co, whereas Ni exhibits a negative
minority spin polarization.
