Large nonlinear magneto-optical effect in the centrosymmetric ferromagnetic semiconductor EuO

Pronounced magnetic-dipole-induced second-harmonic generation (SHG) is observed in epitaxial films of the ferromagnetic semiconductor EuO. The SHG light wave emerges background-free at the Curie temperature and couples linearly to the spontaneous magnetization. The SHG spectrum is determined by spin-allowed and spin-forbidden transitions between the $4f$ ground state and the 5d(t2g) states of the Eu²⁺ ion. At the coercive field, components of the spontaneous magnetization perpendicular to the applied field are observed. Spatially resolved hysteresis measurements reveal that the ferromagnetic domains possess an average extension of <1 μm although pinning effects can stabilize domains that are two orders of magnitude larger.