Cubic magneto-optic Kerr effect in Co(111) thin films

The magneto-optic Kerr effect (MOKE) is often applied as a tool for the magnetic characterization of thin films. Here, the change in polarization upon reflection from the magnetized sample is mainly regarded as being linearly proportional to the magnetization M (LinMOKE). Recently, we reported on third-order MOKE contributions, named cubic MOKE (CMOKE) being ∝ M3 in Ni(111) thin films. This CMOKE manifests itself as an anisotropic contribution to the MOKE signal (with regard to the crystallographic orientation) measured in longitudinal or transversal configuration in full magnetic saturation. While LinMOKE (odd in M) and quadratic MOKE (QMOKE) being ∝M2 (even in M) can easily be separated by methods based on magnetization parity, this no longer holds true for LinMOKE and CMOKE (odd in M). Here, we report on the observation of CMOKE in thin-film heterostructures with structurally twinned and untwinned Co(111) layers, demonstrating that a large CMOKE is not only present in Ni thin films. Additionally, we show that the observed anisotropic contributions cannot stem from LinMOKE by analyzing their dependence on the angle of incidence (AoI) of light. While the QMOKE is almost vanishing in Co(111) using light with wavelengths of 635 and 406 nm, the CMOKE contributions reach up to about 30% of the LinMOKE contribution at an AoI of 45 degrees and become even more dominant toward normal AoI, which emphasizes the importance of higher-order MOKE effects in magneto-optic experiments.