Quadratic and third-order magneto-optic Kerr effect in Ni(111) thin films with and without twinning

Abstract

The linear magneto-optic Kerr effect (LinMOKE) as well as the quadratic MOKE (QMOKE) are important magneto-optic effects of fundamental physics but also powerful tools for thin-film sample characterization, e.g., to analyze magnetocrystalline anisotropies via vectorial magnetometry [1] or to sense the crystallographic ordering in Heusler compounds [2]. In order to separate and study the dependencies of LinMOKE and QMOKE on the crystallographic direction, the so-called eight-directional method can be used [3]. For each crystallographic in-plane direction, a magnetic field is applied in eight in-plane directions (every 45°) and the responses are added up or subtracted depending on the odd and even nature of LinMOKE and QMOKE with respect to the magnetization. So far,this method or similar ones have been utilized to characterize (001)-and (011)-oriented thin films of cubic crystal structure [4,5]. In addition, new setups for QMOKE spectroscopy have been realized for (001)-oriented thin films [6].Within this contribution, we apply the eight-directional method to sputter-deposited Ni(111) thin films and report on a strong three-fold anisotropy in longitudinal MOKE (LMOKE), see Fig. 1. This anisotropy can be explained by theory as an optical interplay of elements in the permittivity tensors of first and second order in M, effectively creating cubic MOKE contributions, i.e., MOKE of third order in M. Furthermore, we studied Ni(111) thin films with and without several multiple (111) phases of 60° in-plane rotation (twinning) which have been characterized by off-specular x-ray diffraction texture maps. We observed substantial reduction of QMOKE oscillations in a twinned sample compared to a thin film with almost no twinning. This indicates that the LMOKE anisotropy truly is of crystallographic origin in the ferromagnetic layer and is not due to other, e.g., interface effects.  QMOKE anisotropy measurement of a 20-nm thick Ni film on MgO(111) at a wavelength of 406 nm. The threefold anisotropy of longitudinal MOKE (∝ ML in black), transverse MOKE (∝ MT in green) and QMOKE (∝ MLMT in blue, as well as ∝ MT2-ML2 in red) are clearly visible.