Stable antiparallel domains in 3D corrugated magnetic thin films

Magnetic nanostructured materials are of great interest in fields such as non-conventional computing or magnetic field sensing due to the possibilities that 3D magnetic textures embedded on metamaterials offer. We present a novel study on the magnetization and magneto-optical properties of a ferromagnetic (permalloy), continuous thin film that is highly corrugated by its deposition on the surface of a triangular silicon nanograting with a low periodicity (250 nm) and a quite large amplitude (180 nm), which leads to the formation of an unusual magnetic texture. This grating profile activates several optical phenomena and thus hinders magnetic characterization, which usually requires the analysis of the magneto-optical Kerr effect (MOKE); however, in this paper we unveil the magnetization and disclose magnetic features sized smaller than the light wavelength. Not only does the optical activity include rotation of the polarization plane upon reflection but also, when using violet light (diffraction regime), there is excitation of surface plasmon polaritons at the metal film and consequently, a strong effect on the magneto-optical activity: the transverse Kerr signal is enhanced up to one order of magnitude and the longitudinal Kerr signal changes sign, in comparison with the values of red light (subwavelength regime). Optical modelling led us to understand that features of the field-cycled MOKE are associated with the non-uniform spatial distribution of magneto-optical activity in the film whereby, firstly, the Kerr effect with red light arises at the lower half of the grating and, secondly, the use of violet light focuses the effect at the film ridges and valleys. The surface-MOKE measured with an in-plane field cycled at different angles indicates a distinctive feature: there is only one magnetic easy axis (groove direction) but two directions symmetrically about the hard axis where the coercive field vanishes. This dependence, in agreement with the micromagnetic simulations, is consistent with the formation of a pattern of antiparallel magnetic domains with nanometric periodicity at the remanent magnetization. We have verified the existence of the magnetic pattern with magnetic force microscopy. Our findings offer a novel way to disentangle the magnetization reversal of 3D corrugated materials by performing Kerr magnetometry at different optical regimes.