Longitudinal magneto-optical Kerr effect in subwavelength thick ferromagnetic films investigated by Mueller matrix ellipsometry

Abstract

Accurately characterizing the magneto-optical (MO) Kerr and optical responses of subwavelength thick ferromagnetic films is of great significance in the field of MO devices. In this paper, an in-situ characterization method based on the Mueller-matrix ellipsometry in a longitudinal MOKE configuration has been proposed, with which the longitudinal MO Kerr effects phenomenon occurring in the sub-wavelength thick ferromagnetic films can be thoroughly investigated. We establish an analytical representation of the MO Kerr responses by simplifying the transfer-matrix model using a more appropriate constraint about the film thickness. By virtue of the high repeatability precision of the measured Mueller matrix better than 0.0002, the MO Kerr response signal of the ferromagnetic films with thicknesses far from the ultra-thin limit 2π|N|∙d/λ<<1 can be reliably captured, despite the well-known weak signal amplitudes. The MO coupling constant, the Kerr ellipticities and rotations can be further determined from the measured Mueller matrix. Experimental results on the Ni and NiFe2O4 films with thicknesses less than 100 nm demonstrate the proposed characterization approach based on the Mueller matrix ellipsometry as a practical and precise technique. As the film thickness increases to larger than the ultrathin film threshold, the proposed method can provide more reasonable characterization results, and the influence of film thickness on the MO responses can be more accurately evaluated.