Thickness dependence of linear and quadratic magneto-optical Kerr effects in ultrathin Fe(001) films

Magneto-optical (MO) Kerr effect (MOKE) magnetometry is one of the most widely employed techniques for the characterization of ferromagnetic thin-film samples. Some information, such as the magnitude of coercive fields or anisotropy strengths, can be readily obtained without any knowledge of the optical and MO properties of the material. On the other hand, a quantitative analysis, which requires precise knowledge of the material's index of refraction n and the MO-coupling constants K and G, is often desirable, for instance, for the comparison of samples which are different with respect to ferromagnetic layer thicknesses, substrates, or capping layers. While the values of the parameters n and the linear MO-coupling parameter K reported by different authors usually vary considerably, the relevant quadratic MO-coupling parameters G even for Fe are completely unknown. Here, we report on measurements of the thickness dependence (0-60 nm) of the linear and quadratic magneto-optical effects in epitaxial bcc-Fe(001) wedge-type samples performed at a commonly used laser wavelength of 670 nm. By fitting the thickness dependence we are able to extract a complete set of parameters n, K, (G(11)-G(12)), and G(44) for the quantitative description of the MOKE response of bcc-Fe(001). We find the parameters n, K, and G to significantly differ for films thinner than about 10 nm as compared to those for thicker films, which is indicative of a thickness dependence of the electronic properties or of surface contributions to the MOKE. The magnitude of the quadratic magneto-optical effect is found to be about one-third of the record values reported recently for Co2FeSi.