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

TL;DR

This study investigates how the thickness of ultrathin Fe(001) films influences linear and quadratic magneto-optical Kerr effects, providing detailed parameter values for accurate material characterization across different film thicknesses.

Contribution

It offers the first comprehensive measurement and analysis of both linear and quadratic MO parameters in ultrathin Fe films, revealing thickness-dependent variations.

Findings

Parameters n, K, G vary significantly below 10nm thickness.

Quadratic MOKE effect is about a third of that in Co2FeSi.

Surface and electronic effects influence MO properties at small thicknesses.

Abstract

Magneto-optical Kerr effect (MOKE) magnetometry is one of the most widely employed techniques for the characterization of ferromagnetic thin-film samples. Some information, such as coercive fields or anisotropy strengths can be obtained without any knowledge of the optical and magneto-optical (MO) properties of the material. On the other hand, a quantitative analysis, which requires a 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 of Fe are completely unknown. Here, we report on measurements of the thickness dependence (0-60nm) of the linear and quadratic MOKE in epitaxial bcc-Fe(001) wedge-type samples performed at a commonly used laser wavelength of 670nm. By fitting the thickness dependence we are able to extract a complete set of parameters n, K, (G11 - G12), and G44 for the quantitative description of the MOKE of bcc-Fe(001). We find sizable different n, K, and G parameters for films thinner than about 10nm as compared to thicker films, which is indicative of a thickness dependence of the electronic properties or of surface contributions to the MOKE. The effect size of the quadratic MOKE is found to be about a third of the record values recently reported for Co2FeSi.