# Quadratic magnetooptic Kerr effect spectroscopy of Fe epitaxial films on MgO(001) substrates

## TL;DR

This study investigates the quadratic magnetooptic Kerr effect (QMOKE) in Fe thin films on MgO substrates, providing detailed spectroscopic measurements and theoretical analysis of the QMOKE parameters and their anisotropy.

## Contribution

It introduces a comprehensive QMOKE spectroscopy method for Fe films, extracting complex parameters and comparing experimental results with ab-initio calculations.

## Key findings

- QMOKE spectra reveal detailed anisotropy parameters.
- Experimental spectra are well described by ab-initio calculations.
- The study advances understanding of second-order magnetooptic effects in cubic crystals.

## Abstract

The magnetooptic Kerr effect (MOKE) is a well known and handy tool to characterize ferro-, ferri- and antiferromagnetic materials. Many of the MOKE techniques employ effects solely linear in magnetization $\bm{M}$. Nevertheless, a higher-order term being proportional to $\bm{M}^2$ and called quadratic MOKE (QMOKE) can additionally contribute to the experimental data. Here, we present detailed QMOKE spectroscopy measurements in the range of 0.8 -- 5.5\,eV based on a modified 8-directional method applied on ferromagnetic bcc Fe thin films grown on MgO substrates. From the measured QMOKE spectra, two further complex spectra of the QMOKE parameters $G_s$ and $2G_{44}$ are yielded. The difference between those two parameters, known as $\Delta G$, denotes the strength of the QMOKE anisotropy. Those QMOKE parameters give rise to the QMOKE tensor $\bm{G}$, fully describing the perturbation of the permittivity tensor in the second order in $\bm{M}$ for cubic crystal structures. We further present experimental measurements of ellipsometry and linear MOKE spectra, wherefrom permittivity in the zeroth and the first order in $\bm{M}$ are obtained, respectively. Finally, all those spectra are described by ab-initio calculations.