Quadratic magnetooptic spectroscopy setup based on photoelastic light modulation

TL;DR

This paper introduces a quadratic magnetooptic spectroscopy setup utilizing photoelastic light modulation to analyze second-order magnetooptic effects, especially in antiferromagnetic materials, with a spectral range of 0.8--5.5 eV.

Contribution

The paper presents a novel spectroscopy setup for quadratic MOKE that enables separation of QMOKE spectra using multiple magnetization directions and spectral analysis.

Findings

Successfully developed and tested the QMOKE separation algorithm.

Spectra reveal quadratic MO parameters G_s and 2G_{44} in the second-order permittivity tensor.

Setup covers a spectral range of 0.8--5.5 eV for detailed MO analysis.

Abstract

In most of the cases the magnetooptic Kerr effect (MOKE) techniques rely solely on the effects 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 experimental data. Handling and understanding the underlying origin of QMOKE could be the key to utilize this effect for investigation of antiferromagnetic materials in the future due to their vanishing first order MOKE contribution. Also, better understanding of QMOKE and hence better understanding of magnetooptic (MO) effects in general is very valuable, as the MO effect is very much employed in research of ferro- and ferrimagnetic materials. Therefore, we present our QMOKE and longitudinal MOKE spectroscopy setup with a spectral range of 0.8--5.5\,eV. The setup is based on light modulation through a photoelastic modulator and detection of second-harmonic intensity by a lock-in amplifier. To measure the Kerr ellipticity an achromatic compensator is used within the setup, whereas without it Kerr rotation is measured. The separation of QMOKE spectra directly from the measured data is based on measurements with multiple magnetization directions. So far the QMOKE separation algorithm is developed and tested for but not limited to cubic (001) oriented samples. The QMOKE spectra yielded by our setup arise from two quadratic MO parameters $G_s$ and $2G_{44}$, being elements of quadratic MO tensor $\bm{G}$, which describe perturbation of the permittivity tensor in the second order in $\bm{M}$.