Measurement of the spin polarization of the magnetic semiconductor EuS with zero-field and Zeeman-split Andreev reflection spectroscopy

TL;DR

This study measures the spin polarization of EuS using zero-field and Zeeman-split Andreev reflection spectroscopy, developing a theoretical model to determine both magnitude and sign of polarization, consistent across methods.

Contribution

It introduces a theoretical model incorporating Maki-Fulde equations into BTK analysis for Zeeman-split ARS, enabling determination of spin polarization's magnitude and sign.

Findings

EuS exhibits ~80% spin polarization in ARS measurements.

The developed model accurately describes Zeeman-split ARS spectra.

Results are consistent between zero-field and Zeeman-split ARS methods.

Abstract

We report measurements of the spin polarization (\textbf{\textit{P}}) of the concentrated magnetic semiconductor EuS using both zero-field and Zeeman-split Andreev reflection spectroscopy (ARS) with EuS/Al planar junctions. The zero-field ARS spectra are well described by the modified (spin-polarized) BTK model with expected superconducting energy gap and actual measurement temperature (no additional spectral broadening). The fittings consistently yield \textbf{\textit{P}} close to 80% regardless of the barrier strength. Moreover, we performed ARS in the presence of a Zeeman-splitting of the quasiparticle density of states in Al. To describe the Zeeman-split ARS spectra, we develop a theoretical model which incorporates the solution to the Maki-Fulde equations into the modified BTK analysis. The method enables the determination of the magnitude as well as the sign of \textbf{\textit{P}} with ARS, and the results are consistent with those from the zero-field ARS. The experiments extend the utility of field-split superconducting spectroscopy from tunnel junctions to Andreev junctions of arbitrary barrier strengths.