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

We report measurements of the spin polarization $(\mathbit{P})$ of the concentrated magnetic semiconductor $\mathrm{EuS}$ using both zero-field and Zeeman-split Andreev reflection spectroscopy (ARS) with $\mathrm{EuS}∕\mathrm{Al}$ planar junctions. The zero-field ARS spectra are well described by the modified (spin-polarized) Blonder-Tinkham-Klapwijk (BTK) model with expected superconducting energy gap and actual measurement temperature (no additional spectral broadening). The fittings consistently yield $\mathbit{P}$ 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 $\mathbit{P}$ 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.