We have carried out a direct measurement of the degree of spin polarization ($\mathit{P}$) of the magnetic semiconductor ${\mathrm{G}\mathrm{a}}_{1-x}{\mathrm{M}\mathrm{n}}_x\mathrm{A}\mathrm{s}$ using Andreev reflection spectroscopy. Analyses of the conductance spectra of high transparency ${\mathrm{G}\mathrm{a}}_{0.95}{\mathrm{M}\mathrm{n}}_{0.05}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}$ junctions consistently yield an intrinsic value for $\mathit{P}$ greater than 85%. Our experiments also revealed an extreme sensitivity of the measured spin polarization to the nature and quality of the interface for this material.

• Received 31 January 2003

DOI:https://doi.org/10.1103/PhysRevLett.91.056602