The spin polarization of ferromagnetic alkali-metal iron antimonides $\mathrm{K}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ and $\mathrm{Na}{\mathrm{Fe}}_4{\mathrm{Sb}}_{12}$ is studied by point-contact Andreev reflection using superconducting Nb and Pb tips. From these measurements an intrinsic transport spin polarization $P_t$ of 67% and 60% for the K and Na compound, respectively, is inferred, which establishes these materials as a class of highly spin-polarized ferromagnets. The results are in accord with band structure calculations within the local spin density approximation that predict nearly 100% spin polarization in the density of states. We discuss the impact of calculated Fermi velocities and spin fluctuations on $P_t$.

• Received 23 July 2005

DOI:https://doi.org/10.1103/PhysRevB.72.180407