Spectral signatures of thermal spin disorder and excess Mn in half-metallic NiMnSb

Abstract

Effects of thermal spin disorder and excess Mn on the electronic spectrum of half-metallic NiMnSb are studied using first-principles calculations. Temperature-dependent spin disorder, introduced within the vector disordered local moment model, causes the valence band at the $\hat{I}“$ point to broaden and shift upwards, crossing the Fermi level and thereby closing the half-metallic gap above room temperature. The spectroscopic signatures of excess Mn on the Ni, Sb, and empty sites (Mn${}_{\mathrm{Ni}}$, Mn${}_{\mathrm{Sb}}$, and Mn${}_{\mathrm{E}}$) are analyzed. Mn${}_{\mathrm{Ni}}$ is spectroscopically invisible. The relatively weak coupling of Mn${}_{\mathrm{Sb}}$ and Mn${}_{\mathrm{E}}$ spins to the host strongly deviates from the Heisenberg model, and the spin of Mn${}_{\mathrm{E}}$ is canted in the ground state. While the half-metallic gap is preserved in the collinear ground state of Mn${}_{\mathrm{Sb}}$, thermal spin disorder of the weakly coupled Mn${}_{\mathrm{Sb}}$ spins destroys it at low temperatures. This property of Mn${}_{\mathrm{Sb}}$ may be the source of the observed low-temperature transport anomalies.

5 pages, 7 figures, updated version with minor revisions and an additional figure, accepted in Phys. Rev. B (Rapid Communication)