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 $\Gamma$ 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.