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

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.