Effect of Nb doping on electronic and magnetic properties of half-metallic CoMnSb semi-Heusler compound from first-principles calculations

We investigate the effect of less 3d electrons on the electronic and magnetic properties of half-Heusler alloys CoMn1-xNbxSb in a wide concentration range using first-principles density-functional calculations within the generalized gradient approximation. Our calculations indicate that for x < 0.5 the doped compounds are half-metallic ferromagnets with the magnetic moments obeying Slater-Pauli rule quite well, while for the larger x the doped compounds become metallic and incompletely compensated ferrimagnets, and finally for x = 1 the corresponding compound is nonmagnetic metal. The local moment of Mn is insensitive to the x concentration as well as the magnetic interaction, revealing the localized nature of the magnetism due to the large exchange splitting of Mn-d states. The total conduction electron spin polarization of Co or Nb and Sb atoms is always negative but drop off slowly with increasing x, suggesting that the ferromagnetic coupling is governed by the ferromagnetic RKKY-type exchange mechanism, while the antiferromagnetic superexchange mechanism becomes important only for x similar to 1, giving rise to the unstable ferromagnetism for x similar to 1. The Curie temperature as well as ferromagnetic intensity decrease with increasing x, originating from the fact that the shift of the Fermi level from the top of the valence band to the bottom of the conduction band leads to the increase of the number of states just above the Fermi level and consequently the superexchange mechanism is more efficient. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim