Ab initio electronic and magnetic properties of half-metallic NiCrSi and NiMnSi Heusler alloys: The role of defects and interfaces

Using state-of-the-art first-principles calculations, we study the properties of the ferromagnetic Heusler compounds NiYSi, where Y stands for V, Cr, or Mn. NiCrSi and NiMnSi contrary to NiVSi are half-metallic at their equilibrium lattice constant exhibiting integer values of the total spin magnetic moment, and thus we concentrate on these two alloys. The minority-spin gap has the same characteristics as for the well-known NiMnSb alloy being around similar to 1 eV. Upon tetragonalization, the gap is present in the density of states even for expansion or contraction of the out-of-plane lattice parameter by 5%. The Cr-Cr and Mn-Mn interactions make ferromagnetism extremely stable and the Curie temperature exceeds 1000 K for NiMnSi. Surface and interfaces with GaP, ZnS, and Si semiconductors are not half-metallic but, in the case of NiCrSi, the Ni-based contacts present spin polarization at the Fermi level around 90%. Finally, we show that there are two cases of defects and atomic-swaps. The first ones, which involve the Cr (Mn) and Si atoms, induce states at the edges of the gap, which persists for a moderate concentration of defects. Defects involving Ni atoms induce states localized within the gap completely destroying the half-metallicity. Based on single-impurity calculations, we associate these states to the symmetry of the crystal. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3005882]