Structure, magnetism and spin polarization in (Ni sub(1-x)Co sub(x)) sub( 2)MnGa alloys: Unusual composition dependences

The effects of Co substitution on various physical properties of (Ni sub(1-x) Co sub(x)) sub(2) MnGa Heusler alloys are systematically investigated using the density functional theory with generalized gradient approximations. The calculated results show that the martensitic phase vanishes at x [> or =] 0.25. The lattice constant and the energy for phase formation in austenite gradually drop with increasing x. The local magnetic moment decreases monotonously for Mn and Co atoms, while it first increases and then decreases for Ni atom. The maximal total moment of 5.0 mu sub(B) is obtained at x = 0.5. Especially, the maximal spin polarization degree of 91% is reached at x =0.5, implying that the (NiCo)MnGa alloy is a half-metal. Finally, the density-of-states calculations reveal that the Co-Ni spin hybridization slightly opens the pseudo-gap and shifts the majority and minority occupied bands towards the high energy.