First principles study of Co and Ni based half Heusler alloys

Abstract

The structural stability, electronic structure, mechanical, magnetic and thermal properties of Co and Ni based half-Heusler (HH) alloys are studied. The most stable phase for CoCrSb, NiCrSb, NiCrBi, CoMnSb, CoMnBi and CoCrBi alloys is found to be γ-phase, whereas α-phase is the most stable phase for NiMnBi and NiMnSb among the three different phases (α, β, γ). As the pressure is increased, structural phase transition from γ-phase to α-phase is predicted for NiCrSb, CoCrBi, NiCrBi, CoMnBi, γ-phase to β-phase for CoMnSb and α-phase to β-phase for NiMnSb, at the pressures of 66.96 GPa, 16.91 GPa, 92.50 GPa, 14.58 GPa, 79.23 GPa and 71.47 GPa respectively. Density of states plot shows that there is no energy gap in spin-up state of all these HH alloys and in the spin-down state, the valence and conduction states are separated by a semiconducting energy gap. Thus, these compounds are half metallic. The spin polarized calculations predicted that these Heusler alloys are ferromagnetic nature. The total magnetic moment per unit cell (μcell) is found to be closer to the integer value of 2 μB, 3 μB and 4 μB. The 100% spin polarization at EF confirms the half-metallic nature of these materials. The dynamical stability of novel HH alloys has also been investigated by computing phonon density of states and it shows that these alloys (except CoCrBi and CoMnBi) are dynamically stable.