First-principles study of Zn-doping effects on phase stability and magnetic anisotropy of Ni-Mn-Ga alloys

Abstract

The effect of Zn doping on Ni-Mn-Ga magnetic shape memory alloy was studied by the first-principles calculations using exact muffin-tin orbital method in combination with the coherent-potential approximation and projector augmented-wave method. Trends in martensitic transformation temperature T M and Curie temperature T C were predicted from calculated energy differences between austenite and nonmodulated martensite, ΔE A-NM, and energy differences between paramagnetic and ferromagnetic state, ΔE PM-FM. Doping upon the Ga-sublattice results in stabilization of martensitic phase which indicates the increase in T M. T C is affected only weakly or slightly decreases, because ΔE PM-FM of martensite does not change significantly with doping. The substitution of Mn atoms by Zn causes the decrease in both T M and T C. Comparing to Cu-doped Ni-Mn-Ga alloys, we predict that doping with Zn results in smaller decrease in T C but also in smaller increase in T M. Moreover, Cu doping upon the Ga-sublattice strongly decreases the magnetic anisotropy energy of martensite, whereas such strong effect was not observed for Zn doping. Based on the calculations of Zn-doped Ni-Mn-Ga alloys we suggest that simultaneous doping with Zn and an element increasing T C can result in significant increase in both transformation temperatures without strong decrease of magnetic anisotropy.