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 firstprinciples calculations using exact muffin-tin orbital method in combination with the coherentpotential 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.

Experimental Observations versus First‐Principles Calculations for Ni–Mn–Ga Ferromagnetic Shape Memory Alloys: A Review

“…The increased stability of NM martensite was also observed for Ni‐rich alloys by Zayak et al [ 95 ] and other authors. [ 62,147 ] The validity of this approach for predicting was also confirmed for alloys with Fe and Co, [ 145,146 ] Al, [ 97 ] Pt, [ 61 ] Cu, [ 148 ] Zn, [ 124,149 ] Ti, [ 150 ] B, [ 122 ] and Co+Cu. [ 151 ] Some predictions without experimental comparisons are also available for Bi [ 150 ] and Mg alloying.…”

“…Such CPA-based analyses are available for off-stoichiometric alloys, [154] as well as alloys with additional Fe, Co, Cu, [155] Al, [168] and Zn. [124] Calculations based on supercells have shown that the excess Mn atoms in the Ga sublattice prefer to be adjacent to each other, [118,121] but such configurations are hardly accessible at low temperatures due to the slow diffusion of Mn atoms. [110] In calculations, it is typically assumed that stoichiometric Ni 2 MnGa exhibits a perfect atomic order.…”

“…[ 80,122 ] However, the stability of individual phases may differ in true paramagnetic states described by the CPA and the approximate nonmagnetic state, as illustrated in the study by Zelený et al [ 123 ] Qualitative predictions of changes in caused by alloying can be based simply on the total energy difference between the paramagnetic and ferromagnetic states, as shown for doping by Co, Cu, [ 123 ] and Zn. [ 124 ] Calculations for noncolinear arrangements, such as spin spirals, can also be used to estimate or magnon dispersion, [ 125 ] and more precise quantitative predictions of can be performed if the magnetic exchange interaction parameters are exactly computed, which is possible for a wide composition range; [ 102,126–129 ] these results can serve as an input for Monte Carlo or spin dynamics simulations. [ 130,131 ] When compared with the experimental data, the ab initio predictions of agree well for Mn‐rich alloys, [ 131 ] as well as for Pt‐ [ 61 ] or Fe‐doped [ 132 ] alloys.…”

“…Enkovaara et al [ 135 ] suggested that the strong MCA originates from the nonsymmetric surrounding of Ni atoms with a maximum in stoichiometric [ 135,136 ] . Although most of the works have focused only on stoichiometric alloys, [ 60,135–138 ] results for Mn‐excess and Co‐, Cu‐, [ 123 ] Zn‐, [ 124 ] or Pt‐doped alloys [ 139 ] are available in the literature, which predicted a decreased MCA with an increasing concentration of the alloying element in all cases. This trend was confirmed experimentally for Co‐ and Cu‐alloyed NM martensite [ 140 ] and partially for Fe‐alloyed NM martensite.…”

Temperature Dependence of Anisotropy in Ti and Gd Doped NiMnGa-Based Multifunctional Ferromagnetic Shape Memory Alloys

“…The substantial hysteresis between the temperature of the martensitic transformation (during cooling) and the austenitic transformation (during heating) was very characteristic for the first-order temperature-driven transformation, especially in polycrystalline materials where the transition takes place in every single grain. Moreover, the martensitic transformation temperature was strongly dependent from the Gibbs free energy difference between the austenite and martensite phases and the addition of alloying elements may have significantly changed these energies [ 36 , 37 , 38 ]. In order to compare the transformation temperatures between the studied alloys, the average value of all temperatures ( M s , M f , A s , A f ) was calculated as: where T M is structural transformation temperature, M s and M f are martensitic transformation start and finish temperature, and A s and A f are austenitic transformation start and finish temperature.…”