Effect of Magnetic Ordering on the Stability of Ni–Mn–Ga(–Co–Cu) Alloys Along the Tetragonal Deformation Path

Abstract

The influence of magnetic ordering on the stability of Ni-Mn-Ga(-Co-Cu) Heusler alloys is investigated using the first-principles exact muffin-tin orbital method in combination with the coherent-potential approximation. The paramagnetic state is described by disordered local moment approach. In stoichiometric Ni 2 MnGa alloy, the total energy profile along the tetragonal deformation path differs between ferromagnetic ground state and paramagnetic state with high energy, where cubic structure of austenite exhibits lower total energy than tetragonally distorted structure of martensite. Martensitic structure is stabilized in ground state by ferromagnetic interaction. In paramagnetic state it can be stabilized by partial substitution of Ni by Co or by partial substitution of Mn/Ga by Cu. Energy difference between paramagnetic and ferromagnetic state ∆E PM-FM can be used for qualitative estimation of Curie temperature T C . Since Co doping to Ni sublattice slightly increases ∆E PM-FM , the T C should also increase, which corresponds to experimental findings. Analogically, Cu doping to Mn sublattice strongly decrease ∆E PM-FM , which corresponds to strong decreasing of T C also confirmed experimentally. For Cu doping in Ga sublattice the decrease in T C is weaker.

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

“…% of Zn are shown in figure 2(b), both magnetic states are related to the energy of the FM austenite of a given composition. The energy profile of Ni 2 MnGa in PM state exhibits a global minimum at c/a=1, whereas NM martensite is represented by a very shallow minimum with higher energy at (c/a) NM =1.14 in this magnetic state [34]. Doping on Mn sublattice has an even stronger suppressing effect on PM martensite and simultaneously lowers the austenite energy minimum, which predicts lower stability of FM states.…”

“…PM FM , has been established for Co and Cu-doping [24,33,34]. Both transformation temperatures can be theoretically predicted by more precise methods including phonon [35] or Monte Carlo [36] calculation, however, with much higher computational cost especially in the case of doped alloys.…”

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

“…A proper description of paramagnetic states with randomly oriented magnetic moments is difficult to achieve in ab initio models; thus, it is often approximated by all atoms having zero magnetic moments. [ 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.…”

“…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.…”