Role of shuffles and atomic disorder in Ni-Mn-Ga

TL;DR

This study uses ab-initio calculations to explore how atomic shuffles and disorder stabilize the low-temperature tetragonal phase of Ni-Mn-Ga, revealing the importance of phonon-related atomic displacements and electronic structure effects.

Contribution

It demonstrates that atomic shuffles and disorder are crucial for stabilizing the tetragonal phase, highlighting the role of phonon coupling and electronic density of states in Ni-Mn-Ga.

Findings

Atomic shuffles form wave-like patterns related to phonon modes.

Atomic disorder also stabilizes the tetragonal structure.

A dip in the minority-spin DOS at the Fermi level is associated with stability.

Abstract

We report results of \textit{ab-initio} calculations of the ferromagnetic Heusler alloy Ni-Mn-Ga. Particular emphasis is placed on the stability of the low temperature tetragonal structure with $c/a = 0.94$. This structure cannot be derived from the parent L2$_1$ structure by a simple homogeneous strain associated with the soft elastic constant $C'$. In order to stabilise the tetragonal phase, one has to take into account shuffles of atoms, which form a wave-like pattern of atomic displacements with a well defined period (modulation). While the modulation is related to the soft acoustic [110]-TA$_2$ phonon mode observed in Ni$_2$MnGa, we obtain additional atomic shuffles, which are related to acoustic-optical coupling of the phonons in Ni$_2$MnGa. In addition, we have simulated an off-stoichiometric systems, in which 25 % of Mn atoms are replaced by Ni. The energy of this structure also exhibits a local minimum at $c/a = 0.94$. This allows us to conclude that both shuffles and atomic disorder stabilize the $c/a = 0.94$ structure. In both cases the stability seems to be associated with a dip in the minority-spin density of states (DOS) at the Fermi level, being related to the formation of hybrid states of Ni-\textit{d} and Ga-\textit{p} minority-spin orbitals.