A first-principles investigation of tetragonal and orthorhombic deformations in the ferromagnetic Heusler alloy Ni$_2$MnGa

Tetragonal and orthorhombic deformations in Ni$_2$MnGa were studied by using the Density Functional Theory formalism with US GGA pseudopotentials implemented in the VASP code in order to find an optimal geometry of the low-temperature structure. Structural and magnetic parameters of the initial L2$_1$ structure and elastic constants are in a good agreement with experimental data and previous calculations. Not all of the low-temperature martensitic structures could be reproduced by applying the tetragonal and the orthorhombic deformations. It was argued that the modulation arising from shuffling of atoms is important for these structures. A simple kind of shuffling was studied. That gives experimentally predicted structural instability associated with a soft phonon mode, but does not stabilize any new structures