The effect of magnetic ordering on mechanical twinning in Mn-excess Ni-Mn-Ga martensite: an ab initio study

This study investigates the effect of magnetic ordering and Mn content on twin boundary (TB) energetics in Ni-Mn-Ga ferromagnetic shape memory alloys using ab initio calculations. Focusing on the (101)[101¯] twinning system in non-modulated (NM) martensite, we analyze generalised planar fault energy (GPFE) curves under various magnetic and compositional configurations. Our results show that the energy barrier for twin nucleation and propagation increases with Mn content when excess Mn atoms substitute Ga sites and adopt antiparallel magnetic moments (denoted further as AFM) alignment, corresponding to the ground state magnetic ordering. This increase is strongly dependent on the local atomic environment, with barriers rising significantly when Mn-excess atoms are located within or near the propagating twin. Furthermore, the otherwise stable two-layer twin becomes destabilised under these conditions. In contrast, parallel magnetic moments (denoted further as FM) alignment of excess Mn lowers the energy barrier, but this configuration is thermodynamically less favourable. The enhanced barriers in the AFM ground state are attributed to strong antiferromagnetic interactions between Mn-excess and other Mn atoms, which hinder TB motion. These findings provide insight into the magnetic and compositional tuning of mechanical responses in Ni-Mn-Ga alloys.