Investigation of the structural competing and atomic ordering in Heusler compounds FeNiSi and NiFeSi under strain condition.

The structural competing and atomic ordering of the full Heusler compounds FeNiSi and NiFeSi under uniform and tetragonal strains have been systematically studied by the first-principles calculation. Both FeNiSi and NiFeSi have the XA structure in cubic phase and they show metallic band structures and large magnetic moments (greater than 3 ) at equilibrium condition. Tetragonal distortion can further decrease the total energy, leading to the possible phase transformation. Furthermore, different atom reordering behaviours have been observed: for FeNiSi, atoms reorder from cubic XA-type to tetragonal L1-type; for NiFeSi, there is only structural transformation without atom reordering. The total magnetic moments of FeNiSi and NiFeSi are mainly contributed by Fe atoms, and Si atom can strongly suppress the moments of Fe atoms when it is present in the nearest neighbours of Fe atoms. With the applied strain, the distance between Fe and Si atoms play an important role for the magnetic moment variation of Fe atom. Moreover, the metallic band nature is maintained for FeNiSi and NiFeSi under both uniform and tetragonal strains. This study provides a detailed theoretical analysis about the full Heusler compounds FeNiSi and NiFeSi under strain conditions.