Physical and electronic structure and magnetism of Mn2 NiGa: Experiment and density-functional theory calculations

Both experimental and theoretical studies have been carried out to study the structure and magnetic properties of Mn2 NiGa alloys. We have found, instead of forming L 21 structure where both A and C sites are occupied by Mn atoms, the alloy favor a structure where the C site is occupied by Ni atoms and Mn atoms at A and B sites. The electronic structures of both cubic austenite and tetragonal martensite Mn2 NiGa were calculated by self-consistent full-potential linearized-augmented plane-wave (FP-LAPW) method. Austenite Mn2 NiGa materials show ferrimagnetism due to antiparallel but unbalanced magnetic moments of Mn atoms at A and B sublattices. The magnetic moment of Mn atoms decrease greatly upon martensitic transformation to a tetragonal structure with a 50% reduction in Mn moments at the A site and almost completely suppressed Mn moments at B sites. Consequently, martensite Mn2 NiGa alloys show ferromagnetic coupling. Different magnetic orderings in martensite and austenite also lead to very different temperature dependence, with which the abnormal behavior of magnetization upon martensitic transformation can be understood. In the off-stoichiometric samples with composition between Ni2 MnGa and Mn2 NiGa, we show that additional Mn atoms that substitute for Ni atoms in Ni2 MnGa have the same magnetic behaviors as Mn in Mn2 NiGa phase, which successfully explains the dependence of the magnetization on Mn composition.