Both experimental and theoretical studies have been carried out to study the structure and magnetic properties of ${\mathrm{Mn}}_2\mathrm{NiGa}$ alloys. We have found, instead of forming $L{2}_1$ 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 ${\mathrm{Mn}}_2\mathrm{NiGa}$ were calculated by self-consistent full-potential linearized-augmented plane-wave (FP-LAPW) method. Austenite ${\mathrm{Mn}}_2\mathrm{NiGa}$ 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 ${\mathrm{Mn}}_2\mathrm{NiGa}$ 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 ${\mathrm{Ni}}_2\mathrm{MnGa}$ and ${\mathrm{Mn}}_2\mathrm{NiGa}$, we show that additional Mn atoms that substitute for Ni atoms in ${\mathrm{Ni}}_2\mathrm{MnGa}$ have the same magnetic behaviors as Mn in ${\mathrm{Mn}}_2\mathrm{NiGa}$ phase, which successfully explains the dependence of the magnetization on Mn composition.

• Received 16 May 2006

DOI:https://doi.org/10.1103/PhysRevB.74.054435