Polymorphic magnetization behavior has been observed experimentally in the Heusler alloy Mn${}_2$NiGa in which Co has been substituted for Ni or Ga. The magnetization of the austenitic phase can be enhanced up to 132 emu/g, when more than 50$\%$ of the antiferromagnetic couplings between Mn atoms are changed to ferromagnetic couplings at the largest composition tolerance for Co substituting for Ga. The effects of the exchange interaction have been investigated based on the corresponding atomic configuration generated by the occupation selectivity of the doped Co atoms. First-principles calculations indicate that a high level of d-electron hybridization can occur when Mn atoms are the nearest neighbors of a Co atom. This causes a strong ferromagnetic exchange interaction in specific atomic configurations and produces a local ferromagnetic structure in the native ferrimagnetic structure matrix. It has been indicated that, based on theoretical work of Stearns, the interatomic distance plays a critical role in producing the local ferromagnetic structure. This has also been used to explain the magnetization behavior through the martensitic transformation in Mn${}_2$NiCoGa alloys.

• Received 4 May 2011

DOI:https://doi.org/10.1103/PhysRevB.84.224404