Factors influencing the martensitic transformation in Ni-Mn-Z (Z = Ga, Sn) Heusler alloys

The martensitic transition temperature of the Ni-Mn-Z (Z = Ga, In, Sn or Sb) generally scales with the average valence electron per atom ( e/a ) ratio. The e/a ratio can be increased either by increasing Ni content in lieu of Mn or Z (Ni-rich) or by substituting Mn for Z (Mn-rich). While the Mn-rich alloys display the expected scaling behaviour of the martensitic transition temperature, the Ni-rich alloys, with the exception of Z = Ga alloys, do not exhibit any dependency on the e/a ratio. By carefully studying and comparing the structure, local structure and the physical properties of the Ni-rich and Mn-rich Ni-Mn-Ga and Ni-Mn-Sn alloys, the cause of the difference in behaviour is identified. The off-stoichiometry brought about by increasing the Ni or Mn content, results in the formation of a new local structural environment in addition to the Heusler-like environment. If either of the two local structural environments is martensitic, then the resulting Ni-rich or Mn-rich alloy also exhibits martensitic transformation.