Temperature dependence of the magnetization of the [Ni.sub.52][Mn.sub.24][Ga.sub.24] alloy in various structural states

Results are presented of a study of the temperature dependence of the magnetization σ(T) of the polycrystalline [Ni.sub.52][Mn.sub.24][Ga.sub.24] alloy in various structural states: in the initial coarse-grained state, after severe plastic deformation by high pressure torsion, and after stepped annealing of the deformed specimen at temperatures from 200 to 700°C for 30 min. As a study of the σ(T) curve shows, in an alloy possessing a coarse-grained initial structure, a martensitic phase transition and a magnetic phase transition are observed in the room temperature interval. The martensitic transformation takes place in the ferromagnetic state of the alloy. This transformation is accompanied by an abrupt lowering of the magnetization of the material, associated with a lowering of the symmetry of the crystalline lattice and a high value of the magnetocrystalline anisotropy constant of the alloy in the martensitic phase. It is shown that as a result of plastic deformation there takes place a destruction of ferromagnetic order and a suppression of the martensitic transformation. Consecutive annealing after deformation leads to a gradual recovery of ferromagnetic order and growth of the magnetization of the material. Recovery of the martensitic transformation begins to be manifested only after annealing of the alloy at a temperature of 500°C, when the mean grain size in the recrystallized structure reaches a value around 1 µm. Keywords: ferromagnetic shape-memory alloys, Heusler alloys, martensitic transformation, severe plastic deformation.