Thermal expansion and magnetization studies of the novel ferromagnetic shape memory alloy Ni2MnGa0.88Cu0.12 in a magnetic field

Thermal expansion, permeability and magnetization measurements of the ferromagnetic shape memory alloys, Ni2MnGa0.88Cu0.12, were carried out across the martensitic transition temperature TM and the reverse martensitic transition temperature TR at atmospheric pressure. When cooling from the austenite phase, a steep decrease in the thermal expansion due to the martensitic transition was found. The permeability indicates a sharp peak around TM and ferromagnetism below TM. Considering the permeability and magnetization results of Ni2MnGa0.88Cu0.12, the region above TM or TR is the paramagnetic-austenite phase and the region below TM or TR is the ferromagnetic-martensite phase. Magnetic phase diagrams were constructed based on the results of the temperature dependence of thermal expansion. TM and TR increased gradually with increasing magnetic field. The shift of TM in magnetic fields (B) around zero magnetic fields was estimated as dTM/dB=1.3 K T-1. The shift of TM indicates that magnetization influences the martensitic transition, and the increase of TM with magnetic field is proportional to the difference in the magnetization between the austenite and martensite phases.