Thermal strain and magnetization of the ferromagnetic shape memory alloy Ni ₅₂Mn ₂₅Ga ₂₃ in a magnetic field

The objective of this paper is the investigation of the correlations between crystal structure and the magnetization of Ni-Mn-Ga Heusler alloy. Thermal strain, permeability, and magnetization measurements of the ferromagnetic shape memory alloy Ni ₅₂Mn ₂₅Ga ₂₃ were performed across the martensite transition temperature T _M and reverse martensite transition temperature T _R at atmospheric pressure. When cooling from the austenite phase, thermal strain steeply decreases because of martensite transition. Permeability suddenly increases at the Curie temperature T _C=358 K, indicating ferromagnetism, and suddenly decreases to around T _M=328 K. This indicates that the lattice transformation and magnetic phase transition correspond to each other. The percentage of contraction by martensite transition at T _M and in a magnetic field is twice that in zero fields. Considering with other Ni-Mn-Ga alloys, it is supposed that the magnetic field influences the orientation of the easy c-axis along the magnetic field, and then the variant rearrangement occurs, and consequently, the variation in the strain between zero fields and non-zero field is observed. The measurement results indicate that the regions above and below T _M or T _R are the ferromagnetic-austenite (Ferro-A) and ferromagnetic-martensite (Ferro-M) phases, respectively. Magnetic phase diagrams were constructed from the results of the temperature dependence of thermal strain. T _M and T _R increase gradually with magnetic field. T _M shift in magnetic fields (B) around zero magnetic field was estimated as dT _M/dB=0.46 K/T, indicating that magnetization influences martensite transition and the dT _M/dB value is the same as that of Ni ₅₂Mn _12.5Fe _12.5Ga ₂₃, thereby suggesting the Ferro-M to Ferro-A transition.