Thermal strain and magnetization of the ferromagnetic shape memory alloy Ni52Mn25Ga23 in a magnetic field

Abstract

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 Ni52Mn25Ga23 were performed across the martensite transition temperature TM and reverse martensite transition temperature TR at atmospheric pressure. When cooling from the austenite phase, thermal strain steeply decreases because of martensite transition. Permeability suddenly increases at the Curie temperature TC=358K, indicating ferromagnetism, and suddenly decreases to around TM=328K. This indicates that the lattice transformation and magnetic phase transition correspond to each other. The percentage of contraction by martensite transition at TM 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 TM or TR 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. TM and TR increase gradually with magnetic field. TM shift in magnetic fields (B) around zero magnetic field was estimated as dTM/dB=0.46K/T, indicating that magnetization influences martensite transition and the dTM/dB value is the same as that of Ni52Mn12.5Fe12.5Ga23, thereby suggesting the Ferro-M to Ferro-A transition.