Effect of temperature on magnetic properties of Ni-Mn-Ga magnetic shape memory (MSM) alloys

Single crystal Ni48Mn31Ga21, magnetic shape memory alloy exhibits giant 5% field-induced strain (MSM effect) due to redistribution of martensitic variants. Magnetization of this alloy was studied as a function of field and temperature. Austenite-to-martensite transition occurs at 299 K. At room temperature the magnetization loop of the martensitic sample exhibits large transient hysteresis in the first quadrant, which is a direct manifestation of the MSM effect. When cooling from room temperature, the observed changes of magnetization curves suggest that the large MSM effect strongly decreases with temperature and about 50 deg. below the martensitic transformation vanishes entirely. The disappearance is mostly due to decreasing mobility of the martensitic twin boundaries. At 210 K, an additional intermartensitie transition occurs which results in a new phase with low magnetic anisotropy.