The Effect of Fe on The Phase Transformation and Magnetic Property of Ni-Mn-Ga Powders Prepared by Ball Milling

The Ni₅₂Mn₂₄Ga₂₄ alloy powders were prepared by dry ball milling method in a planetary ball mill. The ball to powder weight ratio was 10: 1 and the milling speed was 500 rpm/min. The alloy was ball milled for 4 h in the steel vial with hardened steel balls in a vacuum condition. After ball milling, the powders were annealed at 300 ^oC, 400 ^oC, 600 ^oC and 800 ^oC for 1 h. The microstructure, phase transformation, crystal structure and magnetic property of the powders were characterized by SEM, DSC, ac-susceptibility, XRD and PPMS, respectively. It was found that, the as-milled Ni-Mn-Ga powders exhibited flake shape with thickness of ~1 μm, and the EDS showed that the Fe element from the steel vial and balls was introduced into the milled powders during ball milling. The room temperature XRD results showed that the diffraction peaks belonging to Fe were not detected and only a pure Heusler austenite phase existed in the 800 ^oC annealed powders, which indicated that the Fe introduced in the ball milling process had entered into the Ni-Mn-Ga alloy and formed a new structure with the alloy. Compared with the original bulk alloy, the martensitic transformation temperature and Curie temperature of 800 ^oC annealed powders were reduced ~ 30 K and increased ~ 25 K, respectively, which should be related to the introduction of Fe element in the Ni-Mn-Ga alloy. The PPMS showed that the 800 ^oC annealed powders exhibited the similar saturation magnetization to the original bulk alloy at martensite and austenite state.