Comparative Study of Structural and Magnetic Properties of Bulk and Powder Ni $_{52}$ Fe $_{17}$ Ga

The Ni-Fe-Ga-Co system has been proposed as a new magnetic shape memory alloy due to its wide temperature range for martensitic and magnetic transformations (T_m and T_c , respectively) by changing composition. The alloy composition Ni₅₂Fe₁₇Ga₂₇Co₄ is more interesting for practical application due to its relatively high T_m and T_c. Here, martensitic transformations, crystal structures, and magnetic properties of Ni ₅₂Fe ₁₇Ga ₂₇Co ₄ alloys in form of bulk and powder are studied. Bulk samples were prepared by arc-melting followed by annealing at 1473 K for 4 h, while powder samples were filed from this bulk sample and annealed at 673 K for 0-15 h. The martensitic transformation present in the bulk sample totally disappears after powderisation. T_c of the powder sample is about 100 K lower than that of the bulk one. The saturation magnetization of the bulk sample (M_sat = 54 emu/g) is reduced significantly down to 35 emu/g by powderisation, but increased again with increasing annealing time. After annealing the powder for 15 h, the martensitic transformation recovers and T_c is increased close to the level of the bulk sample. From room-temperature X-ray diffraction, it is found that the tetragonal non-modulated martensite in the bulk sample is completely transformed to a disordered fcc phase by powderisation, but gradually changes into a highly ordered L2₁ austenite phase during the subsequent annealing.