Comparative Study of Structural and Magnetic Properties of Bulk and Powder Ni $_{52}$Fe $_{17}$Ga $_{27}$Co $_{4}$ Magnetic Shape Memory Alloy

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 Ni
₅₂
Fe
₁₇
Ga
₂₇
Co
₄
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
₅₂
Fe
₁₇
Ga
₂₇
Co
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
L
2
₁
austenite phase during the subsequent annealing.