Magnetic properties of Ni-Mn-Ga ribbon prepared by rapid solidification

Abstract

Magnetic shape memory Ni/sub 49.7/Mn/sub 28.7/Ga/sub 21.6/ alloy was prepared in the form of the thin ribbon by a planar-flow casting method. Optical microscopy and X-ray diffraction were used to check the microstructure of as-quenched and annealed ribbons (800C/24 h and 72 h). The as-quenched ribbon was microcrystalline with grain size about 1.5-3 /spl mu/m which after annealing increased to about 40 /spl mu/m. Annealing also increased the martensitic transformation and Curie temperatures. The annealed ribbon transformed to martensite at 299 K with a reverse transformation to austenite at about 308 K. Room-temperature martensite had a five-layered modulated structure similar to that observed in bulk material. Magnetization at room temperature, 63 Am/sup 2//kg and Curie temperature, 367 K, were close to that of the master alloy. The magnetization loop of the as-quenched ribbon was very flat due to the high level of quenched-in stresses. The loops of annealed ribbon were round indicating a random distribution of easy magnetization axes. The magnetocrystalline anisotropy constant of the annealed ribbon was K/sub u/=1.6/spl times/10/sup 5/ J/m/sup 3/. Thermoelastic strain due to martensitic transformation was about 0.3%.