Influence of annealing and phase decomposition on the magnetostructural transitions in Ni_(50)Mn_(39)Sr_(11)

摘要

Magnetic and structural transitions in the Ni_(50)Mn_(50-x)Sn_x (x= 10-25) ferromagnetic shape memory alloys are currently of interest. As in Ni-Mn-Ga, these alloys feature high-temperature austenite and low-temperature martensite phases, where the magnetic state is strongly composition dependent. To study the role of chemical ordering in fine-tuning their magnetostructural properties, they were first annealed for 4 weeks/1223 K to achieve structural and compositional homogeneity, and were then further annealed for 1 week (～150 K below the reported B2 to L2_1 transition) at 773 K to increase the degree of chemical ordering. For x=11, this anneal resulted in a dramatic change in the magnetic ordering temperature. Following the 1223 K anneal, the sample exhibited ferromagnetic ordering at 140 K. After the 773 K anneal, the ferromagnetic transition is at 350 K, a characteristic of the ferromagnetic austenite phase with 15 < x < 25. Consistent with the magnetization data, transmission electron microscopy examination confirms that the alloy decomposed into two phases with x=20 and 1. From this result one can conclude that the martensitic transformation occurs only in those compositions where the single phase L2_1 has been retained in a metastable state on cooling.