Local strains, calorimetry, and magnetoresistance in adaptive martensite transition in multiple nanostrips of Ni(39+x)Mn(50)Sn(11−x)(x ⩽ 2) alloys

Ni(39+x)Mn(50)Sn(11−x) (x = 0.5, 1.0, 1.5 and 2) alloys comprise multiple martensite nanostrips of nanocrystallites when cast in small discs, for example, ∼15 mm diameter and 8 mm width. A single martensite phase with a L1(0) tetragonal crystal structure at room temperature can be formed at a critical Sn content of 9.0 at.% (x = 2), whereas an austenite cubic L2(1) phase turns up at smaller x ⩽ 1.5. The decrease in the Sn content from x = 2 to 0.5 also results in a gradual increase in the crystallite size from 11 to 17 nm. Scanning electron microscopy images reveal arrays of regularly displaced multiple martensite strips (x ≽ 1.5) with an average thickness of 20 nm. As forced oscillators, these strips carry over the local strains, magnetic dipoles, and thermions simultaneously in a martensite–austenite (or reverse) phase transition. A net residual enthalpy change ΔH(M↔A) = −0.12 J g(−1) arises in the process that lacks reversibility between the cooling and heating cycles. A large magnetoresistance of (–)26% at 10 T is observed together with a large entropy change of 11.8 mJ g(−1) K(−1), nearly twice the value ever reported in such alloys, in the isothermal magnetization at 311 K. The ΔH(M↔A) irreversibility accounts for a thermal hysteresis in the electrical resistivity. Strain induced in the martensite strips leads them to have a higher electrical resistivity than that of the higher-temperature austenite phase. A model considering time-dependent enthalpy relaxation explains the irreversibility features.