%0 Journal Article
%T Local strains, calorimetry, and magnetoresistance in adaptive martensite transition in multiple nanostrips of Ni39+xMn50Sn11 x(x ¡Ü 2) alloys
%A A A Prasanna and Shanker Ram
%J Science and Technology of Advanced Materials
%D 2013
%I Institute of Physics, National Institute for Materials Science
%X Ni39+xMn50Sn11 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 L10 tetragonal crystal structure at room temperature can be formed at a critical Sn content of 9.0 at.% (x = 2), whereas an austenite cubic L21 phase turns up at smaller x ¡Ü 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 ¡Ý 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¨Caustenite (or reverse) phase transition. A net residual enthalpy change ¦¤HM A =  0.12 J g 1 arises in the process that lacks reversibility between the cooling and heating cycles. A large magnetoresistance of (¨C)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 ¦¤HM 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.
%U http://dx.doi.org/10.1088/1468-6996/14/1/015004