title: Local strains, calorimetry, and magnetoresistance in adaptive martensite transition in multiple nanostrips of Ni39+xMn50Sn11-x(x⩽ 2) alloys.
abstract: 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 aL10tetragonal crystal structure at room temperature can be formed at a critical Sn content of 9.0 at.% (x= 2), whereas an austenite cubicL21phase turns up at smallerx⩽ 1.5. The decrease in the Sn content fromx= 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 ΔHM↔A= -0.12 J g-1arises 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-1K-1, nearly twice the value ever reported in such alloys, in the isothermal magnetization at 311 K. The ΔHM↔Airreversibility 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.
authors.affiliation: Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, India.
authors.forename: A A
authors.initials: AA
authors.lastname: Prasanna
authors.affiliation: Materials Science Centre, Indian Institute of Technology, Kharagpur 721 302, India.
authors.forename: Shanker
authors.initials: S
authors.lastname: Ram
keywords: 10.09; Heusler alloys; magnetocaloric effect; magnetoresistance; martensite transition