Consecutive magnetic and magnetocaloric transitions in herringbone nanostructured Heusler Mn50Ni41Sn9 alloy

A herringbone nanostructured Mn-rich Heusler Mn50Ni50-Sndelta (8 - 9) alloy exhibits tailored magnetocaloric properties in the martensite and ferro <-> paramagnetic transitions concur in a narrow temperature window. In a Sn --> Ni substitution 8 - 9, the martensite (M) <-- austenite (A) transition up-lifts adequately well above the room temperature - 310.5 K in the DSC thermogram and magnetization scanned with temperature. A noninterrupted heating following a cooling in DSC at a given rate gives a smaller enthalpy change deltaH(M <-- A) - deltaH(M --> A approximately equal to 282 mJ/g (deltaC(P)(M <-- A) - deltaC(P)(M --> A) approximately equal to 0.025 mJ/g-K in the heat capacity), i.e., the M <-> A transition process lacks a complete reversibility. Warming a zero-field cooled sample retains lower magnetization (sigma) at low fields B, e.g., by 58% over the field cooled value at 5 mT, wherein merely low field magnetic susceptibility imparts the magnetization process. A reversible thermal hysteresis thus the transition traces in cooling and heating. The field diminishes difference in two sigma-values progressively, e.g., only - 12% lasts at 5 T. The two curves bifurcate below 160 K (B-5 mT) and the gap grows exponentially over lower temperatures before sigma(M <-- A) gets steady near 60 K in a superparamagnetic (SPM) behavior. The SPM feature (follows the Langevin model) below a paramagnetic regime begins (> or = 250 K) before a ferromagnetic A-state lines-up the successive transitions. Temperature and frequency dependence ac and dc susceptibilities describe the surface spins dynamics.