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  Samanta, T.; Taake, C.; Bondzio, L.; Caron, L.: Entropy change reversibility in MnNi_1-xCo_xGe_0.97Al_0.03 near the triple point. JPhys Energy 5 (2023), p. 044002/1-11

10.1088/2515-7655/acf957
Open Access Version

Abstract:
The nature of the phase transition has been studied in MnNi1−xCoxGe0.97Al0.03 (x= 0.20–0.50) through magnetization, differential scanning calorimetry and x-ray diffraction measurements; and the associated reversibility in the magnetocaloric effect has been examined. A small amount of Al substitution for Ge can lower the structural phase transition temperature, resulting in a coupled first-order magnetostructural transition (MST) from a ferromagnetic orthorhombic to a paramagnetic hexagonal phase in MnNi1−xCoxGe0.97Al0.03. Interestingly, a composition-dependent triple point (TP) has been detected in the studied system, where the first-order MST is split into an additional phase boundary at higher temperature with a second-order transition character. The critical-field-value of the field-induced MST decreases with increasing Co concentration and disappears at the TP (x= 0.37) resembling most field-sensitive MST among the studied compositions. An increase of the hexagonal lattice parameter ahex near the TP indicates a lattice softening associated with an enhancement of the vibrational amplitude in the Ni/Co site. The lattice softening leads to a larger field-induced structural entropy change (structural entropy change≫ magnetic entropy change, for this class of materials) with the application of a lower field, which results in a larger reversibility of the low-field entropy change (|ΔSrev| = 6.9 J kg−1 K for Δμ0H = 2 T) at the TP.