Entropy change reversibility in MnNi1-xCoxGe0.97Al0.03 near the triple point

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 a(hex) 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 (vertical bar Delta S-rev vertical bar = 6.9 J kg(-1) K for Delta mu H-0 = 2 T) at the TP.

Automated summary

The nature of phase transition and the reversibility of the magnetocaloric effect have been studied in MnNi1-xCoxGe0.97Al0.03. The introduction of Al substitution lowers the structural phase transition temperature, resulting in a coupled first-order magnetostructural transition. A composition-dependent triple point has been observed in the system, where the first-order phase transition splits into an additional phase boundary with a second-order transition character at higher temperature. The reversibility of the low-field entropy change is enhanced at the triple point due to lattice softening and larger field-induced structural entropy change.