Interplay between the reversibility of the inverse magnetocaloric effect and kinetic arrest in Ni-Co-Mn-Sn Heusler alloys

The nature of martensitic phase transitions and its modification as a function of composition and application of magnetic field have been studied in detail for Ni40Co10Mn40Sn10-xGax (x=0, 0.3, 0.4 and 0.5). The phase transition temperature (TM) increases with increasing x. The kinetic arrest (KA) of the austenitic phase has been observed for x=0 and 0.3, which disappeared for x>0.3. The KA results in a decrease in the change of magnetization (ΔM) across TM. However, the isothermal entropy change ΔS varies almost linearly with dTM/d(μ0H)−1 (field-induced shift in TM) irrespective of the composition variation and thermal cycling. This behavior indicates that the variation of dTM/d(μ0H) is the dominant factor determining the magnitude of the ΔS rather than ΔM. Consequently, the effect of KA on ∆M has a negligible influence on ΔS in the studied Heusler alloys. The reduced field-sensitivity in TM associated with the temperature-induced decrease of magnetization in the ferromagnetic austenitic phase leads to a decrease of the thermal hysteresis and an increase of ΔS. As a result, a large reversible (|ΔSrev|=21 J/kg K for Δμ0H=5 T) has been observed for x=0.5, which exceeds previously reported values for Sn-based Heusler alloys.