PUB - Publikationen an der Universität Bielefeld

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- x Ga-x (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 (OM) across T M . However, the isothermal entropy change OS varies almost linearly with [dT(M)/d (mu 0H)]- 1 (field-induced shift in T (M) ) irrespective of the composition variation and thermal cycling. This behavior indicates that the variation of dTM/d(mu 0H) is the dominant factor determining the magnitude of the OS rather than OM. Consequently, the effect of KA on OM has a negligible influence on OS in the studied Heusler alloys. The reduced field-sensitivity in T (M) 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 OS. As a result, a large reversible (|OSrev|=21 J/kg K for O mu 0H =5 T) has been observed for x =0.5, which exceeds previously reported values for Sn-based Heusler alloys.