Effect of ageing on the structural and magnetic transformations and the related entropy change in a Ni–Co–Mn–Ga ferromagnetic shape memory alloy

The effect of thermal treatments on the structural and magnetic transitions in a Ni42Co8Mn32Ga18 alloy has been studied. The selected composition shows coincidental Curie temperature of the parent phase and martensitic transformation to paramagnetic martensite, thus leading to enhanced magnetization difference between austenite and martensite. Quenching raises the martensitic transformation temperatures, which decrease upon post-quench ageing although an increase is observed for prolonged ageing time. This behaviour is mostly attributed to quenched-in atomic order and its improvement during ageing, the final increase being imputed to a process of elimination of defects occurring at the same time than ordering or to an early stage of precipitation. As the austenite Curie temperature increases with ageing, the structural and magnetic transitions shift in a different way modifying their relative position. On its turn, the transformation entropy change decreases with increasing ageing time, and its evolution can be correlated with the different magnetic order of the austenite when it undergoes the martensitic transformation, as it is modified by ageing.

Research highlights
► Ni42Co8Mn32Ga18 alloy shows martensitic transformation (MT) from ferromagnetic austenite to paramagnetic martensite.
► Water quench and post-quench ageing shift in opposite way the MT and austenite Curie temperatures of the studied alloy.
► The transformation entropy changes for both the forward and reverse MTs decrease with increasing ageing time, although showing different (absolute) values.
► The evolution and relative values of the transformation entropy changes are interpreted in terms of the interaction between the structural and the magnetic transitions.