Ageing effects on structural and magnetic transformations in a Ni–Co–Mn–Ga alloy

Abstract

Partial substitution of Ni by Co in Mn-rich Ni–Mn–Ga alloys has been found to modify the magnetic ordering of the phases, improving in this way the possibility to obtain large magnetization difference between austenite and martensite, an essential requirement to induce the martensitic transformation by application of a magnetic field. Particularly, Ni50−xCoxMnyGa50−y alloys undergo, for Co content below x=9, structural transformation between ferromagnetic austenite and paramagnetic martensite, thus leading to enhanced magnetization difference values.The martensitic transformation temperatures as well as the martensite and austenite Curie temperatures depend on composition, but significant changes can be brought about by selected thermal treatments. In this work, the composition is chosen as Ni42Co8Mn32Ga18 in order to obtain concurrent martensitic transformation and austenite Curie temperature, and the effect of quench and subsequent ageing on the structural and magnetic transitions is studied. Aside from the monotonic transformation temperatures change, which is mostly attributed to atomic ordering taking place upon post-quench ageing, the results show the effect of the relative position of the structural and magnetic ordering reactions on the transformation entropy change.