Martensitic transition and magnetoresistance in a Cu-Al-Mn shape-memory alloy: Influence of ageing

We have studied the effect of ageing within the miscibility gap on the electric, magnetic, and thermodynamic properties of a nonstoichiometric Heusler Cu-Al-Mn shape-memory alloy, which undergoes a martensitic transition from a bcc-based structure $(\ensuremath{\beta}$ phase) towards a close-packed structure $(M$ phase). Negative magnetoresistance that shows an almost linear dependence on the square of magnetization with different slopes in the M and $\ensuremath{\beta}$ phases was observed. This magnetoresistive effect has been associated with the existence of Mn-rich clusters with the ${\mathrm{Cu}}_{2}\mathrm{AlMn}$ structure. The effect of an applied magnetic field on the martensitic transition has also been studied. The entropy change between the $\ensuremath{\beta}$ and M phases shows negligible dependence on the magnetic field, but it decreases significantly with annealing time within the miscibility gap. Such a decrease is due to the increasing amount of ${\mathrm{Cu}}_{2}\mathrm{MnAl}$-rich domains that do not transform martensitically.