Martensitic transformation and magnetocaloric effect in Mn<sub>50</sub>Ni<sub>32</sub><sub>–</sub><sub><italic>x</italic></sub>Cu<sub><italic>x</italic></sub>Co<sub>8</sub>Ti<sub>10</sub> Heusler alloy ribbons

The effects of Cu substitution and annealing on the martensitic transformation behavior and magnetocaloric properties in all-d-metal Mn₅₀Ni_32–xCu_xCo₈Ti₁₀ (x=1, 2) alloy ribbons have been mainly investigated. After the substitution, it is observed that the martensitic transformation temperature of the alloys, which was extremely sensitive to the Cu content, was reduced. Moreover, the ferromagnetism of austenite increased, increasing the magnetization difference across the martensitic transformation. The magnetic-field-induced metamagnetic martensitic transformation also occurred. The magnetic entropy change gradually increased, and the effective refrigeration capacity increased by more than twice than that of the Cu-free substitution. Moreover, the sample with x=1 was utilized as the research object to study the effect of annealing on the martensitic transformation. After annealing, the martensitic transformation became slow and the austenite ferromagnetism decreased. The change in the magnetization across the phase transition was also reduced. The magnetic-field-induced metamagnetic martensitic transformation was also weakened. Consequently, the magnetic entropy change was greatly reduced. However, the effective refrigeration capacity did not reduce significantly because the refrigeration temperature interval increased largely. In this paper, the physical mechanisms of the effects of Cu substitution and annealing on the martensitic transformation were discussed from the change in the d-d interaction and the precipitation in the γ-phase, respectively.