Large inverse magnetic entropy changes and magnetoresistance in the vicinity of a field-induced martensitic transformation in Ni_50-x Co_x Mn_32-y Fe_y Ga₁₈

Significantly large inverse magnetic entropy changes (Δ S _M) and magnetoresistance (MR) were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys: Ni_50-x Co_x Mn_32-y Fe_y Ga₁₈. The crystal structures of alloys were tetragonal at 300 K and the phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion. The maximum peak values of the Δ S_M and MR at H=5 T were determined as ≈ (+) 31 J Kg^-1 K^-1 and ≈-21%, respectively, for x=8 and y=2. The relatively small hysteretic loss and large refrigeration capacity observed in this system make these compounds promising materials for applications.