Magnetocaloric effect enhancement driven by intrinsic defects in a Ni45Co5Mn35Sn15 alloy

The influence of mechanically-induced defects on the magnetostructural properties is analyzed in a Ni-Co-Mn-Sn alloy subjected to soft milling and subsequent annealing treatments. It is found that, opposite to what occurs in Ni-Mn-Sn ternary alloys, the annealing treatment affects the magnetic properties in a different way in martensite and in austenite. In particular, the saturation magnetization significantly increases in martensite after annealing whereas just a very slight variation is observed in austenite. This leads to the interesting fact that the presence of microstructural defects, far for worsening, makes the magnetocaloric effect to be higher in the as-milled state than after annealing. This behavior is explained as the result of the combination of the effect of defects on the Mn-Mn distance, the effect of Co on the magnetic exchange coupling between Mn atoms, and the effect of defects on the vibrational entropy change at the martensitic transformation.