Two magnetic entropy peaks during one-step martensitic transformation in a Ni₄₂Co₈Mn_37.5In_12.5 alloy

Abstract

Magnetic shape memory alloys with multiple entropy change (ΔSM) peaks are promising to extend the working temperature window (ΔTeff) in active magnetic refrigeration. They usually have multi-step martensitic transformations, since the multiple variations of structure and magnetic states lead to the one-to-one corresponding ΔSM peaks. In this work, we report an interesting Ni42Co8Mn37.5In12.5 magnetocaloric alloy, which exhibits only one-step martensitic transformation but generates two considerable ΔSM peaks with the value of 7.2 J·K-1·kg-1 and 9.9 J·K-1·kg-1 under 6 T respectively. This phenomenon is attributed to the restriction internal stresses stemming from the lattice mismatch between martensite and austenite, the random local stresses produced by defects, the interfacial hindrances of wide phase boundaries, and the inhibition of dispersed precipitates, which divide the one-step transformation into two discrete stages. Such multiple ΔSM peaks are beneficial for expanding the working temperature window of magnetocaloric refrigerators.