On the S(T) diagram of magnetocaloric materials with first-order transition: Kinetic and cyclic effects of Heusler alloys

For magnetocaloric materials the total entropy diagram S(T) in different magnetic fields is of central importance. From such a plot the isothermal entropy change ΔST and the adiabatic temperature change ΔTad can be well-defined. Aim of this work is to investigate the reversible magnetocaloric effect of Heusler alloys under cycling in terms of the S(T) diagram. For this purpose we selected three different Heusler alloys of Ni–Mn–In with transition temperatures between 200 k and room temperature. The S(T) diagrams of the three materials cover all conceivable shapes and are therefore representative for inverse magnetocaloric Heusler alloys. First we comprehensively analyzed the magnetocaloric properties of this model system using calorimetry, magnetometry and direct measurements of the adiabatic temperature change ΔTad and subsequently combined those results into the S(T) diagram. It turns out that under certain measurement conditions significant discrepancies can appear. With the help of in-situ optical microscopy, this effect could be attributed to kinetic relaxation effects in the pure martensite phase before the actual ΔTad experiment. Furthermore we found that the S(T) diagram obtained under continuous heating and cooling fails to describe the reversible magnetocaloric properties of minor loops under cycling. Utilizing cyclic measurements of the adiabatic temperature change together with calorimetric data it is however possible to determine the reversible magnetic field induced entropy change ΔST in fast operation comparable to real device conditions which can be applied to every magnetocaloric material with a first-order transition.