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Direct determinations of the isothermal entropy increment, -△ST, in the Heusler alloy Ni50CoMn36Sn13 on demagnetization gave positive values, corresponding to a normal magnetocaloric effect. These values contradict the results derived from heat-capacity measurements and also previous results obtained from magnetization measurements, which indicated an inverse magnetocaloric effect, but showing different values depending on the technique employed. The puzzle is solved, and the apparent incompatibilities are quantitatively explained considering the hysteresis, the width of the martensitic transition and the detailed protocol followed to obtain each datum. The results show that these factors should be analyzed in detail when dealing with Heusler alloys. [ABSTRACT FROM AUTHOR]

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 Δ S T and the adiabatic temperature change Δ T ad 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 Δ T ad 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 Δ T ad 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 Δ S T in fast operation comparable to real device conditions which can be applied to every magnetocaloric material with a first-order transition. [ABSTRACT FROM AUTHOR]