Correlation between entropy diagrams and magnetocaloric parameters in ferromagnetic shape memory alloys using quasi-static and dynamic techniques

Resumen del trabajo presentado a la 20th International Conference on Solid Compounds of Transition Elements, celebrada del 11 al 15 de abril de 2016 en Zaragoza (España).The magnetocaloric (MC) properties of three Heusler alloys with compositions close to Ni50Mn35In15 have been analyzed using calorimetry, magnetometry, and direct measurements of the adiabatic temperature change ΔTad. Entropy curves S(T) at different fields on heating and cooling have been derived from quasi-adiabatic calorimetric measurements. The MC parameters, adiabatic temperature change ΔTad and isothermal entropy change ΔST under magnetic field changes, have been obtained from the entropy curves, from M(T) measurements at different fields using the Maxwell relations, and from direct measurements of ΔT in two different installations using quasi-static and dynamic processes. The three compounds show a first-order transition with high entropy content from paramagnetic martensite to ferromagnetic austenite at different temperatures between 240 K and 300 K, in addition to a second-order transition to paramagnetic state at near 314 K for all the compounds. The thermal hysteresis and the width of the structural transition have similar values for each compound, but differ widely between them in a range between 15 K and 3 K. The compounds present a giant magnetocaloric effects with maximum values of ΔST up to 20 J/(kg·K) and ΔTad up to 5.3 K for a field change of 2 T. Nevertheless, these values decrease sharply on closed cyclic loops due to the thermal hysteresis. The results from magnetization and from quasi-static calorimetric measurements gave similar results for ΔTad and ΔST. Nevertheless, conflicting results have been obtained in some case for the ΔTad values using different heating velocities and different rates in the field application. These results depend on the heating rate used within the martensite phase and not on the rate of the field application to transform from martensite to austenite. This effect has been ascribed to residual stresses and incomplete reorientations of the martensite variants. The differences between full transformation cycles and dynamic partial transformation cycles are crucial for the usefulness of MC materials with first-order transitions in cooling applications.Peer Reviewe