Impact of demagnetising fields on the magnetocaloric effect of gadolinium

Abstract

Gadolinium is a benchmark magnetic refrigerant thanks to its near room temperature Curie point. In this study, the influence of demagnetising fields on the magnetocaloric response in gadolinium plates is quantified from direct measurements of magnetic field-driven adiabatic temperature changes ΔTad. Experimental data are analysed using the geometry-related demagnetising factors in the background of mean-field theory, and are correlated with the critical behaviour of ΔTad(H) at the Curie point. Results evidence that internal magnetic fields can be highly affected by sample orientation and differ significantly from the values of the applied magnetic fields. On the negative side, this translates to a reduction in the magnitude of the magnetocaloric properties that should always be quantified when these effects are enhanced in particular sample geometries. On the positive side, it is shown that demagnetising fields that are induced by the mechanical rotation of the sample at constant applied magnetic fields can be a viable driving force for the magnetocaloric effects, as illustrated by directly collected data of ΔTad = 0.9 ± 0.1 K for μ0ΔH = 1.6 T. These consequences are discussed in Brayton refrigeration cycles.