Application of high-energy Nd-Fe-B magnets in the magnetic refrigeration

The magnetisation M(T, H) of magnetic refrigerants varies strongly with temperature in the vicinity of their Curie temperatures, thus in realistic magnetic refrigerators employing Nd-Fe-B permanent magnets the isofield process of the Ericsson refrigeration cycle cannot be realized by simply keeping a ferromagnetic working body in the gap of a magnetic circuit made up by two tablet-shaped permanent magnets and an iron yoke. In addition, the operational point of Nd-Fe-B magnets is determined by a temperature-dependent load in the magnetic circuit, i.e. the magnetic circuit gap has a low load (ferromagnetic) below the Curie point TCurie of the working substance but a high load (paramagnetic) above the TCurie. To estimate the refrigeration power we consider a pile of equally spaced thin Gadolinium sheets as the working body, which are inserted into (or extracted from) the gap of Nd-Fe-B magnetic circuits. In terms of equivalent reluctance parameters and using iterative calculations, we have determined the average H-field experienced by Gd as a function of temperature for several kinds of Gd sheet pile geometry. The derived adiabatic temperature changes are in good agreement with the experiments