Magnetic refrigeration: phase transitions, itinerant magnetism and spin fluctuations

Magnetic refrigeration at around ambient temperatures has become of considerable technical and commercial interest over the last few years. It depends upon the magnetocaloric effect, and suitable working materials are those that undergo a phase transition which can be driven by a modest magnetic field. We focus here on one attractive family of intermetallics based on the compound La(Fe,Si)(13). Its metamagnetic phase transition is accompanied by a peak in the heat capacity that can be several times larger than the background and, for certain compositions and fields, also a well-defined first order transition with associated latent heat. It seems that some key aspects drawn from the bestiary of magnetism are particularly helpful in optimising magnetocaloric performance, namely itinerant electron magnetism and spin fluctuations. They appear to assist in maximising the entropy change at the phase transition without incurring the penalty of unduly large hysteresis. Many of these features are shared by other groups of compounds that have attractive performance.