Magnetocaloric effect in superparamagnets

The magnetocaloric effect is calculated for superparamagnetic materials as a function of temperature, field and cluster size. Assuming classical behavior, a universal curve is calculated from which an optimum cluster moment may be found for maximum entropy change upon application of a given field H at a given temperature T. Quantum effects are shown to be small for temperatures above 10 K and fields less than a few tesla. A comparison with results for a spin- 7 2 paramagnet such as Gd 3Ga 5O 12 (GGG) is made, which indicates that superparamagnetic materials such as magnetic nanocomposites offer the possibility of extending the upper useful temperature limit of paramagnetic materials for magnetic refrigeration.