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The magnetocaloric effect of super-paramagnetic nanocomposites composed of iron-oxide nanograins dispersed in a silver matrix was studied by evaluating the magnetic entropy change ΔS induced by demagnetization. The nanocomposites were synthesized by inert gas condensation techniques, and their grain sizes are 10–35 nm. The values of ΔS obtained by calculation from magnetization data sets and from a theoretical formula based on the Langevin super-paramagnetism model were compared. The behavior of ΔS agrees fairly well with that predicted by the Langevin model at relatively higher temperatures T and lower magnetic fields H. At lower T or higher H, ΔS deviated from the model. The deviation was examined and ascribed to exchange coupling and crystalline anisotropy, which hinder the fully random spin orientation state at zero field. The evaluated ΔS values of the nanocomposites were larger than those of the paramagnet containing Fe3+ by about two orders of magnitude, indicating a clear enhancement of the magnetocaloric effect due to the nanostructure.

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Magnetocaloric effect of nanocomposites composed of iron-oxide or iron-nitride grains dispersed in a silver matrix was studied by calculating magnetic entropy change ΔS induced by a change in applied magnetic field H. These nanocomposites were synthesized by the inert gas condensation technique and nitridation by heat treatment in an ammonia gas stream. Average sizes of the iron-containing grains were 10-35 nm. Magnetic phases in the materials were Fe3O4 or γ -Fe2O3 for the oxide-composites and γ-Fe4N or ε -Fe3N for the nitride-composites. Values of the ΔS were obtained by applying a thermodynamic Maxwell's relation, (∂S / ∂H)T = (∂M / ∂T)H, to data set of magnetization M measured at various temperatures T. They clearly indicated significant enhancement due to the nanostructure as predicted.

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