Magnetocaloric effect of super-paramagnetic nanocomposite composed of iron-oxide and silver

The magnetocaloric effect of super-paramagnetic nanocomposites composed of iron-oxide nanograins dispersed in a silver matrix was studied by evaluatin g the magnetic entropy change Delta S induced by demagnetization. The nanoc omposites were synthesized by inert gas condensation techniques, and their grain sizes are 10-35 nm. The values of Delta S obtained by calculation fro m magnetization data sets and from a theoretical formula based on the Lange vin super-paramagnetism model were compared. The behavior of Delta S 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, Delta S deviated from the model. The deviation was examined and ascribed to excha nge coupling and crystalline anisotropy, which hinder the fully random spin orientation state at zero field. The evaluated Delta S values of the nanoc omposites were larger than those of the paramagnet containing Fe3+ by about two orders of magnitude, indicating a clear enhancement of the magnetocalo ric effect due to the nanostructure.