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 evaluating the magnetic entropy change Delta S induced by demagnetization. The nanocomposites were synthesized by inert gas condensation techniques, and their grain sizes are 10-35 nm. The values of Delta S obtained by calculation from magnetization data sets and from a theoretical formula based on the Langevin 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 exchange coupling and crystalline anisotropy, which hinder the fully random spin orientation state at zero field. The evaluated Delta S 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.