Inert gas condensation synthesis of magnetic nanocomposites for working substance of future magnetic refrigeration

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Magnetic nanocomposites in which magnetic iron oxide or nitride nanograins are dispersed in a silver matrix were synthesized with the inert gas condensation method combined with in-situ compaction technique. By varying evaporation conditions, we obtained nanocomposite samples with various iron contents and grain sizes. Their material evaluation was performed with various characterization techniques, x-ray absorption near edge structure (XANES), induction coupled plasma spectroscopy (ICP), transmission electron microscopy, X-ray diffraction, superconducting quantum interference device (SQUID) magnetometer, etc. Magnetocaloric effect of the samples was evaluated by calculating magnetic entropy changes ΔS on the basis of Max well's equation, (∂S/∂H )_r=(∂M/∂T)_H, to which dc-magnetization data measured at various temperatures and externally applied fields were substituted. Our data clearly indicated enhancement of magnetocaloric effect, in other words, magnitudes of ΔS, caused by grain size reduction to nanoscale dimension. Degree of enhancement was evaluated by comparing values of ΔS of the present samples with that possessed by an iron-containing alum, a paramagnetic salt. Enhancement of nanocomposites of iron-oxide was almost of an order in ΔS magnitude, and that of iron nitride was of another one order. But these gains are still smaller than those expected from the Langevin superparamagnetism model. The reason of this reduced gain was discussed and concluded that grain size distribution and magnetic moment size distribution caused the reduction. One of next targets of development in working substance for advanced magnetic refrigeration system is thus pointed out. That is synthesis of magnetic nanocomposite with narrow size distribution of magnetic moment size.