Magnetic properties of thin-film Ag+iron-oxide granular metals

The magnetic properties of iron-oxide particulates dispersed in silver (prepared by rf diode sputtering silver and iron oxide over a wide composition range) have been investigated using the Mössbauer effect, electron microscopy, and vibrating sample magnetometry. The sputtered films are nanocrystalline mixtures of immiscible particles of the two constituents (even when only a very small amount of silver is present). At room temperature, the Mössbauer effect data show that for most compositions (even those with low silver content) the majority of the film is superparamagnetic, with the fraction of material in the superparamagnetic state increasing with increasing Ag content. Consistent with these results, very narrow hysteresis loops were observed. On decreasing the temperature to near 100 K, the fraction of material in the superparamagnetic state decreased and the magnetically ordered component increased. Electron diffraction patterns clearly showed the presence of polycrystalline Ag and Fe3 O4 in the films, but composition analysis (0.03 μm resolution) was unable to separate these regions.