Abstract
Magnetic nanocomposites containing magnetite (Fe3O4) nanoparticles were prepared from iron oxide microtubules produced by Leptothrix ochracea, a species of water-habitant iron-oxidizing bacteria. The microtubules were mainly composed of Si-containing ferric hydroxide that shows a broad X-ray diffraction pattern similar to that of 2-line ferrihydrite. After moderate heat treatment in a reductive atmosphere above 325°C, the ferric ions were partially reduced to a ferrous state, and nanocrystalline Fe3O4 with a spinel-type structure was formed in a noncrystalline silicate matrix. The average crystallite size of the Fe3O4 nanoparticles was estimated to be in the order of a few nanometers. The sample heat-treated at 500°C exhibited considerable magnetization together with superparamagnetic behavior at room temperature, and super-spin-glass interaction occurred at low temperature. On further heat treatment above 530°C, Fe3O4 was reduced to wüstite (Fe1−xO) and finally crystallized into iron metal (α-Fe) and ferrous silicate (Fe2SiO4).
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