Synthesis and magnetic properties of magnetite-silicate nanocomposites derived from iron oxide of bacterial origin

Magnetic nanocomposites containing magnetite (Fe ₃O ₄) 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 Fe ₃O ₄ with a spinel-type structure was formed in a noncrystalline silicate matrix. The average crystallite size of the Fe ₃O ₄ 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, Fe ₃O ₄ was reduced to wüstite (Fe _{1 -x}O) and finally crystallized into iron metal (α-Fe) and ferrous silicate (Fe ₂SiO ₄).