Spin Glass Magnetic Behavior of Iron/Silica Gel Nanocomposites

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Abstract
Homogeneous gelled composites of iron and silica containing 5-40 wt.%Fe prepared by low temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst) were heated to 380°C in the presence of hydrogen gas. X-ray diffraction and M6ssbauer effect measurements, and transmission electron microscope (TEM) observations show these materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. Magnetic susceptibility data indicate the materials became either superparamagnetic or ferromagnetic at room temperature. On cooling, the magnetization data furthermore show that the hydrogenated materials containing ll-30% Fe become magnetic spin glasses at temperatures less than 30 K. Magnetic history effects are observed in addition to displaced hysteresis loops below their spin freezing temperatures (Tf). For field-cooled materials at 10 K, the displacement of the hysteresis loops along the field axis indicates the presence of a unidirectional anisotropy which decreases with the cooling field. Both superparamagnetic-to-spin glass and ferromagnetic-to-spin glass transitions are observed in these nanocomposites. Tf varies with the Fe content from ∼30 K for the 11%Fe nanocomposite to ∼10 K for a content near 33%Fe.