Iron magnetic moments in iron/silica gel nanocomposites

Homogeneous gelled composites of iron and silica containing 11–40 wt. % Fe have been prepared by low-temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X-ray diffraction, electron microscopy, Mössbauer effect, and magnetization measurements have been used to show that these bulk materials are paramagnetic composites at room temperature and remain in that state to 10 K. In this condition the Fe is present in nanometer-sized regions and exists in ionic form (both Fe3+ and Fe2+ ). It possesses a large magnetic moment which decreases linearly from 3.9 μB/ Fe atom to 2.8 μB /Fe atom as the Fe content increased from 11% to 40%. For this composition increase, a negative Curie-Weiss temperature was found which increased in magnitude linearly from −13 to −46 K. It is suggested that many of the iron atoms in the as-cured nanocomposites interact antiferromagnetically, and that the magnitude of the effect increases with the Fe concentration. After treatment in hydrogen, the state of the Fe changes.