CHANGE IN MAGNETIC STATE OF FE + SILICA-GEL NANOCOMPOSITES DUE TO LOW-TEMPERATURE TREATMENT IN AMMONIA

Homogeneous gelled composites of iron and silica containing 11-40 wt. % Fe were prepared by room temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). Previous electron microscopy, x-ray diffraction, and Mossbauer effect data showed these bulk materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. They were also all paramagnetic below 300 K. Here the effect on the magnetic state of these nanocomposites following a low temperature (T < 400-degrees-C) treatment in 1 atm of ammonia (after a prior anneal in 1 atm of hydrogen) is presented, along with the dependence on the H-2 pretreatment. In all cases the room temperature Mossbauer spectra for the material in the NH3-treated and H-2-pretreated conditions were similar. However, when treated in H-2 at 770-degrees-C (2 h) the Mossbauer spectra also contained a significant component having a large isomer shift (approximately 1.3 mm/s) and quadrupole splitting (approximately 3.2 mm/s). This material was also slightly ferromagnetic at all temperatures below 300 K. For materials pretreated in hydrogen below 400-degrees-C, a threefold enhancement in the magnetic susceptibility was measured following treatment in ammonia. In addition, both the field and temperature dependence of the susceptibility indicated the presence of spin-glass behavior at 10 K for NH3-treated samples containing up to 40% Fe. At room temperature, these latter ammonia-treated nanocomposites were either superparamagnetic (Fe contents, C(Fe), up to 25%) or ferromagnetic (C(Fe) > 25%).