Change in magnetic state of Fe+silica gel nanocomposites due to low temperature treatment in ammonia

Abstract

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 Mössbauer 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 °C) treatment in 1 atm of ammonia (after a prior anneal in 1 atm of hydrogen) is presented, along with the dependence on the H2 pretreatment. In all cases the room temperature Mössbauer spectra for the material in the NH3-treated and H2-pretreated conditions were similar. However, when treated in H2 at 770 °C (2 h) the Mössbauer spectra also contained a significant component having a large isomer shift (∼1.3 mm/s) and quadrupole splitting (∼3.2 mm/s). This material was also slightly ferromagnetic at all temperatures below 300 K. For materials pretreated in hydrogen below 400 °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, CFe, up to 25%) or ferromagnetic (CFe≳25%).