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 silicagel 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, C Fe, up to 25%) or ferromagnetic (C Fe≳25%).