Magnetic studies of mesoporous nanostructured iron oxide materials synthesized by one-step soft-templating

Abstract

A combined magnetization and (57)Fe spin-echo nuclear magnetic resonance (NMR) study has been carried out on mesoporous nanostructured materials consisting of the magnetite (Fe3O4) and maghemite (γ-Fe2O3) phases. Two series of samples were synthesized using a recently developed one-step soft-templating approach with systematic variations in calcination temperature and reaction atmosphere. Nuclear magnetic resonance has been shown to be a valuable tool for distinguishing between the two magnetic iron oxide spinel phases, Fe3O4 and γ-Fe2O3, on the nanoscale as well as monitoring phase transformation resulting from oxidation. For the Fe3O4 and γ-Fe2O3 phases, peaks in the NMR spectra are attributed to Fe in the tetrahedral (A) sites and octahedral (B) sites. The magnetic field dependence of the peaks was observed and confirmed the site assignments. Fe3O4 on a nanoscale readily oxidizes to form γ-Fe2O3 and this was clearly evident in the NMR spectra. As evidenced by transmission electron microscope (TEM) images, the porous mesostructure for the iron oxide materials is formed by a random close-packed aggregation of nanoparticles; correspondingly, superparamagnetic behavior was observed in the magnetic measurements. Although X-ray diffraction (XRD) shows the spinel structure for the Fe3O4 and γ-Fe2O3 phases, unlike NMR, it is difficult to distinguish between the two phases with XRD. Nitrogen sorption isotherms characterize the mesoporous structures of the materials, and yield BET surface area values and limited BJH pore size distribution curves.

Nanocasting of Superparamagnetic Iron Oxide Films with Ordered Mesoporosity

“…Moreover, the blocking temperature T B shifts to about 260 K (Figure h), above which the material becomes superparamagnetic. This increase in blocking temperature is in line with reports of, e.g., Jin et al, who observed a similar shift from about 170 to about 330 K after reduction of aggregated maghemite nanoparticles to magnetite.…”

“…However, above T B the thermal energy is high enough to overcome magnetic anisotropy and randomize the orientation of the magnetic domains, inducing paramagnetism. The observed temperature dependence agrees well with aggregated maghemite nanoparticles of similar crystallite sizes whereas (silica derived) hard‐templated maghemite shows a much higher blocking temperature of 340 K …”

“…When measured at a temperature of 100 K, a strong saturation magnetization can be induced ( M s100 = 55 emu ), and a small hysteresis indicative of remaining ferrimagnetism is evident ( H c = 200 Oe). This coercivity is about twice as high as reported by Jin et al for aggregated maghemite nanoparticles. The hysteresis vanishes when measured at 300 K, which confirms a complete transition to superparamagnetic behavior.…”

“…In temperature‐dependent measurements, the absolute values are lower, but the relative shape of the curves is identical and results the same blocking temperature. Comparing the magnetite prepared by Jin et al to their maghemite sample the saturation magnetization and the coercivity at 100 and 300 K increased drastically as well as the blocking temperature. The maghemite/magnetite sample (ii) prepared by us shows little higher saturation magnetization at both temperatures, but is roughly comparable.…”

“…Jin et al performed magnetic measurements on aggregated nanoparticles. Comparing the field‐dependent measurements for maghemite, the saturation magnetization at 100 K is roughly a third of the samples we obtained in our experiments.…”