Characterization of Iron Oxide Nanoparticles in an Fe₂O₃−SiO₂ Composite Prepared by a Sol−Gel Method

Abstract

An Fe2O3−SiO2 composite was prepared by a gelation method that adopts tetraethoxysilane and iron(III) nitrate as starting materials. The dried gel was treated at increasing temperatures, and the samples were characterized by XRD, TEM, magnetic susceptibility measurements, and EPR and Mössbauer spectroscopies. Nanometer size (3−4 nm) X-ray-amorphous iron(III) oxide particles are observed in the samples treated at low temperature. These particles display superparamagnetic behavior in the Mössbauer spectra and susceptibility measurements, and their magnetic moments indicate antiferromagnetic clustering. The occurrence of two sites for iron ions, one in the bulk and one on the surface of nanoparticles, is suggested by EPR and Mössbauer spectroscopies. Heating of the samples to higher temperatures (T > 700 °C) gives rise to a small increase of the particle size. Simultaneously XRD and TEM exhibit the formation of γ-Fe2O3 crystalline particles, Mössbauer spectra reveal a large change in the magnetization, magnetic measurements show the transition from antiferro- to ferrimagnetic behavior, and the signals of ferrimagnetic resonance appear in the EPR spectra. Superparamagnetic relaxation in these particles was investigated by ac susceptibility measurements and paramagnetic resonance experiments at variable temperatures. Further increase in temperature gives rise to the formation of α-Fe2O3. Atomic reorganization processes taking place during these transitions are discussed.

Magnetic phases of imperfectly crystalline Co2SiO4

“…This thermal irreversibility was assumed by the authors to be indicative of a spin-glass property. In another related system, Fe 2 O 3 ±SiO 2 , which is a classical paramagnet for temperatures above 20 K, a similar hysteresis was found at lower temperatures [10,11], although the irreversibility was attributed to a blocking mechanism, common to superparamagnets. Nevertheless these irreversibilities constitute important data related to the magnetic state of the system, they are insucient for an unique classi®cation.…”

Hard Magnetic Ferrite: ε-Fe2O3

“…These observations are consistent with the literature. 53,54 Note that the superparamagnetism in £-Fe 2 O 3 is due to the small particle size of ca. 6 nm.…”

On the behavior of Ni Magnetic Nanowires as studied by FMR and the effect of “blocking”.

“…The consequence would be a sudden intensity drop. Yet this effect cannot be related to the blocking: if it were so then experiments at higher frequencies would not be able to yield absorption signals, which is not what has been found in experiments [Ennas et al, 1998]. …”

“…In previous works [Antoniak et al, 2005;Berger et al, 2001;Sánchez et al, 1999] this intensity loss was associated with "blocking" in the sense that the magnetic system was not able to follow the microwave excitation frequency   10 10 GHz (X band) below this "blocking" temperature. If that would be the case then, at the same temperature, the system would not be able to show any sign of resonance at a higher frequency, such as Q-band (35GHz), which is not the case (see also, [Ennas et al, 1998]). In this work we use this ideal "blocked" Ni NWs to study the system at three different frequencies and as a function of the sample history.…”