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We report the results of comparative analysis of magnetic properties of the systems based on ϵ-Fe2O3, nanoparticles with different average sizes (from ~3 to 9 nm) and dispersions. The experimental data for nanoparticles higher than 6-8 nm in size are consistent with the available data, specifically, the transition to the magnetically ordered state occurs at a temperature of ~500K and the anomalies of magnetic properties observed in the range of 80-150K correspond to the magnetic transition. At the same time, Mőssbauer and ferromagnetic resonance spectroscopy data as well as the results of static magnetic measurements show that at room temperature all the investigated samples contain ϵ-Fe2O3 particles that exhibit the superparamagnetic behavior. It was established that the magnetic properties of nanoparticles significantly change with a decrease in their size to ~6 nm. According to high-resolution electron microscopy and Mőssbauer spectroscopy data, the particle structure can be attributed to the ϵ-modification of trivalent iron oxide; meanwhile, the temperature of the magnetic order onset in these particles is increased, the well-known magnetic transition in the range of 80-150K does not occur, the crystallographic magnetic anisotropy constant is significantly reduced, and the surface magnetic anisotropy plays a decisive role. This is apparently due to redistribution of cations over crystallographic positions with decreasing particle size, which was established using Mőssbauer spectra. As the particle size is decreased and the fraction of surface atoms is increased, the contribution of an additional magnetic subsystem formed in a shell of particles smaller than ~4 nm becomes significant, which manifests itself in the static magnetic measurements as paramagnetic contribution. [ABSTRACT FROM AUTHOR]

The magnetization dynamics of ϵ-Fe2O3 nanoparticles with an average size of about 9 nm is investigated. From comparison of the hysteresis loops obtained in quasi-static conditions and under pulse fields with amplitudes up to 200 kOe and pulse lengths 8-32ms, it follows that the effective coercivity increases considerably with the variation rate of the imposed magnetic field. A theoretical explanation of this behavior is proposed. The model takes into account the superparamagnetic effects as well as the fact that magnetic anisotropy of the nanoparticles, along with the bulk term, includes a surface contribution. The latter, being of minor importance for the observed magnetic behavior of 25-100 nm particles, becomes essential when the particle size is below 10 nm. From the experimental data, a reference value of the surface anisotropy of nanodisperse ϵ-Fe2O3 is established, and evidence is presented to the effect that below 300K this contribution does not significantly depend on temperature. [ABSTRACT FROM AUTHOR]

Magnetic properties of &eh;-Fe2O3 nanoparticles supported on silica with the average size of few nanometers, narrow size distribution and no admixture of any other iron oxide polymorphs are investigated. The investigation of the temperature behavior of magnetization within the temperature range from 4.2 to 1000 K revealed the presence of several magnetic subsystems in the species under study. The temperatures' behavior of the magnetic moment value indicates ferrimagnetic ordering in the &eh;-Fe2O3 nanoparticles with a Curie temperature of about 800 K and points to the existence of a significant paramagnetic contribution becoming apparent at low temperatures. According to the electron spin resonance data, the particles possess superparamagnetic behavior at temperature higher ∼120 K. The model of the magnetic structure of monophase system of few nanometers &eh;-Fe2O3 nanoparticles supported on silica is proposed. [ABSTRACT FROM AUTHOR]

Electron spin resonance in situ was applied to study the initial stages of ferrofluid formation in combination with the X-ray diffraction, high-resolution transmission electron microscopy and Fourier transform infrared spectroscopy studies data. The ultrafine (d = 4 nm) magnetite in dimethylsulfoxide ferrofluid with a narrow size nanoparticles distribution was obtained in a single step at room temperature by admixing butylene oxide to a solution of iron (II) and (III) chlorides in dimethylsulfoxide. It has been shown that at the first step of the ferrofluid synthesis antiferromagnetically ordered iron containing phase is formed, which can be correlated with iron hydroxides. [ABSTRACT FROM AUTHOR]

The stability of a catalyst for partial H2S oxidation has been studied by the ferromagnetic resonance (FMR) technique combined with transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and magnetostatic investigations. The ε-Fe2O3 iron oxide nanoparticles supported on silica have been examined for their stability under the selective H2S oxidation conditions. The combination of the physicochemical methods has been used to study the state of reacted catalysts. The ε-Fe2O3 phase has been found to remain stable under the selective H2S oxidation conditions at temperatures up to 300 °C. The active phase state during the catalytic reaction has been explored using in situ FMR experiments. It has been established that the ε-Fe2O3 nanoparticles retain their structure and magnetic properties in the presence of H2S at high temperatures. During the in situ FMR experiments, the ε-Fe2O3 sulfidation process has been studied. [ABSTRACT FROM AUTHOR]

Evolution of the local environment of Fe3+ ions in deposited Fe2O3/SiO2 nanoparticles formed in samples with different iron contents was investigated in order to establish the conditions for obtaining the stable ε-Fe2O3/SiO2 samples without impurities of other iron oxide polymorphs. Microstructure of the samples with an iron content of up to 16% is studied by high-resolution transmission electron microscopy, X-ray diffraction analysis, and Mössbauer spectroscopy, and their magnetic properties are examined. At iron concentrations below 6%, calcinations of iron-containing precursor nanoparticles in a silica gel matrix lead to the formation of the ε-Fe2O3 iron oxide polymorphic modification without foreign phase impurities, while at the iron concentration in the range of 6-12%, the hematite phase forms in the sample in the fraction of no more than 5%. It is concluded on the basis of the data obtained that the spatial stabilization of iron-containing particles is one of the main factors facilitating the formation of the ε-Fe2O3 phase in a silica gel matrix without other iron oxide polymorphs. It is demonstrated that the increase in the iron content leads to the formation of larger particles in the sample and gradual changes of the Fe3+ ion local environment during the phase transition ε-Fe2O3 → α-Fe2O3. [ABSTRACT FROM AUTHOR]

A new metamaterial is obtained on the basis of ε-Fe2O3 nanoparticles immobilized in a xerogel matrix. Samples are synthesized by impregnating SiO2 hydrogel with iron (II) salts with subsequent drying and calcination. The structure and magnetic properties of the composites are probed using transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy, and static magnetic measurements. [ABSTRACT FROM AUTHOR]

The ferromagnetic resonance (FMR) method in situ is used to study the initial stages of the formation of ɛ iron oxide nanoparticles deposited on silica gel at temperatures up to 600°C. It is shown that at high-temperature treatment of starting samples obtained by impregnation with an iron(II) sulfate solution, supermagnetic ɛ-Fe 2O 3/SiO 2 nanoparticles form with a narrow size distribution. An analysis of the FMR data in comparison with the data of other methods enables the formulation of the formation conditions for systems of deposited ɛ-Fe 2O 3 nanoparticles without other polymorph impurities. [ABSTRACT FROM AUTHOR]

The magnetic properties of [variant_greek_epsilon]-Fe2O3 nanoparticles with an average size of 3.4 nm in a silica gel matrix are investigated using Mössbauer technique, electron spin resonance, and magnetic measurements. Two magnetic subsystems reveal in magnetic measurements. Paramagnetic subsystem is formed by the Fe(III) ions in the smallest (<3.5 nm) particles' shell, while ferrimagnetic ordering in the 'core' of the particles results in superparamagnetic behavior. The superparamagnetic behavior of the investigated samples is observed up to ∼800 K. The magnetic moment of the particles is formed by both the ferrimagnetic ordering characteristic of [variant_greek_epsilon]-Fe2O3 and the additional effect of uncompensated sublattices (planes). [ABSTRACT FROM AUTHOR]