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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]

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]