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 8–32 ms, 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 300 K this contribution does not significantly depend on temperature.
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