Abstract
Film composites based on the magnetite Fe3O4 nanoparticles in the polyvinyl alcohol matrix were studied by means of the Mössbauer spectroscopy in a weak magnetic field, mainly at room temperature. The Mössbauer spectra in the absence of a magnetic field show a superposition of minor (poor-resolved) magnetic hyperfine structure and major collapsed spectrum typical for superparamagnetic particles. Such a pattern evidences for the mean particles size (<D> ≈ 4 nm) being close to the 'blocking' size for this material at a given temperature. The spectra taken in a weak (0.34 T) magnetic field show almost complete restoration of the magnetic hyperfine structure characteristic for the nanoparticles of the kind. The experimental Mössbauer spectra have been analyzed within the recently developed three-level relaxation model of magnetic dynamics in an ensemble of single-domain particles. This model takes into account the precession and diffusion of particle's uniform magnetization as well as the particles size distribution.
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