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Magnetic and structural properties of magnetite nanoparticles stabilized in polyvinylalcohol thin films are investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) and static magnetometry techniques. The nanoparticles have well-defined crystallinity, and are superparamagnetic at room temperature. Their size distribution is characterized by the distinct log-normal law (with average diameters near 5-7 nm) and slight maximum near 70-80 nm. The EPR spectra and static magnetization data demonstrated pronounced anomalies in the interval between 130 K (corresponding to Verwey transition) and 200 K. The experimental data obtained can be understood on the basis of the half-metallic electronic structure, complex temperature behavior of the magnetic anisotropy, along with effects of "weak magnetic-electron" sublattice of the magnetite.

This paper describes the magnetic behaviour of polymer nanocomposites containing nanoparticles (∽10 nm) of maghemite γ‐Fe2O3 and magnetite Fe3O4 in a polyvinyl alcohol matrix. The compositions were synthesised using an original method of in situ preparation, the reaction of oxide formation taking place inside the polymer matrix itself. It is the one‐domain‐per‐particle nature of magnetism in the given polymer nanocomposites that was found to be responsible for such a distinctive feature as superparamagnetism detected using Mössbauer spectroscopy and magnetisation measurements. A giant negative magnetoresistance was found in the highly filled compositions. As far as we know, this is the first time that this effect has been observed in polymer composites based on ferrite nanoparticles. Copyright © 1999 John Wiley & Sons, Ltd.

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