Magnetic metallopolymeric nanocomposites have recently been considered as an important materials because of their potential application, especially in medicine, biotechnology and ecology. In this work we report on the investigations of low temperature magnetic properties of a polymer matrix nanocomposite produced by thermal decomposition of Fe(III) acrylate complex ([Fe₃O(CH₂=CHCOO)₆]OH·3H₂O) at 663 K. After such pyrolysis process a polyphase composite have been obtained, where the main product are magnetite nanocrystalites randomly distributed within the polymer matrix. The Mössbauer spectroscopy investigations, performed in the temperature range 80-295 K, show the superimposition of two contributions - a magnetically split component and a quadrupolar doublet. With increasing temperature the relative intensity of the doublet increases, indicating that it can largely be ascribed to isolated superparamagnetic particles. However, even at room temperature the magnetically split component is still observed pointing to the existence of large grains of particle agglomerates. The ZFC-FC magnetization curves exhibit irreversibility up to 300 K, but their shape is characteristic of system dominated by interparticle interactions rather than superparamagnetism. The magnetization curves display the coercivity up to 300 K and its decrease with temperature cannot be described by a simple thermally activated process. The strongest changes of several magnetic parameters (ZFC magnetization, coercivity, remanence) are observed below ~100 K. This can be a sign of a transition of isolated particles into the superparamagnetic state. Considering, however, that at low temperatures the exchange anisotropy appears (as evidenced by a displacement of the FC hysteresis loop) the other possible explanation is the freezing process of a non-collinear spin structures either related to the disordered surface layer of particles or cluster-glass-like behaviour of particle agglomerates.

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