Magnetic interactions in γ-Fe2O3@SiO2 nanocomposites

摘要

Structural and magnetic properties of γ-Fe2O3 nanoparticles of 4 nm diameter, dispersed into silica SiO2 matrix with a wide range value of volume fraction (0.05 to 1), were investigated. Produced γ-Fe2O3@SiO2 nanocomposites consist of an assembly of very small single domain magnetic object (<10 nm) with a random distribution of both inter-particle distance and direction of particle magnetic moment. We focused on the determination of a magnetic percolation threshold, defined as the magnetic particles concentration value above which the magnetic properties of isolated particles vanished in favor of a magnetic collective behavior induced by magnetic interactions. A percolation threshold value of 0.63 was obtained by a local probe technique such as 57Fe Mössbauer spectrometry and confirmed by global magnetic measurements through zero-field cooled, field-cooled, and ac susceptibility data. Below this threshold, dynamic ac magnetization measurements show a thermally activated Arrhenius dependence of the blocking temperature of superparamagnetic nanoparticles and above this critical value, one observes a slowing down of their dynamic properties, which lead toward the establishment of a spin-glass like state.