Magnetothermal behavior of a nanoscale Fe/Fe oxide granular system

The low-temperature magnetic properties of samples obtained by cold-compacting core-shell Fe/Fe oxide nanoparticles have been investigated, and their dependence on the structure, composition, and mean particle size D has been discussed. Samples with different D, varying from 6 to 15 nm, and different Fe to oxide ratio were analyzed by means of transmission electron microscopy, x-ray diffraction, and magnetization measurements in the 5–300-K temperature range. The results support the existence of a low-temperature (below $T_1\sim 20\mathrm{K})$ frozen, disordered magnetic state, characterized by a strong exchange coupling between the structurally disordered, spin-glass-like oxide matrix and the Fe nanocrystallites. Above $T_1,$ a different regime is distinguished, characterized by the coexistence of a quasi-static, ferromagnetic component, given by the Fe particles, and a relaxing component, represented by regions of exchange-interacting spins of the oxide matrix. As the temperature is increased above $T_1,$ the net moments of the oxide magnetic regions become able to thermally fluctuate and they tend to be polarized by the Fe particle moments. The above picture well accounts for the composition, particle size, and thermal dependence of the coercivity and of the exchange field, which strongly increase with reducing temperature in correspondence with the freezing of most of the moments of the oxide magnetic regions.