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.

The spin-flop transition of acicular hematite nanoparticles of different size synthesized by chemical route and annealed at different temperatures has been investigated as a function of temperature. Transmission electron microscopy (TEM) measurements show that particles have an ellipsoidal shape, with a major axis of $330\pm 50\mathrm{nm}$ and a minor axis of $70\pm 10\mathrm{nm}.$ TEM and x-ray diffraction experiments show that nanoparticles are made of hematite crystallites, which grow on increasing the annealing temperature. Both the Morin transition temperature ${(T}_M)$ and the spin-flop transition field ${(H}_{\mathrm{sf}})$ have been found to increase for increasing crystallite size $\mathrm{}\left(d\right):$ for instance, for $d=36\mathrm{}\mathrm{nm}$ particles $T_M=164\mathrm{}\mathrm{K}$ and the value of $H_{\mathrm{sf}}$ extrapolated at $T=0\mathrm{}$ ${(H}_{\mathrm{sf}0\mathrm{}})$ is 1.7 T, whereas for bulk hematite $T_M=263\mathrm{}\mathrm{K}$ and $H_{\mathrm{sf}0\mathrm{}}=6.5\mathrm{}\mathrm{T}.$ Both $H_{\mathrm{sf}0\mathrm{}}$ and $T_M$ follow a $1/d$ dependence (at a faster rate for $H_{\mathrm{sf}0\mathrm{}}),$ indicating that their variation is mainly driven by surface effects.

The exchange bias properties of a granular system composed of $\mathrm{Fe}$ nanoparticles (mean size $\sim 6\mathrm{nm}$) embedded in an $\mathrm{Fe}$ oxide matrix have been studied. The effect of the cooling field on the exchange field, coercivity and remanent magnetization at $T=5\mathrm{K}$ has been investigated. The observed variations are explained in terms of interfacial exchange coupling between a ferromagnetic phase (the $\mathrm{Fe}$ particles) and a disordered magnetic phase (the oxide matrix), whose spin configuration is strongly affected by the cooling field.