The magnetic properties of ultra-small (3 nm) CoFe(2)O(4) nanoparticles have been investigated by DC magnetization measurements as a function of temperature and magnetic field. The main features of the magnetic behaviour are blocking of non-interacting particle moments (zero-field-cooled magnetization T(max) approximately 40 K), a rapid increase of saturation magnetization (up to values higher than for the bulk material) at low T and an increase in anisotropy below 30 K due to the appearance of exchange bias. The low temperature behaviour is determined by a random freezing of surface spins. Localized spin-canting and cation distribution between the two sublattices of the spinel structure account quantitatively for the observed increase in saturation magnetization.

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.

Magnetic interactions in silica coated spherical nanoporous assemblies of CoFe(2)O(4) nanoparticles have been investigated by low temperature field dependent remanent magnetization (M(DCD) and M(IRM)) and magnetization relaxation measurements. The synthesis procedure leads to the formation of spherical aggregates of about 50-60 nm in diameter composed of hexagonal shaped nanocrystals with shared edges. The negative deviation from the non-interacting case in the Henkel plot indicates the predominance of dipole-dipole interactions favouring the demagnetized state, although the presence of exchange interactions in the porous system cannot be excluded. The activation volume, derived from time dependent magnetization measurements, turns out to be comparable with the particle physical volume, thus indicating, in agreement with static and dynamic irreversible magnetization measurements, that the magnetization reversal actually involves individual crystals.