The remanence and coercivity of an assembly of single-domain ferromagnetic particles are studied using the Monte Carlo simulation technique. The particles have random locations, possess random uniaxial anisotropy, and are coupled through dipolar interactions. The dependence of the magnetic properties on the packing density, the size of the particles, and the temperature are examined systematically. The role of the packing geometry (sc, fcc) and the sample boundaries are discussed. Dipolar interactions are shown to reduce the coercivity with respect to values for the noninteracting assembly in all cases except for strongly dipolar systems below the percolation threshold. An enhancement of the remanence is found in weakly dipolar systems and is attributed to the macroscopic Lorentz field. The fcc packing of the particles leads to more pronounced ferromagnetic behavior than the sc packing. The sample free boundaries and the corresponding demagnetizing field have a strong effect on the remanence of the assembly while they produce a minor reduction to the coercivity. The results from the simulations are compared with magnetic measurements on frozen ferrofluids and granular metal solids.

Magnetization and AC susceptibility measurements have been performed on ∼3 nm NiO nanoparticles in powder form. The results indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core, with an uncompensated magnetic moment, and a magnetically disordered surface shell. The core magnetic moments block progressively with decreasing temperature, according to the distribution of their anisotropy energy barriers, as shown by a broad maximum of the low field zero-field-cooled magnetization (M(ZFC)) and in the in-phase component χ' of the AC susceptibility, centred at ∼70 K. On the other hand, surface spins thermally fluctuate and freeze in a disordered spin-glass-like state at much lower temperature, as shown by a peak in M(ZFC) (at 17 K, for H = 50 Oe) and in χ'. The temperature of the high temperature χ' peak changes with frequency according to the Arrhenius law; instead, for the low temperature maximum a power law dependence of the relaxation time was found, τ = τ(0)(T(g)/(T(ν)-T(g)))(α), where α = 8, like in spin glasses, τ(0) = 10(-12) s and T(g) = 15.9 K. The low temperature surface spin freezing is accompanied by a strong enhancement of magnetic anisotropy, as shown by the rapid increase of coercivity and high field susceptibility. Monte Carlo simulations for core/shell antiferromagnetic particles, with an antiferromagnetic core and a disordered shell, reproduce the qualitative behaviour of the temperature dependence of the coercivity. Interparticle interactions lead to a shift to a high temperature of the distribution of the core moment blocking temperature and to a reduction of magnetization dynamics.