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, fcc͒ 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. ͓S0163-1829͑98͒03241-X͔

No abstract

We study the slow phase of thermally activated magnetic relaxation in finite two-dimensional ensembles of dipolar interacting ferromagnetic nanoparticles whose easy axes of magnetization are perpendicular to the distribution plane. We develop a method to numerically simulate the magnetic relaxation for the case that the smallest heights of the potential barriers between the equilibrium directions of the nanoparticle magnetic moments are much larger than the thermal energy. Within this framework, we analyze in detail the role that the correlations of the nanoparticle magnetic moments and the finite size of the nanoparticle ensemble play in magnetic relaxation.Comment: 21 pages, 4 figure

No abstract

Antiferromagnetic particles can exhibit the phenomenon of superparamagnetism if the number of spins in the two sublattices is unequal (“uncompensated spins”). Monte Carlo simulations have been performed on spherical particles in which the spins interact with classical Heisenberg interactions. Anisotropy effects are also included with distinct forms in the core and at the surface. The magnetization was calculated as a function of both temperature and applied magnetic field for a range of particle sizes. Different types of behavior are found which depend on the number of uncompensated spins. Certain field dependent behavior is shown to scale with the ratio of the number of uncompensated to the total number of spins.