Using molecular dynamics simulations we investigate the translational dynamics of particles with dipolar interactions in homogenous external fields. For a broad range of concentrations, we find that the anisotropic, yet normal diffusive behavior characterizing weakly coupled systems becomes anomalous both parallel and perpendicular to the field at sufficiently high dipolar coupling and field strength. After the ballistic regime, chain formation first yields cagelike motion in all directions, followed by transient, mixed diffusive-superdiffusive behavior resulting from cooperative motion of the chains. The enhanced dynamics disappears only at higher densities close to crystallization.

Using molecular dynamics simulations, we demonstrate that the layering of confined colloidal particles with dipolar interactions, such as ferrofluids, in slablike geometries can be controlled by homogeneous external fields. For suitable surface separations, strong fields directed perpendicular to the film plane do not only align the particles but create additional layers in the system. The reverse effect occurs with an in-plane field which can induce a collapse of layers. Both effects are accompanied by pronounced particle rearrangements in lateral directions. Our simulation results are consistent with recent experiments of ferrofluids at surfaces.

We report molecular dynamics simulations results for model ferrofluid films subject to an external, homogeneous magnetic field directed parallel or perpendicular to the film surfaces. The interactions between the magnetic nanoparticles are modeled via the Stockmayer potential. In a previous study [J. Jordanovic and S. H. L. Klapp, Phys. Rev. Lett. 101, 038302 (2008)] we have shown that an external field can control the number and internal structure of the layers characterizing the fluid films, in qualitative agreement with experiments. Here we explore the dependence of the layering effects on thermodynamic conditions, and we analyze the results from an energetic (microscopic and macroscopic) perspective. As a special case we investigate a monolayer to bilayer transition induced via a perpendicular field.

Through evaporation of dense colloids of ferromagnetic ~13 nm ε-Co particles onto carbon substrates, anisotropic magnetic dipolar interactions can support formation of elongated particle structures with aggregate thicknesses of 100–400 nm and lengths of up to some hundred microns. Lorenz microscopy and electron holography reveal collective magnetic ordering in these structures. However, in contrast to continuous ferromagnetic thin films of comparable dimensions, domain walls appear preferentially as longitudinal, i.e., oriented parallel to the long axis of the nanoparticle assemblies. We explain this unusual domain structure as the result of dipolar interactions and shape anisotropy, in the absence of inter-particle exchange coupling.

We simulate the formation of domain walls in two-dimensional assemblies of magnetic nanoparticles. Particle parameters are chosen to match recent electron holography and Lorentz microscopy studies of almost monodisperse cobalt nanoparticles assembled into regular, elongated lattices. As the particles are small enough to consist of a single magnetic domain each, their magnetic interactions can be described by a spin model in which each particle is assigned a macroscopic “superspin.” Thus, the magnetic behaviour of these lattices may be compared to magnetic crystals with nanoparticle superspins taking the role of the atomic spins. The coupling is, however, different. The superspins interact only by dipolar interactions as exchange coupling between individual nanoparticles may be neglected due to interparticle spacing. We observe that it is energetically favorable to introduce domain walls oriented along the long dimension of nanoparticle assemblies rather than along the short dimension. This is unlike what is typically observed in continuous magnetic materials, where the exchange interaction introduces an energetic cost proportional to the area of the domain walls. Structural disorder, which will always be present in realistic assemblies, pins longitudinal domain walls when the external field is reversed, and makes a gradual reversal of the magnetization by migration of longitudinal domain walls possible, in agreement with previous experimental results.