Simulations of super-structure domain walls in two dimensional assemblies of magnetic nanoparticles

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. (C) 2015 AIP Publishing LLC

Phase transitions and ordering of confined dipolar fluids

We apply a modified mean-field density functional theory to determine the phase behavior of Stockmayer fluids in slitlike pores formed by two walls with identical substrate potentials. Based on the Carnahan-Starling equation of state, a fundamental-measure theory is employed to incorporate the effects of short-ranged hard sphere - like correlations while the long-ranged contributions to the fluid interaction potential are treated perturbatively. The liquid-vapor, ferromagnetic liquid - vapor, and ferromagnetic liquid - isotropic liquid first-order phase separations are investigated. The local orientational structure of the anisotropic and inhomogeneous ferromagnetic liquid phase is also studied. We discuss how the phase diagrams are shifted and distorted upon varying the pore width.Comment: 15 pages including 8 figure

Longitudinal domain wall formation in elongated assemblies of ferromagnetic nanoparticles

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

Magnetic Self-Assembling of spherical Co nanoparticles used as building blocks: Syntheses, properties and theory

International audienceIn this chapter, we show that thanks to the use of micellar and organometallic approaches, one can favor the growth of uniform spherical Co NPs with controlled surface passivation (dodecanoic acid or oleylamine), tunable size (from around 4 to 9 nm) and tunable nanocrystallinity (from fcc to hcp structure). As a result of the balance between van der Waals attractions between the metallic NPs, magnetic interactions between the magnetic NPs and solvent-mediated interactions between ligands, these uniform colloidal NPs can be used as building units to form a full set of assemblies which morphology depends on the deposition strategy, involving solvent evaporation. In the case of spontaneous self-assembling of magnetic NPs, compact hexagonal 2D arrays and 3D superlattices called supercrystals can form. In the latter case, either face-centered cubic supercrystalline films or single colloidal crystals can be obtained. Mesostructures of hexagonally ordered columns, labyrinths and void structures can result from assisted self-assembling, induced by the application of an external magnetic field. In highly ordered superlattices, individual NPs act as “artificial atoms” and occupy the lattice sites to form repetitive, periodic “artificial planes". From a fundamental point of view, these artificial solids constitute good models for investigating crystallization behavior. Resulting from collective interactions between neighboring NPs, they exhibit novel magnetic properties. The magnitude of these interactions, and then, the magnetic properties, can be tuned by various parameters including (1) the (crystallographic) nature of the magnetic NP, (2) the NP size, (3) the nature of the coating agent, (4) the nature of the solvent, (5) the evaporation rate and (6) if appropriate, the application of an external field during the solvent evaporation. On the one hand, simulations based on a flory-type solvation theory using Hansen solubility colloidal parameters allow to predict the cobalt NP size. On the other hand, Monte Carlo simulations and free energy theories are able to predict the size and type of patterns appearing during the evaporation of a solution of magnetic NPs under a magnetic fiel