Abstract
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.
Highlights
Magnetic nanoparticles have potential as building blocks to fabricate materials and devices as, for example, sensors,1 storage media,2 and permanent magnets,3–5 with new magnetic properties that are not present in bulk magnets
We simulate the formation of domain walls in two-dimensional assemblies of magnetic nanoparticles
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.” the magnetic behaviour of these lattices may be compared to magnetic crystals with nanoparticle superspins taking the role of the atomic spins
Summary
Magnetic nanoparticles have potential as building blocks to fabricate materials and devices as, for example, sensors, storage media, and permanent magnets, with new magnetic properties that are not present in bulk (continuous) magnets. In combination with the application of different nanofabrication techniques, it is possible to achieve columnar structures, closed-packed grids, and thin film aggregates.. In combination with the application of different nanofabrication techniques, it is possible to achieve columnar structures, closed-packed grids, and thin film aggregates.10,11 Methods for creating these low-dimensional structures are, for example, polymer-mediated or dry-mediated fabrication techniques. When single-domain magnetic particles form lattice structures, there is the possibility that magnetic interactions leads to magnetic ordering of the nanoparticle moments. Dipolar ferromagnetic ordering was first predicted by Luttinger and Tisza in 1946 for an fcc lattice of point dipoles Such magnetic ordering is sometimes referred to as “superferromagnetic” to indicate that it is referring to magnetic ordering in the “superlattice” of nanoparticles, and not the magnetic ordering of the atoms in the nanoparticles itself. We shall omit the prefix “super-,” as there is no risk of confusion between the nanoparticle structures and atomic structures
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