Abstract
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.
Highlights
Represent distinct materials, which deserve interest, both in terms of physical understanding, and in terms of exploitation of potential applications, e.g. in magnetic devices that can be designed bottom-up
We show by use of transmission electron microscopy (TEM) based techniques for magnetic imaging[21] that collective magnetic ordering and unconventional domain structures exist in these structures
Magnetic nanoparticle systems may in general be considered a new class of magnetic materials, since their properties may be very different from conventional magnetic materials, and can be tailored by means that are not accessible in other systems
Summary
Elongated continuous strips of Co with comparable dimensions, e.g. nanowires made by focused electron beam induced deposition[24], do not support longitudinal domain walls. A longitudinal domain wall appears favorable in the simulations for wider particle ropes (L/W ~2–7) where the demagnetizing field of the entire assembly of dipoles is sufficient to compete with the local ferromagnetic alignment. From this we find that the disorder has very little effect on the magnetic phase diagram (Fig. 3b vs 3a); again, only the relaxed F- and L- states are favored. We started the calculations from the configuration of a disordered structure of a relaxed F- and L-state (Fig. 4a,d) and applied a reverse magnetic field up to 50 mT in steps of 5 mT. The relaxed L-state (Fig. 4d–f) shows propagation of the domain wall mainly in the transversal direction These results are similar to what we have observed experimentally (Figs 2 and S3). The simulations predict a “coercive”/de-stabilizing field of the order of 35 mT for the F- and L-states, comparable with the observations (Figs 2, S3)
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