Directed self-assembly offers an excellent route for the fabrication of nano-structured materials over large length scales. The final structure is determined by the interaction between the different building blocks, which can be, for example, steric, Coulomb or hydrophobic. From magnetic nano-particles the magnetic induction provides an additional design opportunity, with the particular advantage that the forces can be tuned and manipulated by the application of an external magnetic field. A detailed understanding of the relation between the different interactions as well as potential tuning via magnetic fields is crucial for the Taylor designing of future functional and smart materials. As many such materials rely on the deposition of thin films and coatings the self-assembly of particles at a solid-liquid interface is of high relevance.
We have studied the self-assembly of magnetic particles at differently terminated solid substrates by using neutron reflectivity measurements. Neutron reflectivity allows to study buried and, in particular, solid-liquid interfaces and provides information on the magnetic induction in samples. From our results we conclude that the self-assembly of magnetic particles at an interface can be controlled by the interface coating. While for an attractive interaction and chemical binding dense layers form, no interfacial layering is reported for a repulsive interaction. Once formed the first wetting layer results in magnetic field gradients attracting further particles and building up subsequent layers. This effect strongly depends on the inherent magnetic properties of the particles. Interestingly, a very well-ordered interfacial layer is found for a substrates coated with a magnetic seed layer with out-of-plane anisotropy. In this case even intensive rinsing and cleaning of the substrate did not allow to remove the self-assembled layers.
Figure 1: Schematics of the self-assembly of magnetic particles at solid substrates. For strong and attractive interaction between the magnetic nano-particles and the coating of the substrate a densely packed monolayer forms. Once and out of plane field is applied the magnetic stray fields attract more particles which may result in a three-dimensional well-ordered structure.
We have been able to resolve the interfacial-structure of magnetic nano-particles self-assembled at solid boundaries. As next steps we will Taylor design magnetic substrates to provide templates for self-assembly along the lateral direction as well as we will synthesis new types of nano-particles with specific magnetic properties to promote better defined self-assembly. Moreover, till now we were not able to resolve the magnetic structure and induction in our layers as the spin asymmetry was small for our sample. In future work we will focus on resolving the magnetic structure to reveal the role of the magnetic interaction and compare it to the other interactions present in the system.