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We show how the non-collinear magnetic arrangement of ferromagnetic Janus particles enables the construction of a rich phase diagram via static self-assembly in two dimensions. Branched clusters of staggered chains, compact clusters, linear chains and single particles can be formed and interconverted via constant and low-frequency external magnetic fields. The magnetic anisotropy of the presented particles fundamentally adds new possibilities to controlled self-assembly

Self-assembly of magnetic colloidal particles in solution has succesfully been simulated by hard- or soft-sphere models with a set of embedded magnetic point dipoles, and the position and orientation of each dipole are adapted to mimic the magnetization distribution. The present study introduces a conceptually simpler approach for magnetically capped colloidal particles, which replaces the set of dipoles by the magnetization distribution of a single conductive loop. Only two parameters are required to characterize the magnetization distribution: the diameter of the loop and its radial off-center shift within the sphere. This approach reflects the radial symmetry and the spatial extension of the magnetic cap. In the far-field and in the limit of very small loops the model exactly reproduces the magnetization distribution and the particle arrangements obtained with the single, radially shifted dipole model. For larger loop radii additional stable assembly patterns are obtained, which occur in experiments, but can not be simulated with a single shifted dipole model.

Colloidal self-assembly bears significant potential for the bottom-up fabrication of advanced materials and micromechanical structures. A wide range of particles with different types of anisotropy have been recognized as promising precursors for controlled structure engineering. Here, we concentrate on particles that interact via polar fields, which are intrinsically anisotropic. More specifically, we focus on the assembly of micron-sized silica spheres which are partly covered by a thin ferromagnetic layer with an out-of-plane magnetic anisotropy. To study assemblies of such magnetic particles, we introduce a simple two-parameter model: The extended magnetization distribution is approximated by a current-carrying coil enclosed inside a hard sphere. The far field of that current reproduces the stray field of a point dipole model, the near field reflects an extended magnetization. Such a model employs only two parameters to describe the shape of the magnetization distribution: The radius and the position of the coil inside the sphere. We present stable assemblies as a function of both parameters. In the limit of very small coils the analytical solution for two particles with shifted point dipoles is correctly reproduced. By increasing the radius of the coil, we reproduce experimentally observed particle arrangements not covered by models based on single shifted dipoles.

In this article, we demonstrate how colloidal self-assembly and non-equilibrium dynamic processes can be enhanced by anisotropic particles. As an example, we study spherical particles with radially off-centered net magnetic moment in an oscillating field. Based on complementary data from a numerical simulation of spheres with shifted dipole and experimental observations from particles with hemispherical ferromagnetic coating, it is explained how this magnetic asymmetry gives rise to dynamic structural and orientational phenomena on a two-particle basis. We further present the behavior of ensembles of coated particles. It illustrates the potential for controlled reconfiguration based on the presented two-particle dynamics.

The microscopic interactions between atomic magnetic moments determine the macroscopic magnetic properties of matter. For strongly correlated magnetic systems the local spin configuration plays a key role. High relaxation times, however, make direct investigations of dynamic processes such as crystallization very difficult. Here, we present an artificial spin system of magnetic colloids, which are often discussed as potential mesoscopic model systems for condensed matter. The very low relaxation rates of interacting colloids enable the visualization of phase transitions or crystallization processes by video microscopy. The used colloids have a predened net magnetic moment, as analogue to the atomic spin. These micromagnets show a quasiantiferromagnetic interaction. They form two-dimensional hexagonal clusters with a spin configuration similar to the 120° antiferromagnetic Neel state in the cluster center and strong deviations along the edges. The cluster size emerged as critical parameter for the occurrence of spin defects. Dur-ing the cluster growth the total magnetization of the system increased in discrete steps. Further we obtained a linear increase of the inverse spin pair correlation for particles in the center. The influence of an external constant or uctuating magnetic field is investigated as control tool for cluster growth and defect formation.

In the last decades thin film technology has led to a vast increase in the variety of fabricated magnetic nano- and microstructures with desired shape, size and properties. Usually flat substrates are used to grow structures. Recently, a study on magnetic thin films with perpendicular anisotropy on curved substrates, for instance on silica spheres [1], has shown that the film curvature enormously influences the magnetic properties. In contrast to structures on flat substrates those so-called magnetic caps exhibit a spatial change of the magnetic easy axis and a film thickness variation across the cap. These features extend the possibilities in tailoring the magnetic specifications such as the magnetic anisotropy [2,3] and coercivity [1,4] or magnetization reversal process [1,2,5]. This complex structure geometry, however, leads to difficulties in characterizing the magnetic properties of the caps using standard techniques: In the case of micrometer-sized structures magnetic imaging methods suffer, for example, from beam deflection or the huge structure height due to the surface curvature. We will present our results on 4.5µm and 0.33µm sized magnetic caps consisting of [Co(0.28nm)/Pd(0.9nm)]x8-multilayers with perpendicular magnetic anisotropy. First, we concentrate on the interior materials’ properties by studying the magnetic switching process of the cap in external fields and the spin dynamics. Second, the stray field is investigated, which determines the cap’s interaction with the environment. The magnetization reversal in micron-sized caps has been studied by both, superconducting quantum interference device and magnetic force microscopy, for two distinct directions of an externally applied magnetic field: normal and parallel to the substrate surface. We observe clear deviations from curves measured for the flat film: The caps also show a hysteretic behavior with a very small coercivity of 150Oe in the field parallel to the substrate surface. In the normal field there is no abrupt switching as known from the flat film, but a continuous, two-step like reversal. The origin of the switching features is investigated with in-field and remanent MFM. We will present the magnetic reversal as a multi-step process across the different regions of the cap. In the parallel field two different nucleation processes during the magnetization switching have been observed: a ring nucleation, which is also mentioned in [2], and isolated domain nucleation. The dynamic properties of the [Co/Pd]-caps have been investigated with ferromagnetic resonance. Due to the small particle size the use of conventional FMR setups does not allow single particle detection. Therefore we prepared micron sized resonators using electron beam lithography [6]. The angular dependence of the FMR modes is investigated. The results are compared with FMR measurements on geometrically similar permalloy caps with in-plane magnetic anisotropy. The curvature-induced spatial modulation of the magnetic anisotropy further leads to a complicated stray field around the cap. So far, micromagnetic simulations [2] have been carried out to calculate the spin distribution giving rise to the stray field. Here we experimentally visualize the magnetic stray field of 0.33 µm particles by off-axis electron holography (figure 2, left). This technique provides access to the projected in-plane component of the magnetic induction by reconstructing the phase shift of the electron wave when passing through the magnetic stray field of the sample [7]. The projected stray field can be obtained by the gradual rotation of the cap. In contrast to standard measurements on flat samples, for magnetic caps the interpretation of the electron holograms is not straightforward since the stray field is not constant along the electron path. Therefore, we compare the experimentally obtained results with the outcome of micromagnetic simulations performed with the finite element based simulation software SpinFlow 3D (figure 2, right). The qualitative agreement proves the reliability of the simulation.

Gallium phosphide thin films on cheap silicon substrates are a promising III-V/IV heterostructure to be used in optoelectronic devices. As an almost lattice matched system with a band gap difference of 1.14 eV it includes applicability for multi-junction solar cells. The present study concerns discontinuities emerging at the boundaries of a GaP thin layer on a silicon substrate. The optical anisotropy of the (001) interface has been determined by an optical model applied to reflectance anisotropy spectroscopy measurements of the GaP/Si heterostructure. Density-functional calculations of the interface have been performed with both, the pseudopotential plane wave code ABINIT [1] and the all-electron augmented plane wave code Wien2K [2]. The study distinguishes between the Ga-rich and the P-rich interface termination. At the perfectly flat interface, the latter exhibits higher stability as indicated by the work of separation. More complex interface models also consider defects. The calculated density of states projected onto in-plane directions gives an indication for anisotropy. It aims at distinguishing interface termination and defects as origin of the experimentally observed reflectance anisotropy. [1] www.abinit.org [2] www.wien2k.at