Martin P. Magiera

Trapping of interacting propelled colloidal particles in inhomogeneous media.

A trapping mechanism for propelled colloidal particles based on an inhomogeneous drive is presented and studied by means of computer simulations. In experiments this method can be realized using photophoretic Janus particles driven by a light source, which is partially blocked by a shading mask. This leads to an accumulation of particles in the passive part. An equation for an accumulation parameter is derived using the effective inhomogeneous diffusion constant generated by the inhomogeneous drive. The impact of particle interaction on the trapping mechanism is studied, as well as the interplay between passivity-induced trapping and the emergent self-clustering of systems containing a high density of active particles. The combination of both effects makes the clusters more controllable for applications.

Spin excitations in a monolayer scanned by a magnetic tip

Energy dissipation via spin excitations is investigated for a hard ferromagnetic tip scanning a soft magnetic monolayer. We use the classical Heisenberg model with Landau-Lifshitz-Gilbert (LLG) dynamics including a stochastic field representing finite temperatures. The friction force depends linearly on the velocity (provided it is small enough) for all temperatures. For low temperatures, the corresponding friction coefficient is proportional to the phenomenological damping constant of the LLG equation. This dependence is lost at high temperatures, where the friction coefficient decreases exponentially. These findings can be explained by properties of the spin polarisation cloud dragged along with the tip.

Spin waves cause non-linear friction

Energy dissipation is studied for a hard magnetic tip that scans a soft magnetic substrate. The dynamics of the atomic moments are simulated by solving the Landau-Lifshitz-Gilbert (LLG) equation numerically. The local energy currents are analysed for the case of a Heisenberg spin chain taken as substrate. This leads to an explanation for the velocity dependence of the friction force: The non-linear contribution for high velocities can be attributed to a spin wave front pushed by the tip along the substrate.

Magnetic Vortices Induced by a Monopole Tip

A ferromagnetic monolayer with an easy-plane anisotropy scanned by a magnetic tip that is moved with constant velocity ν is studied using atomistic computer simulations. The spin dynamics are treated using the Landau-Lifshitz-Gilbert equation. To study the influence of the tips field, it is modeled by a monopole field instead of a dipole field, which is a common near-field approximation of a scanning probe microscopy cantilever. The magnetic structures induced by the moving tip are analyzed with respect to the strength of the coupling as well as the scanning velocity, and the energy dissipation is calculated. The results agree with calculations in a continuum model using Thieles equation, as well as with earlier results obtained from simulations using a dipolar tip. The quantitative influence of the field is illustrated using energetic arguments.

Magnetic friction: From Stokes to Coulomb behavior

We demonstrate that in a ferromagnetic substrate, which is continuously driven out of equilibrium by a field moving with constant velocity

Energy dissipation of moved magnetic vortices

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Simulation of magnetic friction

, at least two types of friction may occur when

Magnetic Friction and the Role of Temperature

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Trapping of propelled colloidal particles

goes to zero: The substrate may feel a friction force proportional to

Trapping of photophoretic particles

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