Programmable Locomotion Mechanisms of Nanowires with Semi-Hard Magnetic Properties Near a Surface Boundary.

We report on the simplest magnetic nanowire-based surface walker able to change its propulsion mechanism near a surface boundary as a function of the applied rotating magnetic field frequency. The nanowires are made of CoPt alloy with semi-hard magnetic properties synthesized by means of template-assisted galvanostatic electrodeposition. The semi-hard magnetic behavior of the nanowires allows for programming their alignment with an applied magnetic field as they can retain their magnetization direction after pre-magnetizing them. By engineering the macroscopic magnetization, the nanowires' speed and locomotion mechanism is set to tumbling, precession, or rolling depending on the frequency of an applied rotating magnetic field. Also, we present a mathematical analysis that predicts the translational speed of the nanowire near the surface, showing very good agreement with experimental results. Interestingly, the maximal speed is obtained at an optimal frequency (~10 Hz), which is far below the theoretical step-out frequency (~345 Hz). Finally, vortices are found by tracking polystyrene microbeads, trapped around the CoPt nanowire, when they are propelled by precession and rolling motion.