  We present a combined experimental and theoretical study describing the
dynamical regimes displayed by a paramagnetic colloidal particle externally
driven above a stripe-patterned magnetic garnet film. A circularly polarized
rotating magnetic field modulates the stray field of the garnet film and
generates a translating periodic potential which induces particle motion.
Increasing the driving frequency, we observe a transition from a phase-locked
motion with constant speed to a sliding dynamics characterized by a lower speed
due to the loss of synchronization with the traveling potential. We explain the
experimental findings with an analytically tractable theoretical model and
interpret the particle dynamics in the presence of thermal noise. The model is
in good quantitative agreement with the experiments.
