  The vortex dynamics in a low temperature superconductor deposited on top of a
rectangular array of micrometer size permalloy triangles is investigated
experimentally. The rectangular unit cell is such that neighboring triangles
physically touch each other along one direction. This design stabilizes
remanent states which differ from the magnetic vortex state typical of
individual non-interacting triangles. Magnetic Force Microscopy images have
revealed that the magnetic landscape of the template can be switched to an
ordered configuration after magnetizing the sample with an in-plane field. The
ordered phase exhibits a broad flux flow regime with relatively low critical
current and a highly anisotropic response. This behavior is caused by the
spontaneous formation of two separated rows of vortices and antivortices along
each line of connected triangles. The existence of a clear flux flow regime
even for zero external field supports this interpretation. The density of
induced vortex-antivortex pairs is directly obtained using a high frequency
measurement technique which allows us to resolve the discrete motion of
vortices. Strikingly, the presence of vortex-antivortex rows gives rise to a
self organized synchronized motion of vortices which manifests itself as field
independent Shapiro steps in the current-voltage characteristics.
