  We study the collective transport of paramagnetic colloids driven above a
magnetic bubble lattice by an external rotating magnetic field. We measure a
direct ratchet current which rises in integer and fractional steps with the
field amplitude. The stepwise increase is caused by excluded volume
interactions between the particles, which form composite clusters above the
bubbles with mobile and immobile occupation sites. Transient energy minima
located at the interstitials between the bubbles cause the colloids to hop from
one composite cluster to the next with synchronous and period doubled modes of
transport. The colloidal current may be polarized to make selective use of type
up or type down interstitials.
