  Magnetic skyrmions are whirl-like nano-objects with topological protection.
When driven by direct currents (DC), skyrmions move but experience a transverse
deflection. This so-called skyrmion Hall effect is often regarded a drawback
for memory applications. Herein, we show that this unique effect can also be
favorable for spintronic applications: We show that in a racetrack with a
broken inversion symmetry, the skyrmion Hall effect allows to translate an
alternating current (AC) into a directed motion along the track, like in a
ratchet. We analyze several modes of the ratchet mechanism and show that it is
unique for topological magnetic whirls. We elaborate on the fundamental
differences compared to the motion of topologically trivial magnetic objects,
as well as classical particles driven by periodic forces. Depending on the
exact racetrack geometry, the ratchet mechanism can be soft or strict. In the
latter case, the skyrmion propagates close to the efficiency maximum.
