  We report on the observation and study of the magneto-ratchet effect in a
graphene-based two-dimensional metamaterial formed by a graphite gate that is
placed below a graphene monolayer and patterned with an array of triangular
antidots. We demonstrate that terahertz/gigahertz excitation of the
metamaterial leads to sign-alternating magneto-oscillations with an amplitude
that exceeds the ratchet current at zero magnetic field by orders of magnitude.
The oscillations are shown to be related to the Shubnikov-de Haas effect. In
addition to the giant ratchet current oscillations we detect resonant ratchet
currents caused by the cyclotron and electron spin resonances. The results are
well described by the developed theory considering the magneto-ratchet effect
caused by the interplay of the near-field radiation and the nonuniform periodic
electrostatic potential of the metamaterial controlled by the gate voltages.
