  We report on the observation and systematic study of polarization sensitive
magnetic quantum ratchet effects induced by alternating electric fields in the
terahertz frequency range. The effects are detected in (Cd,Mn)Te-based quantum
well (QW) structures with inter-digitated dual-grating-gate (DGG) lateral
superlattices. A dc electric current excited by cw terahertz laser radiation
shows 1/B-periodic oscillations with an amplitude much larger than the
photocurrent at zero magnetic field. Variation of gate voltages applied to
individual grating gates of the DGG enables us to change the degree and the
sign of the lateral asymmetry in a controllable way. The data reveal that the
photocurrent reflects the degree of lateral asymmetry induced by different gate
potentials. We show that the magnetic ratchet photocurrent includes the Seebeck
thermoratchet effect as well as the effects of "linear" and "circular"
ratchets, which are sensitive to the corresponding polarization of the driving
electromagnetic force. Theoretical analysis performed in the framework of
semiclassical approach and taking into account Landau quantization describes
the experimental results well.
