  We report on the magneto-gyrotropic photogalvanic effect (MPGE) in n-doped
(110)-grown GaAs/AlGaAs quantum-well (QW) structures caused by free-carrier
absorption of terahertz radiation in the presence of a magnetic field. The
photocurrent behavior upon variation of the radiation polarization state,
magnetic field orientation and temperature is studied. The developed theory of
MPGE describes well all experimental results. It is demonstrated that the
structure inversion asymmetry can be controllably tuned to zero by variation of
the delta-doping layer positions. For the in-plane magnetic field the
photocurrent is only observed in asymmetric structures but vanishes in
symmetrically doped QWs. Applying time-resolved Kerr rotation and polarized
luminescence we investigate the spin relaxation in QWs for various excitation
levels. Our data confirm that in symmetrically doped QWs the spin relaxation
time is maximal, therefore, these structures set the upper limit of spin
dephasing in GaAs/AlGaAs QWs.
