  The magneto-gyrotropic photogalvanic and spin-galvanic effects are observed
in (0001)-oriented GaN/AlGaN heterojunctions excited by terahertz radiation. We
show that free-carrier absorption of linearly or circularly polarized terahertz
radiation in low-dimensional structures causes an electric photocurrent in the
presence of an in-plane magnetic field. Microscopic mechanisms of these
photocurrents based on spin-related phenomena are discussed. Properties of the
magneto-gyrotropic and spin-galvanic effects specific for hexagonal
heterostructures are analyzed.
