Spin-controlled negative magnetoresistance resulting from exchange interactions

We studied conductivity of AlGaAs–GaAs quantum well structures (where centers of the wells were doped by Be) at temperatures higher than $4$ K in magnetic fields up $10$ T. Throughout all the temperature region considered the conductivity demonstrated activated behavior. At moderate magnetic fields $0.1 T < H < 1 T$, we observed negative isotropic magnetoresistance, which was linear in magnetic field while for magnetic field normal with respect to the plane of the wells the magnetoresistance was positive at $H > 2T$. To the best of our knowledge, it was the first observation of linear negative magnetoresistance, which would be isotropic with respect to the direction of magnetic field. While the isotropic character of magnetoresistance apparently evidences role of spins, the existing theoretical considerations concerning spin effects in conductance fail to explain our experimental results. We believe that such a behavior can be attributed to spin effects supported by exchange interactions between localized states.