Slowly generated anomalously large nuclear field in bulk n-AlGaAs

This study investigated the formation and relaxation dynamics of nuclear spin polarization in three Al_xGa_1-xAs bulk samples with different aluminum concentrations x of 0.00, 0.05, and 0.15. The time-resolved Kerr rotation technique was primarily used. The samples with x = 0.15 and 0.05 exhibited anomalously large nuclear magnetic fields BN exceeding 1 T, approximately twice the applied magnetic field. Further investigations revealed that BN formation occurred in two-stages, a rapid initial rise followed by a gradual increase toward a saturation value. Relaxation measurements revealed that the relaxation time of BN was longer for AlGaAs than for GaAs. The comparison of the results obtained under strong and weak magnetic fields indicated the suppression of quadrupole-induced relaxation. We modified the dynamics model of nuclear spin polarization and explained the two-stage formation and the accompanying large BN in AlGaAs bulks.