Spontaneous Spin Coherence in n-GaAs Produced by Ferromagnetic Proximity Polarization

We find that photoexcited electrons in an n-GaAs epilayer rapidly (50 ps) spin polarize due to the proximity of an epitaxial ferromagnetic metal. Comparison between MnAs/GaAs and Fe/GaAs structures reveals that this coherent spin polarization is aligned antiparallel and parallel to their magnetizations, respectively. In addition, the GaAs nuclear spins are dynamically polarized with a sign determined by the spontaneous electron-spin orientation. In Fe/GaAs, competition between nuclear hyperfine and applied magnetic fields results in complete quenching of electron-spin precession.