  We have studied spin dephasing in a high-mobility two-dimensional electron
system (2DES), confined in a GaAs/AlGaAs quantum well grown in the [110]
direction, using the resonant spin amplification (RSA) technique. From the
characteristic shape of the RSA spectra, we are able to extract the spin
dephasing times (SDT) for electron spins aligned along the growth direction or
within the sample plane, as well as the $g$ factor. We observe a strong
anisotropy in the spin dephasing times. While the in-plane SDT remains almost
constant as the temperature is varied between 4 K and 50 K, the out-of-plane
SDT shows a dramatic increase at a temperature of about 25 K and reaches values
of about 100 ns. The SDTs at 4 K can be further increased by additional, weak
above-barrier illumination. The origin of this unexpected behavior is
discussed, the SDT enhancement is attributed to the redistribution of charge
carriers between the electron gas and remote donors.
