  We present a simultaneous investigation of coherent spin dynamics in both
localized and itinerant carriers in Fe/GaAs heterostructures using ultrafast
and spin-resolved pump-probe spectroscopy. We find that for excitation
densities that push the transient Fermi energy of photocarriers above the
mobility edge there exist two distinct precession frequencies in the ob-served
spin dynamics, allowing us to simultaneously monitor both localized and
itinerant states. For low magnetic fields (below 3 T) the beat frequency
between these two excitations evolves linearly, indicating that the nuclear
polarization is saturated almost immediately and that the hyperfine coupling to
these two states is comparable, despite the 100x enhancement in nuclear
polarization provided by the presence of the Fe layer. At higher magnetic
fields (above 3 T) the Zeeman energy drives reentrant localization of the
photocarriers. Subtracting the constant hyperfine contribution from both sets
of data allows us to extract the Lande g-factor for each state and estimate
their energy relative to the bottom of the conduction band, yielding -2.16 meV
and 17 meV for localized and itinerant states, respectively. This work advances
our fundamental understanding of spin-spin interactions between electron and
nuclear spin species, as well as between localized and itinerant electronics
states, and therefore has implications for future work in both spintronics and
quantum information/computation.
