  We employ optical pump-probe spectroscopy to investigate the voltage
dependence of spontaneous electron and nuclear spin polarizations in hybrid
MnAs/n-GaAs and Fe/n-GaAs Schottky diodes. Through the hyperfine interaction,
nuclear spin polarization that is imprinted by the ferromagnet acts on
conduction electron spins as an effective magnetic field. We demonstrate tuning
of this nuclear field from <0.05 to 2.4 kG by varying a small bias voltage
across the MnAs device. In addition, a connection is observed between the diode
turn-on and the onset of imprinted nuclear polarization, while traditional
dynamic nuclear polarization exhibits relatively little voltage dependence.
