  Time-resolved Faraday rotation is used to measure the coherent electron spin
precession in a GaAs/InGaAs quantum well below an interdigitated magnetized Fe
grating. We show that the electron spin precession frequency can be modified by
applying a gate voltage of opposite polarity to neighboring bars. A tunability
of the precession frequency of 0.5 GHz/V has been observed. Modulating the gate
potential with a gigahertz frequency allows the electron spin precession to be
controlled on a nanosecond timescale.
