  Electron spins in an InGaAs semiconductor quantum well are used as a
magnetometer of magnetic stray-fields from patterned Fe stripes. Using
time-resolved Faraday rotation, the coherent precession of quantum-well spins
in the inhomogeneous field below the Fe stripes is measured for varying
magnetic fields. Comparing with reference stripes made of Au, we find an
enhancement of the spin precession frequency proportional to the Fe
magnetization, in line with a decrease of the spin decay time, which is
attributed to the inhomogeneous magnetic stray-field in the quantum well layer.
