Annealing dependence of exchange bias in MnO/Ga_1-xMn_xAs heterostructures

We discuss the annealing-dependence of exchange bias in Ga_1-xMn_xAs epilayers that are overgrown with Mn. Although pure Mn is a known antiferromagnet, we find that the asgrown Mn overlayer does not produce any exchange coupling with Ga_1-xMn_xAs. Rather, our studies indicate that annealing in air is essential for creating the standard signatures of exchange bias (a clear shift in the magnetization hysteresis loop and an increased coercive field). We use X-ray photoelectron spectroscopy to characterize the compositional depth profile of both as-grown and rapid thermal annealed samples. These studies demonstrate that the cleanest exchange bias arises when the Mn overlayer is completely converted into MnO.