  We performed a detailed study of the training effect in exchange biased
CoO/Co bilayers. High-resolution measurements of the anisotropic
magnetoresistance (AMR) are consistent with nucleation of magnetic domains in
the antiferromagnetic CoO layer during the first magnetization reversal. This
accounts for the enhanced spin rotation observed in the ferromagnetic Co layer
for all subsequent reversals. Surprisingly, the AMR measurements as well as
magnetization measurements reveal that it is possible to partially reinduce the
untrained state by performing a hysteresis measurement with an in plane
external field perpendicular to the cooling field. Indeed, the next hysteresis
loop obtained in a field parallel to the cooling field resembles the initial
asymmetric hysteresis loop, but with a reduced amount of spin rotation
occurring at the first coercive field. This implies that the antiferromagnetic
domains, which are created during the first reversal after cooling, can be
partially erased.
