  We use nanometer-sized point contacts to a Co/Cu spin valve to study the
giant magnetoresistance (GMR) of only a few Co domains. The measured data show
strong device-to-device differences of the GMR curve, which we attribute to the
absence of averaging over many domains. The GMR ratio decreases with increasing
bias current. For one particular device, this is accompanied by the development
of two distinct GMR plateaus, the plateau level depending on bias polarity and
sweep direction of the magnetic field. We attribute the observed behavior to
current-induced changes of the magnetization, involving spin transfer due to
incoherent emission of magnons and self-field effects.
