  A new characteristic is observed in the training effect of perpendicular
exchange bias. For Pt/Co/Pt/IrMn multilayers with perpendicular magnetic
anisotropy, the magnetization reversal process is accompanied by pinned domain
wall motion and the asymmetry of hysteresis loop is always equal to zero during
subsequent measurements. It is interesting to find that the exchange field
decreases greatly as a function of the cycling number while the coercivity
almost does not change. It is clearly demonstrated that the training effect of
perpendicular exchange bias strongly depends on the magnetization reversal
mechanism of the ferromagnetic layer.
