Magnetic reversal of perpendicularly-biased Co/Pt multilayers

We have investigated the magnetic reversal behavior of [(Co/Pt)(4)Co/CoO](N) multilayers that are exchange biased perpendicular to the film plane. We find clear differences in the nucleation properties of the reverse domains of the ascending and descending branches of the hysteresis loops. However, the evolution of the reverse domains, once nucleated, is symmetric to positive and negative field sweeps. This behavior is in contrast to many in-plane biased experiments and can be understood from the collinear uniaxial magnetic anisotropy and unidirectional exchange-bias axis.