  The effect of the saturation field on the magnetization reversal of
FePt(L10)/FeCo/CoO/FeCo multilayer (ML) has been investigated to understand the
origin of asymmetric magnetization reversal and its correlation with exchange
bias (EB). In the ML structure, the bottom FeCo layer is coupled to the hard
FePt(L10) layer, and the top FeCo layer is comparatively free due to the
relatively more distance from it. The ML has been deposited under UHV
conditions and characterized at each stage of growth using magneto-optical Kerr
effect and x-ray reflectivity techniques. Magnetization reversal is further
studied through domain imaging using the Kerr microscopy technique. The
experimental findings reveal that ML exhibits asymmetrical magnetization
reversal for a certain range of azimuthal angles for both 1.5kOe and 50kOe
saturation fields; however, this angular range of asymmetry decreases with the
increase in the saturation field. Furthermore, EB was absent at the low
saturation field, whereas, EB, in addition to asymmetry, is observed at the
large saturation field. The origin of asymmetry is attributed to
non-collinearity between magnetic anisotropy axes of both FeCo layers. It
results from the proximity effect through short-range Heisenberg exchange
interaction via the CoO barrier layer. On the other hand, EB arises due to
unidirectional anisotropy induced in the FePt layer due to the high saturation
field. It is further proposed that asymmetry would disappear when
unidirectional anisotropy is strong enough to align both the FeCo layers in the
saturation direction leading to loss of the non-collinearity between them.
