Study of Asymmetric Magnetization Reversal and Exchange Bias in FePt(L10)/FeCo/CoO/FeCo Magnetic Multilayer

TL;DR

This study investigates how the saturation field influences asymmetric magnetization reversal and exchange bias in FePt/FeCo/CoO/FeCo multilayers, revealing the roles of non-collinearity and unidirectional anisotropy.

Contribution

It provides new insights into the origin of asymmetry and exchange bias in multilayers, linking them to magnetic anisotropy and saturation field effects.

Findings

Asymmetrical magnetization reversal depends on azimuthal angles and saturation field.

Exchange bias appears only at high saturation fields.

Non-collinearity between magnetic axes causes asymmetry.

Abstract

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.