Influence of Underlayer Thickness on Gilbert Damping in Exchange-Biased [Pd/CoFeB]₄/MnIr Multilayers

We have studied the influence of underlayer thickness on the static and dynamic magnetic properties in perpendicular exchange-biased (PEB) [Pd/CoFeB]₄/MnIr multilayers for both Pd and Cu cases. The PEB field Heb decreases and the coercivity field Hc increases with increasing thickness of Cu and Pd underlayers. The dynamic results demonstrate that the Gilbert damping constant α increases with increasing the underlayer thickness in both cases. We attribute the enhancement in α to the increased interfacial spin disordering due to the higher roughness. The increasing of spin disordering in the dead layer enhances the interfacial spin scattering or spin dephasing which makes an additional contribution to the damping constant α. In addition, we find that the intrinsic damping constant becomes larger when exchange bias effect is established, because the introduction of anti-ferromagnetic (AFM) layer adds another transfer channel of angular momentum of ferromagnetic (FM) layer.