Damping Reduction in Rough Fe/Al/Fe Trilayers via a Transition from Exchange to Dipolar Coupling

The static and dynamic magnetic responses of symmetric Fe/Al/Fe trilayers were investigated as a function of the thickness of the nonmagnetic spacer layer thickness, with tAl ranging from 0 to 2 nm. Samples showed ferromagnetic coupling between Fe layers for all values of tAl, despite presenting characteristics of low-quality thin films, including high roughness and low-saturation magnetic moments. However, it was demonstrated that inclusion of a thin nonmagnetic Al spacer is an effective method to reduce the effective apparent damping parameter (αapp) of the dominant acoustic mode of the multilayered system. Specifically, αapp was reduced from 0.030 to 0.013 when the Al spacer thickness exceeded the characteristic roughness of the layers (tAl ≥ 1.4 nm). This reduction coincided with the appearance of distinct acoustic and optical resonance modes, indicating a transition from a direct exchange-coupled regime dominated by pinholes to a regime dominated by dipolar coupling. This suggests that decoupling the ferromagnetic layers is a viable strategy for developing low-damping Fe-based materials, even in systems with significant structural imperfections.