N superlattice multilayer: A real optical mode ferromagnetic resonance thick-film

Abstract

A pure optical mode (OM) resonance with ultrahigh resonance frequency (f r O) and permeability μ at zero bias field was observed in 50-nm FeCoB/Ru/FeCoB sandwich trilayers due to the presence of strong antiferromagnetic interlayer exchange coupling. However, the necessary thickness for creating practical microwave devices cannot be readily achieved by increasing the FeCoB layer thickness or stacking FeCoB/Ru units due to dramatically deteriorated high-frequency performance. In this study, several 50-nm FeCoB/Ru/FeCoB OM units were stacked to form a superlattice structure, with 10-nm ZnO insulator spacers separating the units. It is interesting that the superlattice multilayers not only retain excellent high-frequency OM resonance performance comparable to the single OM unit but also increase the effective magnetic film thickness by a factor of 5. This can be attributed to the positive superposition of the almost identical OM units as well as the effective decoupling between them by thick ZnO spacers. This study provides a promising way to fabricate thicker films while still maintaining excellent OM resonance performance.