Anomalous increasing of the intensity of field dependence optical mode ferromagnetic resonance in the exchange coupled bilayer system

Acoustic and optical ferromagnetic resonance (FMR) in the interlayer exchange coupled Fe$_{20}$Ni$_{80}$/Co bilayer have been investigated. In the optical mode, unexpected increasing tendencies of peak value at the resonance frequency has been observed under an increasing magnetic field. We presented analytical calculations with which the exchange coupling between Co and Fe$_{20}$Ni$_{80}$ layers, the magnetization and the in-plane uniaxial anisotropy are taken into account, to interpret the increasing of the maximum values of the optical permeability. Both experimental measurements and theoretical calculation show that such tendencies are dependent on the layer thickness t, and that there is a critical field above which the optical peak value begins to decrease. These results might help us to understand the mechanism of interlayer exchange coupling induced optical FMR and might enlighten us to find new possibility of high frequency applications of magnetic materials.