Thickness dependent optical mode ferromagnetic resonance in Co/FeNi bilayer

We investigate the thickness dependent magnetic properties of an exchange coupled bilayer consisting of Co/FeNi. Inside these bilayers, both acoustic and optical modes of ferromagnetic resonance exist, which are produced by the in-phase and out-of-phase precession of the spins in the different layers. The optical mode ferromagnetic resonance, with a much higher resonance frequency, can be tuned significantly by changing the thickness of the FeNi layer. We show that the thickness dependent optical ferromagnetic resonance frequency is due to the variations of the interlayer exchange coupling with the changes in thickness.