Dispersion relation of interlayer exchange coupled tailored ferrimagnets

Different ferromagnetic resonance (FMR) modes in micron-sized antiferromagnetically interlayer exchange coupled SiO2/Ta(5nm)/Py(t)/Ru(0.85 nm)/Py(3nm)/Ru(3nm) structures, with t = 3, 6 and 9 nm, were studied by means of the electrically detected ferromagnetic resonance (ED-FMR).
The main magnetoresistance effect used in ED-FMR was anisotropic magnetoresistance (AMR). Bilinear and biquadratic coupling strengths for each sample were determined by fitting SQUID-VSM measurements on 4×4 mm² thin films, using equilibrium total energy minimization. The existence of two different resonance modes (in-phase (acoustic) and out-of-phase (optic)) is shown for asymmetric samples. For the symmetric sample only the acoustic mode was observed, due to the compensation of AMR response from Py layers for the out-of-phase mode. The obtained dispersion relations show a clear dependence of the acoustic mode frequency minimum on the bilinear coupling strength.