Microwave spectroscopy with vector network analyzer for interlayer exchange-coupled symmetrical and asymmetrical NiFe/Ru/NiFe

Vector network analyzer ferromagnetic resonance spectroscopy (VNA-FMR) is used here to study the different excited modes of sputtered asymmetrical NiFe(13.6 nm)/Ru(d Ru )/ NiFe(27.2 nm) exchange-coupled films with variable Ru thicknesses.The obtained results have been compared to those of the symmetrical NiFe(30 nm)/Ru(d Ru )/NiFe(30 nm).In both cases, the measurements show the existence of an optic and an acoustic precessional mode.The optic mode was only observed over limited field ranges, especially in the symmetrical trilayers.To overcome such a limitation, we developed a new technique similar to the longitudinal FMR, where the bias and the rf field are parallel to each other and perpendicular to the pinning field.Interestingly, and in contrast to the symmetrical trilayers, we observed a mode anti-crossing in the dispersion relation of the asymmetrical layers that we attributed to the thickness difference between the two NiFe layers.Our experimental results on the effect of the biquadratic coupling on the mode frequency variations are in good agreement with the theory.