Interlayer coupling in Ni80Fe20/Ru/Ni80Fe20 multilayer films: Ferromagnetic resonance experiments and theory

We present a systematic study of the static and dynamic magnetization behavior of interlayer-coupled Ni80Fe20(200 angstrom)/Ru(t(Ru))/Ni80Fe20(100 angstrom) trilayers as a function of the Ru spacer layer thickness t(Ru). As t(Ru) was varied in the range from 0 to 15.8 angstrom, we observe a strong antiferromagnetic (AFM) exchange coupling between the two ferromagnetic (FM) layers for t(Ru) = 5 angstrom, which becomes weak for t(Ru) = 10 angstrom. For t(Ru) = 14.1 angstrom, the coupled magnetic system changes from AFM to FM ordering. Using broadband ferromagnetic resonance spectroscopy, we have probed the effects of the different coupling mechanisms on both the acoustic and optic magnetic modes. We found that the biquadratic exchange coupling has a negligible effect compared to Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange coupling, while the uniaxial anisotropy at the Ni80Fe20/Ru interfaces also plays an important role in determining the behaviors of the modes. A mode anticrossing phenomenon is observed when the RKKY exchange interaction term is above a critical value. A theoretical framework developed is in very good agreement with our experimental results.