Ultrahigh Frequency and Anti-Interference Optical-Mode Resonance with Biquadratic Coupled FeCoB/Ru/FeCoB Trilayers

Microwave soft magnetic films (SMFs) are the key materials to effectively miniaturize and multifunctionalize the microwave electromagnetic components and devices. However, currently, single-layer SMFs encounter a frequency bottleneck at around 10 GHz. The ferromagnet/nonmagnetic spacer/ferromagnet sandwiched films with strong interlayer exchange coupling are possible solutions to break through that frequency limitation because they exhibit ultrahigh optical-mode (OM) resonance frequency f_r^O up to 50 GHz, while the tiny permeability and the limited thickness are their own obstacles to overcome. In this study, biquadratic coupled FeCoB_25nm/Ru_0.25nm/FeCoB_25nm sandwiched films with uniaxial magnetic anisotropy were deposited by a composition gradient sputtering method. Pure OM resonance with self-bias f_r^O up to 18.21 GHz and a relative permeability μ_r^O as high as 169 at the cut-off frequency was achieved. Moreover, both the f_r^O and μ_r^O remain unchanged in the magnetic field range of 0-80 Oe, indicating a strong anti-interference capability to small interference field. These results demonstrate that the biquadratic coupled OM resonance can solve the current frequency bottleneck of microwave SMFs by providing ultrahigh resonance frequency while maintaining considerable permeability, thus leading to potential applications of OM resonance in Ku-band microwave magnetic components.