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 frO up to 50 GHz, while the tiny permeability and the limited thickness are their own obstacles to overcome. In this study, biquadratic coupled FeCoB25nm/Ru0.25nm/FeCoB25nm sandwiched films with uniaxial magnetic anisotropy were deposited by a composition gradient sputtering method. Pure OM resonance with self-bias frO up to 18.21 GHz and a relative permeability μrO as high as 169 at the cut-off frequency was achieved. Moreover, both the frO and μrO 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.