Ferromagnetic resonance (FMR) in soft magnetic films (SMFs) to a large extent determines the maximum working frequency of magnetic devices. The FMR frequency (fr) in an optical mode is usually much higher than that in the corresponding acoustic mode for exchange coupled ferromagnet/nonmagnet/ferromagnet (FM/NM/FM) trilayers. In this study, we prepared a 50 nm FeCoB film with uniaxial magnetic anisotropy (UMA), showing a high acoustic mode fr of 4.17 GHz. When an ultrathin Ru spacer was inserted in the very middle of the UMA-FeCoB film, the zero-field FMR was abruptly switched from an acoustic mode to an optical one with fr dramatically enhanced from 4.17 GHz to 11.32 GHz. Furthermore, the FMR mode can be readily tuned to optical mode only, acoustic mode only, or double mode by simply varying the applied filed, which provides a flexible way to design multi-band microwave devices.

Heusler Co2FeSi films with a uniaxial magnetic anisotropy and high ferromagnetic resonance frequency f r were deposited by an oblique sputtering technique on Ru underlayers with various thicknesses t Ru from 0 nm to 5 nm. It is revealed that the Ru underlayers reduce the grain size of Co2FeSi, dramatically enhance the magnetic anisotropy field H K induced by the internal stress from 242 Oe (1 Oe = 79.5775 A⋅m−1) to 582 Oe with an increment ratio of 2.4, while a low damping coefficient remains. The result of damping implies that the continuous interface between Ru and Co2FeSi induces a large in-plane anisotropic field without introducing additional external damping. As a result, excellent high-frequency soft magnetic properties with f r up to 6.69 GHz are achieved.

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