Stress-Enhanced Interlayer Exchange Coupling and Optical-Mode FMR Frequency in Self-Bias FeCoB/Ru/FeCoB Trilayers

Nowadays, the most popular method to increase ferromagnetic resonance (FMR) frequency ( f_r) in self-bias soft magnetic films is to improve the anisotropy field H_K. However, to push f_r to higher frequencies only via raising H_K becomes increasingly challenging because f_r is already higher than 10 GHz by now. In this study, we fabricated a series of magnetically anisotropic FeCoB/Ru/FeCoB sandwich films possessing antiferromagnetic-like coupling and gradually increased uniaxial stress in the FeCoB sublayers from 52 to 110 MPa. It is quite remarkable that the acoustic mode of FMR gradually disappears, whereas the optical mode is enhanced in these structures. We observed simultaneous enhancement of H_K and interlayer coupling field ( J_IEC) with the uniaxial stress, which leads to a very pronounced optical-mode frequency increase from 8.67 to 11.62 GHz with a very sensitive stress response of 51 Hz/Pa. In contrast, the f_r in a FeCoB single layer (acoustic mode) only varies from 3.47 to 5.05 GHz under similar stress. We believe that the strain-induced electron density variation of the Ru spacer's Fermi surface in the out-of-plane direction is responsible for the enhancement of J_IEC. This study demonstrates that the antiferromagnetic coupling is a new route to achieve higher f_r and provides the possibility of engineering and manipulating optical-mode resonance simply by controlling the interlayer coupling strength via stress.