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

Abstract

Nowadays, the most popular method to increase ferromagnetic resonance (FMR) frequency ( f) in self-bias soft magnetic films is to improve the anisotropy field H. However, to push f to higher frequencies only via raising H becomes increasingly challenging because f 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 and interlayer coupling field ( J) 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 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. This study demonstrates that the antiferromagnetic coupling is a new route to achieve higher f and provides the possibility of engineering and manipulating optical-mode resonance simply by controlling the interlayer coupling strength via stress.

Interlayer RKKY coupling in bulk Rashba semiconductors under topological phase transition

“…The detection of the interlayer exchange coupling in the topological phase and the topological phase transition requires pressure-controlled measurements of magneto-resistance, spin susceptibility or ferromagnetic resonances. These methods have been previously utilized to study the pressure effects on the interlayer exchange coupling in Fr/Cr multilayers 85 , 2D ferromagnets 86 and FeCoB/Ru/FeCoB heterostructures 87 . These well-established experimen-tal methods coupled with the controlled growth of bulk Rashba semiconductors [8][9][10][11][12] should make the observation of the unconventional interlayer exchange interaction mediated by these exotic materials readily accessible.…”

Zero-field Optic Mode Beyond 20 GHz in a Synthetic Antiferromagnet

“…This optic mode has been studied by several groups due to its potential to significantly enhance self-biased ferromagnetic resonances towards low-terahertz frequencies [36]. In recent works, notable enhancements to the f r frequency were demonstrated by Li et al [37] who reported a zero-field optic mode resonance of 11.32 GHz.…”

“…The presence of this interlayer coupling has a significant impact on the precessional dynamics of the SAF system. Here the resonance frequency is enhanced by the addition of an effective interlayer coupling term (H ex ) in the dispersion relation that introduces an optic mode resonance (f r o ) in the AF configuration [36], shown schematically in Fig. 1 and given by…”

Strong coupling of quantized spin waves in ferromagnetic bilayers

“…The integral-quantized PSSW approximation is valid in the limit of weak IEC, where the deviation of the wave vectors from nπ/d is negligibly small. It has been demonstrated that the frequency splitting between the hybrid modes is linear with the IEC based on the macrospin model [28][29][30][31]. Moreover, it has been shown that a phase shift was induced when spin waves propagate through a hetero-structured interface due to the mismatch of the spin-wave dispersion [32][33][34].…”