Giant Magnetoelastic Coupling in a Love Acoustic Waveguide Based on TbCo2/FeCo Nanostructured Film on ST-Cut Quartz

In this work, we propose a theoretical and experimental investigation of the interaction of guided pure shear-horizontal (SH) wave within a uniaxial multilayered TbCo2/FeCo thin film deposited on ST-X90. cut quartz in a delay-line configuration. We evaluate theoretically the evolution of phase velocity as a function of magnetic field and experimentally the variation of S-21 transmission coefficient (amplitude and phase). An equivalent piezomagnetic model based on pure magnetoelastic coupling is used (developed to allow us) to calculate the elastic stiffness constants of the multilayer as a function of the bias magnetic field. The model is also implemented for the calculation of acoustic waves' dispersion curves. We show that the evolution of the phase velocity with respect to the bias magnetic field is dominated by the C-66 elastic stiffness constant as expected for the case of shear-horizontal surface acoustic wave. In the fabricated device, both fundamental and third-harmonic shear mode are excited at 410 MHz and 1.2 GHz, respectively. For both modes, the theoretical and experimental results are in agreement. At 1.2 GHz, the guiding of the acoustic wave in the ferromagnetic thin film enhances the sensitivity to the bias magnetic field with a maximum phase-velocity shift close to 2.5% and an attenuation reaching 500 dB/cm, for a sensitivity as high as 250 ppm/Oe, which is better than what has been reported in the literature so far. We also report that, from a specific ratio between the thin-film thickness and the acoustic wavelength, the bias magnetic field can induce a breaking of the acoustic wave polarization, leading to an acoustic mode conversion.