Multilayer repetition controls surface acoustic wave propagation and anisotropic elastic response in CoFeB/Au systems

Understanding how nanoscale multilayer ML architectures influence the propagation of surface acoustic waves (SAWs) is essential for advancing acousto-electronic and sensing technologies. In this work, we investigated how the repetition number of [CoFeB/Au] bilayers affected SAW propagation and the anisotropy of MLs deposited on Si(001). Brillouin light scattering was used to measure the dispersion and in-plane angular dependence of SAW modes, and the results were supported by finite-element simulations. Increasing the number of repetitions led to a systematic decrease in SAW phase velocity and a gradual reduction of effective elastic anisotropy. These trends arose from the depth-dependent sensitivity of SAWs, which led to a transition from substrate-dominated to multilayer-dominated elastic response as the total film thickness increased. The strongest variations occurred at low repetition numbers, while thicker MLs converged toward an elastically homogeneous response. These results demonstrated that ML repetition is a powerful and precise control parameter for engineering acoustic and elastic properties in nanoscale heterostructures, providing a pathway toward tunable, high-performance acousto-electronic and sensing devices.