Model calculations of lattice strains and rotations associated with surface acoustic waves

Abstract

Magnon–phonon coupling has garnered increasing interest in condensed matter physics due to its rich physics and potentials for applications in devices with novel functionalities. Surface acoustic waves (SAWs) are widely used to generate coherent acoustic phonons, with magnetoelastic and magnetorotation interactions often mediating the coupling between magnons and phonons. Since these interactions are governed by the strain and rotational fields of the lattice, both of which depend on the material properties and the SAW device structure, understanding their behavior is essential. However, conventional SAW devices are built on anisotropic piezoelectric substrates, such as LiNbO3, making experimental analyses and modeling challenging. In this work, we present a numerical analysis of SAWs on Y-cut and Y+128°-cut LiNbO3 substrates. Key parameters, including SAW velocity, excitation efficiency, lattice displacement, strain, and rotation, are numerically calculated against the SAW propagation direction relative to the crystal axes and the electrical boundary conditions at the substrate surface. The in-plane shear strain, which contributes to phononic angular momentum, reaches nearly half the amplitude of the longitudinal strain when SAWs propagate at ±45° from the crystal X-axis on Y+128°-cut LiNbO3. The out-of-plane shear strain, relevant to nonreciprocal SAW transmission via magnetoelastic coupling, can be either enhanced or suppressed depending on the surface condition. Additionally, we find that the out-of-plane rotational field, which mediates magnetorotation coupling in systems with perpendicular magnetic anisotropy, can exceed the longitudinal strain by nearly a factor of three. These findings provide useful reference for designing experiments on magnon–phonon coupling and advanced magnonic and phononic devices.