Magnetoelastic coupling between surface acoustic waves and various types of spin waves

Abstract

We theoretically investigate magnetoelastic (ME) coupling between surface acoustic waves (SAWs) and spin waves (SWs) in yttrium iron garnet (YIG)/gadolinium gallium garnet (GGG) bi-layers. Two SW types propagating in an in-plane magnetized film are considered: surface spin waves (SSWs), backward volume spin waves (BVSWs). A general analytical expression for ME coupling is derived via interacting wavevector modes and transversal profiles. The coupling is systematically analyzed via applied magnetic fields and geometric parameters. Optimal conditions for coupling exceed both damping rates of the SAW and SWs, enabling observation of coherent coupling phenomena. The calculated spectra of the hybrid ME waves show anti-crossing gaps. Coupling SAW-SSW vanishes for Damon-Eschbach geometry, where the magnetic film is magnetized at 90◦ relative to direction of wave propagation. Optimal conditions for SAW-SSW interactions occur at 45◦, where SWs maintain a surface characteristic with non-reciprocal wave profiles. This non-reciprocity makes coupling highly non-reciprocal. The ratio of coupling for opposite propagating waves exceeds 5 for practically achievable parameters that can be exploited to develop non-reciprocal ME devices as isolators or filters. Coupling SAWs-BVSWs exhibit intriguing phenomena. The waves possess dispersion curves of opposite slopes, which opens a forbidden frequency band (bandgap) where wave propagation is entirely prohibited. This bandgap, where coupling occurs, is controllable via magnetic field either for a localized spatial region or short time interval. Spatially inhomogeneous fields, implemented via current-carrying wire atop YIG, transforms SAWs into BVSWs propagating in the opposite direction. This mechanism offers a route toward voltage-controlled SW transducers and SAW filters. Current-carrying wires create an effective potential well for hybrid ME waves, yielding ultra-narrow transmission bands for SAWs corresponding to ME eigenmodes of the well. SAW-BVSW transduction is also achieved applying a short, spatially homogeneous magnetic field pulse. The BVSW shifts its frequency while preserving the wavevector of the incident SAW. This behavior contrasts the spatially inhomogeneous case, by frequency conservation and a shift in the wavevector. Combining spatially and temporally non-uniform configurations, operations of time reversal and frequency inversion is successfully realized opening a path forward to novel hybrid ME space-time metamaterials tailored for advanced microwave signal processing at GHz frequencies.