Nonreciprocity of spin waves is essential for components such as magnetic isolators and circulators used in spin-wave-based computing. A ferromagnetic (FM) bilayer exhibits significant frequency nonreciprocity and has attracted attention in recent years. Prior research on bilayers has predominantly focused on the high-wavenumber regime, where spin waves display significant nonreciprocity and are accessible through Brillouin light scattering (BLS). Although nonreciprocity at lower wavenumbers, which enables rapid magnon propagation, has also been expected, the exploration of the spin-wave modes in the low-wavenumber regime using electrical excitation methods, compatible with devices, has remained elusive. Here, we investigate spin-wave propagation in the bilayer using coplanar waveguides (CPWs) and demonstrate that increasing the bilayer thickness enhances nonreciprocity even at low wavenumbers, which leads to the high group velocity originating from the Damon-Eshbach (DE) mode. Furthermore, our analysis reveals a trade-off between spin-wave nonreciprocity and group velocity. These findings establish design principles for high-speed, low-loss spin-wave-based information processing.

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• Magnetic coupling
• Magnons
• Non-reciprocal propagation