Spin-wave propagation in a bilayer of van derWaals magnet and ferrimagnetic insulator

Spin waves in van der Waals magnets hold promise for magnonic devices and circuits down to the two-dimensional limit. However, their short decay lengths pose challenges for practical applications. Here, we report on a material platform consisting of a van der Waals magnet, Fe₅GeTe₂ (FGT), and a ferrimagnetic insulator of yttrium iron garnet, Y₃Fe₅O₁₂ (YIG), which supports the low-loss propagation of spin waves. Using broadband spin-wave spectroscopy, we observed an increase in spin-wave group velocity with decreasing temperature, which peaks at 30 K in the YIG and FGT/YIG films. This effect is ascribed to a change in the saturation magnetization of YIG and FGT/YIG at low temperature, resulting in a change in the spin-wave dispersion relations. Using micromagnetic simulations, we further investigated spin-wave propagation in an FGT/YIG bilayer and revealed a longer spin-wave decay length in the bilayer than in a single FGT layer, which is due to the lower effective damping in the bilayer. Moreover, asymmetric spin-wave dispersion, induced by a chiral dipolar interaction between the YIG and FGT layers, enables nonreciprocal control of spin-wave propagation.