  We report broadband spectroscopy and numerical analysis by which we explore
propagating spin waves in a magnetic bilayer consisting of a 23 nm thick
permalloy film deposited on 130 nm thick $Y_{3}Fe_{5}O_{12}$. In the bilayer,
we observe a characteristic mode that exhibits a considerably larger group
velocity at small in-plane magnetic field than both the magnetostatic and
perpendicular standing spin waves. Using the finite element method, we confirm
the observations by simulating the mode profiles and dispersion relations. They
illustrate the hybridization of spin wave modes due to exchange coupling at the
interface. The high-speed propagating mode found in the bilayer can be utilized
to configure multi-frequency spin wave channels enhancing the performance of
spin wave based logic devices.
