  We propose to use a soft/hard exchange-spring coupling bilayer magnetic
structure to introduce a narrow channel for spin-wave propagation. We show by
micromagnetic simulations that broad-band Damon-Eshbach geometry spin waves can
be strongly localized into the channel and propagate effectively with a proper
high group velocity. The beamwidth of the bound mode spin waves is almost
independent from the frequency and is smaller than 24nm. For a low-frequency
excitation, we further investigate the appearance of two other spin beams in
the lateral of the channel. In contrast to a domain wall, the channel formed by
exchange-spring coupling can be easier to realize in experimental scenarios and
holds stronger immunity to surroundings. This work is expected to open new
possibilities for energy-efficient spin-wave guiding as well as to help shape
the field of beam magnonics.
