  We investigate spin-wave propagation in a microstructured magnonic-crystal
waveguide fabricated by localized ion implantation. The irradiation caused a
periodic variation in the saturation magnetization along the waveguide. As a
consequence, the spin-wave transmission spectrum exhibits a set of frequency
bands, where spin-wave propagation is suppressed. A weak modification of the
saturation magnetization by 7% is sufficient to decrease the spin-wave
transmission in the band gaps by a factor of 10. These results evidence the
applicability of localized ion implantation for the fabrication of efficient
micron- and nano-sized magnonic crystals for magnon spintronic applications.
