  In the developing field of magnonics,it is very important to achieve
tailoring of spin wave propagation by both a proper combination of materials
with different magnetic properties and their nanostructuring on the
submicrometric scale. We have exploited deep ultra-violet lithography, in
combination with tilted shadow deposition technique, to fabricate arrays of
closely spaced bi-layer nanowires (NWs), with separation d=100 nm and
periodicity a=440 nm, having bottom and top layers made of Permalloy and Iron,
respectively. The NWs have either rectangular cross section (bottom and upper
layers of equal width) or L-shaped cross section (upper layer of half width).
The frequency dispersion of collective spin wave excitations in the above
bi-layered NWs arrays has been measured by the Brillouin light scattering
technique while sweeping the wave vector perpendicularly to the wire length
over three Brillouin zones of the reciprocal space. For the rectangular NWs,
the lowest frequency fundamental mode, characterized by a quasi- uniform
profile of the amplitude of the dynamic magnetization across the NW width,
exhibits a sizeable and periodic frequency dispersion. A similar dispersive
mode is also present in L-shaped NWs, but the mode amplitude is concentrated in
the thin side of the NWs. The width and the center frequency of the magnonic
band associated with the above fundamental modes has been analyzed, showing
that both can be tuned by varying the external applied field. Moreover, for the
L-shaped NWs it is shown that there is also a second dispersive mode,at higher
frequency,characterized by an amplitude concentrated in the thick side of the
NW. These experimental results have been quantitatively reproduced by an
original numerical model that includes two-dimensional Green's function of the
dipole field of the dynamic magnetization and interlayer exchange coupling
between the layers.

Materials Science;Mesoscale and Nanoscale Physics