We have created spin-wave nanochannels with a width of down to about 70 nm in magnonic wave guides. We have found propagation velocities of up to 2.2 km/s which is controlled by the orientation and strength of an applied magnetic field. Our findings promise a new device, i.e., the velocity-modulation transistor for spin waves.

(a) Spin precession profile (top) of a confined Damon-Eshbach mode in a 300 nm wide Permalloy wire extending into y direction. The internal field (bottom) is almost constant in the center of the wire and decreases near the edges. (b) Spin precession amplitude (top) when a zig-zag-like magnetic configuration is present in the wire. Here the profile consists of two narrow channels reflecting the inhomogeneous internal field (bottom). The width of each channel is approximately 65 nm.

This is reproducibly achieved by changing the waterfall-like magnetic configuration of a transversely magnetized wire to a well-tailored zig-zag-shaped magnetic configuration.

(a) Spin wave eigenfrequencies in a 300 nm wide and 20 nm thick Permalloy wire as a function of in-plane magnetic field H (simulated spectra). H is applied 12° off from the hard axis direction. Mode A2 is in particular interesting. It reflects two spin-wave channels which are remote from the edges and each about 65 nm wide. This is illustrated in (b). Here, white color indicates a large spin precssion amplitude. As the absolute value of H increases the sub-100 nm wide spin-wave channels move to the edges of the wire. The vertical axis is along the cross section of the 300 nm wide wire.

At the same time we found that the magnon beams in the nanochannels were remote from geometrical edges. If, in future experiments, indeed a reduced magnon scattering rate will be found, the zig-zag-magnetized magnon waveguide might be compared to the graded-index fiber known from optical communication. The graded-index optical fiber was a breakthrough for loss-less data transmission by light.