  Conventional magnonic devices use three classes of magnetostatic waves that
require detailed manipulation of magnetization structure that makes the design
and the device/circuitry scalability difficult tasks. Here, we demonstrate that
devices based on topological exchange spin waves do not suffer from the problem
with additional nice features of nano-scale wavelength and high frequency. Two
results are reported. 1) A perpendicular ferromagnet on a honeycomb lattice is
generically a topological magnetic material in the sense that topologically
protected chiral edge spin waves exist in the band gap as long as spin-orbit
induced nearest-neighbor pseudodipolar interaction (and/or next-nearest
neighbor Dzyaloshinskii-Moriya interaction) is present. 2) As a proof of
concept, spin wave beamsplitters and spin wave interferometers are designed by
using domain walls to manipulate the propagation of topologically protected
chiral spin waves. Since magnetic domain walls can be controlled by magnetic
fields or electric current/fields, one can essentially draw, erase and redraw
different spin wave devices and circuitry on the same magnetic plate so that
the proposed devices are reconfigurable and tunable. Devices made from magnetic
topological materials are robust against both internal and external
perturbations such as the spin wave frequency variation and device geometry as
well as defects.
