  Electrostatic gating confines and controls the transport of electrons in
integrated circuits. Magnons, the quanta of spin waves of the magnetic order,
are promising alternative information carriers, but difficult to gate. Here we
report that superconducting strips on top of thin magnetic films can totally
reflect magnons by its diamagnetic response to the magnon stray fields. The
induced large frequency shifts unidirectionally blocks the magnons propagating
normal to the magnetization. Two superconducting gates parallel to the
magnetization create a magnonic cavity. The option to gate coherent magnons
adds functionalities to magnonic devices, such as reprogrammable logical
devices and increased couplings to other degrees of freedom.
