  Topological states of matter have been a subject of intensive studies in
recent years because of their exotic properties such as the topologically
protected edge and surface states. The initial studies were exclusively for
electron systems. It is now known that topological states can also exist for
other particles. Indeed, topologically protected edge states have already been
found for phonons and photons. In spite of active searching for topological
states in many fields, the studies in magnetism are relatively rare although
topological states are apparently important and useful in magnonics. Here we
show that the pyrochlore ferromagnets with the Dzyaloshinskii-Moriya
interaction are intrinsic magnonic Weyl semimetals. Similar to the electronic
Weyl semimetals, the magnon bands in a magnonic Weyl semimetal are nontrivially
crossing in pairs at special points (called Weyl nodes) in momentum space. The
equal energy contour around the Weyl nodes gives rise to the Fermi arcs on
sample surfaces due to the topologically protected surface states between each
pair of Weyl nodes. Additional Weyl nodes and Fermi arcs can be generated in
lower energy magnon bands when an anisotropic exchange interaction is
introduced.
