  Using first-principles electronic structure calculations, we show that
ferromagnetic Heusler compounds Co$_2$MnX (X= Si, Ge, Sn) present non-trivial
topological characteristics and belong to the category of Weyl semimetals.
These materials exhibit two topologically interesting band crossings near the
Fermi level. These band crossings have complex 3D geometries in the Brillouin
zone and are characterized by non-trivial topology as Hopf links and chain-like
nodal lines, that are protected by the perpendicular mirror planes. The
spin-orbit interaction split these nodal lines into several zero-dimensional
Weyl band crossings. Unlike previously known topologically non-trivial Heusler
materials, these majority-spin band crossings lie in the band gap of minority
spin bands, potentially facilitating its experimental realization.
