  Weyl semimetals (WSMs) have recently stimulated intensive interest because
they exhibit fascinating physical properties and also promise exciting
technological applications. So far, however, the few conrmed magnetic WSMs
generally have a large number of Weyl points either located away from the Fermi
level (EF ) or shrouded by nontopological Fermi surface pockets. Based on first
principles density functional theory calculations, we establish cubic double
perovskite BaEuNiOsO6 to be a high Curie temperature (Tc) ferromagnetic WSM
with magnetization along the [111] direction, just two pairs of Weyl points at
the EF and Tc = 325 K. The strong ferromagnetism is attributed to the strong
ferromagnetic Ni 3d-Eu 4f-Os 5d coupling induced by the substitution of half of
Ba atoms with Eu atoms in double perovskite Ba2NiOsO6. Moreover, the momentum
separation of one Weyl point pair is large, thus giving rise to not only a long
(001) surface Fermi arc but also large anomalous Hall conductivity.
Intriguingly, as a unique physical result of a ferromagnetic WSM, the (111)
BaEuNiOsO6 monolayer superlattice (BaEuNiOsO6)/(BaTiO3)10, being its (111)
quantum-well structure, is found to be a high temperature (Tc = 210 K) Chern
insulator with a large band gap of 190 meV. Therefore, cubic double perovskite
BaEuNiOsO6 will provide a superior high temperature material plotform for
exploring fundamental physics of Weyl fermions and its (111) monolayer
superlattices will offer a high temperature magnetic topological insulator for
studying exotic quantum phenomena such as quantum anomalous Hall effect.
