  Magnetic topological quantum materials have attracted great attention due to
their exotic topological quantum physics induced by the interplay among
crystalology, magnetism, and topology, which is of profound importance to
fundamental research and technology applications. However, limited materials
are experimentally available, most of whom are realized by magnetic impurity
doping or heterostructural constructions. In this work, based on the
first-principles calculations, we predict that double perovskite Ba2CdReO6 is
an intrinsic ferromagnetic topological semi-half-metal, while the ferrimagnetic
double perovskite with space group symmetry Fm-3m, such as Ba2FeMoO6, belongs
to a topological half-metal. One pair of Weyl points and fully spin-polarized
nodal-ring states are found in the vicinity of the Fermi level in Ba2CdReO6.
Its two-dimensional nearly flat drumhead surface states are fully
spin-polarized. In Ba2FeMoO6, however, there exist four pairs of Weyl points
and two fully spin-polarized nodal-rings near the Fermi level. These
topological properties are stable in the presence of spin-orbit coupling. This
makes these materials be an appropriate platform for studying the emerging
intriguing properties, especially for the applications in spintronics,
information technology, and topological superconductivity.
