  Magnetic Weyl semimetal (WSM) is of great importance both for fundamental
physics and potential applications due to its spontaneous magnetism, robust
band topology, and enhanced Berry curvature. It possesses many unique quantum
effects, including large intrinsic anomalous Hall effect, Fermi arcs, and
chiral anomaly. In this work, using ab initio calculations, we propose that Ni
doped pyrochlore Tl$_2$Nb$_2$O$_7$ is a magnetic WSM caused by exchange field
splitting on bands around its quadratic band crossing point. The exchange field
tuned by Ni 3d on-site Coulomb interaction parameter $U$ drives the evolution
of Weyl nodes and the resulting topological phase transition. Since Weyl nodes
can exist at generic point in Brillouin zone and are hard to be exactly
identified, the creation and annihilation of them, i.e., the change in their
number, chirality and distribution, have been consistently confirmed with a
combined theoretical approach, which employs parity criterion, symmetry
indicator analysis and the Wilson loop of Wannier center. We find that Weyl
nodes remain in a quite large range of $U$ and are close to Fermi level, which
makes the experimental observation very possible. We think this method and our
proposal of magnetic WSM will be useful in finding more WSMs and adding our
understanding on the topological phase transition.
