  Interplay between magnetism and electronic band topology is of central
current interest in topological matter research. We use quantum oscillations as
powerful tool to probe the evolution of band topology in the Dirac semimetal
EuMnSb$_2$. The Eu local 4f magnetic moments display different
antiferromagnetic states below 25 K and a field-polarized phase above 16 T.
Upon cooling from 65 K into the field-polarized state, an exotic temperature
dependent shift of oscillation peaks arises, accompanied by the development of
non-zero Berry phase and a huge unconventional splitting of the oscillations.
Band-structure calculations confirm the change from trivial to non-trivial band
topology induced by the ferromagnetic Eu state, classifying EuMnSb$_2$ as
unique magnetic topological semimetal.
