  Interplay of magnetism and electronic band topology in unconventional magnets
enables the creation and fine control of novel electronic phenomena. In this
work, we use scanning tunneling microscopy and spectroscopy to study thin films
of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually
large number of densely-spaced spectroscopic features straddling the Fermi
level. These are consistent with signatures of low-energy Weyl fermions and
associated topological Fermi arc surface states predicted by theory. By
measuring their response as a function of magnetic field, we discover a
pronounced evolution in energy tied to the magnetization direction. Electron
scattering and interference imaging further demonstrates the tunable nature of
a subset of related electronic states. Our experiments provide the first
visualization of how in-situ spin reorientation drives changes in the
electronic density of states of the Weyl fermion band structure. Combined with
previous reports of massive Dirac fermions, flat bands and electronic
nematicity, our work establishes Fe3Sn2 as a unique platform that harbors an
extraordinarily wide array of topological and correlated electron phenomena.
