  The discovery of topological states of matter has led to a revolution in
materials research. When external or intrinsic parameters break certain
symmetries, global properties of topological materials change drastically. A
paramount example is the emergence of Weyl nodes under broken inversion
symmetry, acting like magnetic monopoles in momentum space. However, while a
rich variety of non-trivial quantum phases could in principle also originate
from broken time-reversal symmetry, realizing systems that combine magnetism
with complex topological properties is remarkably elusive due to both
considerable experimental and theoretical challenges. Here, we demonstrate that
giant open Fermi arcs are created at the surface of ultrathin hybrid magnets.
The Fermi-surface topology of an atomically thin ferromagnet is substantially
modified by the hybridization with a heavy-metal substrate, giving rise to
Fermi-surface discontinuities that are bridged by the Fermi arcs. Due to the
interplay between magnetism and topology, we can control both the shape and the
location of the Fermi arcs by tuning the magnetization direction. The
hybridization points in the Fermi surface can be attributed to a non-trivial
"mixed" topology and induce hot spots in the Berry curvature, dominating spin
and charge transport as well as magneto-electric coupling effects.
