  The ability to imprint a given material property to another through proximity
effect in layered two-dimensional materials has opened the way to the creation
of designer materials. Here, we use molecular-beam epitaxy (MBE) for a direct
synthesis of a superconductor-magnet hybrid heterostructure by combining
superconducting niobium diselenide (NbSe$_2$) with the monolayer ferromagnetic
chromium tribromide (CrBr$_3$). Using different characterization techniques and
density-functional theory (DFT) calculations, we have confirmed that the
CrBr$_3$ monolayer retains its ferromagnetic ordering with a magnetocrystalline
anisotropy favoring an out-of-plane spin orientation. Low-temperature scanning
tunneling microscopy (STM) measurements show a slight reduction of the
superconducting gap of NbSe$_2$ and the formation of a vortex lattice on the
CrBr$_3$ layer in experiments under an external magnetic field. Our results
contribute to the broader framework of exploiting proximity effects to realize
novel phenomena in 2D heterostructures.
