  Van der Waals heterostructures combining two-dimensional magnetic and
semiconducting layers constitute a promising platform for interfacing
magnetism, electronics, and optics. Here, we use resonant optical reflection
spectroscopy to the observe magnetic proximity effect in a gate-tunable
MoSe$_2$/CrBr$_3$ heterostructure. High quality of the interface leads to a
giant zero-field splitting of the K and K' valley excitons in MoSe$_2$,
equivalent to an external magnetic field of 12 T, with a weak but distinct
electric field dependence that hints at potential for electrical control of
magnetization. The magnetic proximity effect allows us to use resonant optical
spectroscopy to fully characterize the CrBr$_3$ magnet, determining the
easy-axis coercive field, the magnetic anisotropy energy, and critical
exponents associated with spin susceptibility and magnetization.
