  Magnetic proximity interactions (MPIs) between atomically-thin semiconductors
and two-dimensional magnets provide a means to manipulate spin and valley
degrees of freedom in nonmagnetic monolayers, without the use of applied
magnetic fields. In such van der Waals (vdW) heterostructures, MPIs originate
in the nanometer-scale coupling between the spin-dependent electronic
wavefunctions in the two materials, and typically their overall effect is
regarded as an effective magnetic field acting on the semiconductor monolayer.
Here we demonstrate that this picture, while appealing, is incomplete: The
effects of MPIs in vdW heterostructures can be markedly asymmetric, in contrast
to that from an applied magnetic field. Valley-resolved optical reflection
spectroscopy of MoSe$_{2}$/CrBr$_{3}$ vdW structures reveals strikingly
different energy shifts in the $K$ and $K'$ valleys of the MoSe$_2$, due to
ferromagnetism in the CrBr$_3$ layer. Strong asymmetry is observed at both the
A- and B-exciton resonances. Density-functional calculations indicate that
valley-asymmetric MPIs depend sensitively on the spin-dependent hybridization
of overlapping bands, and as such are likely a general feature of such hybrid
vdW structures. These studies suggest routes to selectively control
\textit{specific} spin and valley states in monolayer semiconductors.
