  The integration of magnetic material with semiconductors has been fertile
ground for fundamental science as well as of great practical interest toward
the seamless integration of information processing and storage. Here we create
van der Waals heterostructures formed by an ultrathin ferromagnetic
semiconductor CrI3 and a monolayer of WSe2. We observe unprecedented control of
the spin and valley pseudospin in WSe2, where we detect a large magnetic
exchange field of nearly 13 T and rapid switching of the WSe2 valley splitting
and polarization via flipping of the CrI3 magnetization. The WSe2
photoluminescence intensity strongly depends on the relative alignment between
photo-excited spins in WSe2 and the CrI3 magnetization, due to ultrafast
spin-dependent charge hopping across the heterostructure interface. The
photoluminescence detection of valley pseudospin provides a simple and
sensitive method to probe the intriguing domain dynamics in the ultrathin
magnet, as well as the rich spin interactions within the heterostructure.
