  Monolayer transition metal dichalcogenides have attracted great attentions
for potential applications in valleytronics. However, the valley polarization
degree is usually not high because of the intervalley scattering. Here, we
demonstrate a largely enhanced valley polarization up to 80\% in monolayer
WS$_2$ under non-resonant excitation at 4.2 K using WS$_2$/LaMnO$_3$ thin film
heterostructure, which is much higher than that for monolayer WS$_2$ on
SiO$_2$/Si substrate with a valley polarization of 15\%. Furthermore, the
greatly enhanced valley polarization can be maintained to a high temperature of
about 160 K with a valley polarization of 53\%. The temperature dependence of
valley polarization is strongly correlated with the thermomagnetic curve of
LaMnO$_3$, indicating an exciton-magnon coupling between WS$_2$ and LaMnO$_3$.
A simple model is introduced to illustrate the underlying mechanisms. The
coupling of WS$_2$ and LaMnO$_3$ is further confirmed with an observation of
two interlayer excitons with opposite valley polarizations in the
heterostructure, resulting from the spin-orbit coupling induced splitting of
the conduction bands in monolayer transition metal dichalcogenides. Our results
provide a pathway to control the valleytronic properties of transition metal
dichalcogenides by means of ferromagnetic van der Waals engineering, paving a
way to practical valleytronic applications.
