Tuning valley polarization in two-dimensional ferromagnetic heterostructures

Two-dimensional (2D) heterostructures can generate new properties that are not possessed in their constituents. Monolayer transition metal dichalcogenides (TMDs) MX₂ (M = Mo, W; X = Se, Te) are an ideal system to produce the valley degree of freedom, furthermore, the magnetic proximity effect in MX₂/Cr₂Ge₂Te₆ heterostructures can lift the valley degeneracy and produce nonvolatile valley polarization (VP). Taking the WSe₂/Cr₂Ge₂Te₆ heterostructure as an example, we demonstrate that the normal strain can modify the magnetic proximity effect and tune the VP. In addition, comparing the different MX₂/Cr₂Ge₂Te₆ heterostructures, we find that the VP not only relies on the transition metal atom M, but also depends on the super-super exchange "-X-Te-" which connects the M atom and the magnetic Cr atom. The induced VP in MX₂ follows the sequence: WTe₂ > WSe₂ > MoTe₂ > MoSe₂. Our investigation reveals the tunability of VP in MX₂/Cr₂Ge₂Te₆ heterostructures, which will be of great significance in valleytronics.