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.