  Magnetoelectric (ME) effect, the phenomenon of inducing magnetization by
application of an electric field or vice versa, holds great promise for
magnetic sensing and switching applications. Studies of the ME effect have so
far focused on the control of the electron spin degree of freedom (DOF) in
materials such as multiferroics and conventional semiconductors. Here, we
report a new form of the ME effect based on the valley DOF in two-dimensional
(2D) Dirac materials. By breaking the three-fold rotational symmetry in
single-layer MoS2 via a uniaxial stress, we have demonstrated the pure
electrical generation of valley magnetization in this material, and its direct
imaging by Kerr rotation microscopy. The observed out-of-plane magnetization is
independent of in-plane magnetic field, linearly proportional to the in-plane
current density, and optimized when the current is orthogonal to the
strain-induced piezoelectric field. These results are fully consistent with a
theoretical model of valley magnetoelectricity driven by Berry curvature
effects. Furthermore, the effect persists at room temperature, opening
possibilities for practical valleytronic devices.
