Surface magnetoelectric effects in monolayer Cr2Ge2Te6 and Cr2Ge2Te3Se3

Two-dimensional (2D) Janus materials possess two distinct surfaces, which may give rise to entirely new physical properties. In this work, based on 2D magnets Cr2Ge2Te6 and Cr2Ge2Te3Se3, we find that the surface atoms play the critical role in electric control of magnetism. For Cr2Ge2Te6 with mirror symmetry, an external electric field induces opposite contributions from the top and bottom surface layers to the MAE modulation, suppressing the net magnetoelectric (ME) effect, while for the Janus Cr2Ge2Te3Se3, a large net MAE modulation is obtained because the Te-surface shows a much larger response to an external electric field than the Se-surface. In addition, spin-projected band structures reveal the combined effect of Rashba- and Zeeman-type spin-splitting features in Cr2Ge2Te3Se3. Our work demonstrates that Janus engineering is a powerful strategy for achieving surface-driven magnetoelectric effects, offering new opportunities for 2D spintronics.