Out‐of‐Plane Polarization Control in Monoclinic Ferroelectrics CuInP₂S₆ via In‐Plane Electric Field

Most studies of ferroelectric domain switching in CuInP 2 S 6 (CIPS) primarily focus on the vertical electrode geometry to reversibly switch the out‐of‐plane polarization along the van der Waals stacking direction. Taking the crystalline symmetry into account, a monoclinic ferroelectric with a non‐zero dielectric tensor component k 13 suggests an in‐plane electric field has the opportunity to control out‐of‐plane polarization, which plays a crucial role in lateral devices. Here, through piezoresponse force microscopy combined with phase‐field simulations, in‐plane (IP) electric fields are demonstrated inducing progressive changes in both IP and out‐of‐plane (OOP) domain, leading to monodomain formation. The phase‐field simulations reveal that it is the k 13 that drives OOP polarization by IP electric stimuli. Repeated switching cycles reveal repeatable domain wall nucleation and movement, demonstrating nonvolatile memory functionality. The findings demonstrate that in‐plane electric fields can effectively control out‐of‐plane polarization in CIPS, suggesting new possibilities for reconfigurable device architectures by integrating van der Waals ferroelectrics.