Ferrielectricity controlled widely-tunable magnetoelectric coupling in van der Waals multiferroics.

The discovery of various primary ferroic phases in atomically-thin van der Waals crystals have created a new two-dimensional wonderland for exploring and manipulating exotic quantum phases. It may also bring technical breakthroughs in device applications, as evident by prototypical functionalities of giant tunneling magnetoresistance, gate-tunable ferromagnetism and non-volatile ferroelectric memory etc. However, two-dimensional multiferroics with effective magnetoelectric coupling, which ultimately decides the future of multiferroic-based information technology, has not been realized yet. Here, we show that an unconventional magnetoelectric coupling mechanism interlocked with heterogeneous ferrielectric transitions emerges at the two-dimensional limit in van der Waals multiferroic CuCrPS with inherent antiferromagnetism and antiferroelectricity. Distinct from the homogeneous antiferroelectric bulk, thin-layer CuCrPS under external electric field makes layer-dependent heterogeneous ferrielectric transitions, minimizing the depolarization effect introduced by the rearrangements of Cu ions within the ferromagnetic van der Waals cages of CrS and PS octahedrons. The resulting ferrielectric phases are characterized by substantially reduced interlayer magnetic coupling energy of nearly 50% with a moderate electric field of 0.3 V nm, producing widely-tunable magnetoelectric coupling which can be further engineered by asymmetrical electrode work functions.