Exchange bias effect in ferro-/antiferromagnetic van der Waals heterostructures.

Recent discovery of magnetic van der Waals (vdW) materials provides a platform to answer fundamental questions on the two-dimensional (2D) limit of magnetic phenomena and applications. An important question in magnetism is the ultimate limit of antiferromagnetic-layer thickness in ferromagnetic (FM)/antiferromagnetic (AFM) heterostructures to observe the exchange bias (EB) effect, of which origin has been subject to a long-standing debate. Here, we report that the EB effect is maintained down to atomic bilayer of AFM in FM (Fe3GeTe2)/AFM (CrPS4) vdW heterostructure but it vanishes at the single-layer limit. Given that CrPS4 is of A-type AFM and thus the bilayer is the smallest unit to form an AFM, this result clearly demonstrates the 2D limit of EB; only one unit of AFM ordering is sufficient for a finite EB effect. Moreover, the semiconducting property of AFM CrPS4 allows us to electrically control the exchange bias, providing an energy-efficient knob for spintronic devices. Our observation of ultimate thickness limit of EB effect in vdW heterostructures will not only extend the understanding of spin interaction across a vdW gap, but will also invigorate research for 2D spintronics in atomic limit.