Two-dimensional (2D) van der Waals (vdW) materials are extremely thin materials composed of only a few atomic layers. Particularly, 2D vdW magnets have attracted significant attention because of their potential applications in next-generation spintronic devices, and various novel 2D vdW magnets and their physical properties are being actively explored. A fascinating feature of 2D vdW materials is that different 2D materials can be stacked to build vdW heterostructures, which are artificial materials that do not exist in nature and can possess novel physical properties by interacting with each other through proximity effects at their interfaces. In heterostructures involving 2D vdW magnets, while the proximity effect from 2D magnets toward adjacent non-magnetic materials has been extensively studied, proximity effects from non-magnetic materials toward 2D vdW magnets have been less investigated.

Now, a research team led by Dr. Shigemi Terakawa (University of Osaka), Mr. Binbin Liu (École Polytechnique Fédérale de Lausanne), Prof. Titus Neupert (University of Zurich), Prof. Stuart S. P. Parkin and Prof. Niels B. M. Schröter (Max Planck Institute of Microstructure Physics), and Dr. Amilcar Bedoya-Pinto (University of Valencia), has discovered that the preferred direction of magnetization in 2D vdW (the magnetic easy axis) can be altered by a proximity effect from a substrate.

The results reveal that the magnetic properties and interfacial electronic states of 2D vdW magnets can be strongly influenced by proximity effects with substrates, offering new strategies for engineering the magnetic and electronic properties of spintronic devices based on 2D vdW magnets and their heterostructures.

In the study, researchers focus on Iron(II) Chloride (FeCl₂), which is a layered antiferromagnet in its bulk crystal. They successfully fabricated ultrathin epitaxial films of FeCl₂ on a Bismuth substrate using molecular beam epitaxy.

X-ray magnetic circular dichroism (XMCD) measurements performed at BOREAS beamline in ALBA and DEIMOS beamline in SOLEIL synchrotron revealed that the magnetic easy axis of the monolayer FeCl₂ is reoriented to a predominantly in-plane direction, in contrast to the out-of-plane magnetic anisotropy observed in the bulk FeCl₂. This reorientation vanishes in bilayer FeCl₂, where the magnetic easy axis recovers its intrinsic out-of-plane orientation.

This magnetic anisotropy reorientation is not attributed to the “monolayer” thickness itself but to a “proximity effect” from the Bismuth substrate, because the out-of-plane easy axis remains down to the monolayer in the FeCl₂ film grown on a Gold substrate. This is the first experimental demonstration that a nonmagnetic material can induce a reorientation of magnetic anisotropy in a 2D vdW magnet.

In addition, the research revealed interface electronic states between the monolayer FeCl₂ and the Bismuth surface using angle-resolved photoelectron spectroscopy (ARPES) performed at Bloch beamline of Max IV Laboratory. Detailed analysis of the Fermi surface showed that the interface states arise from the Bismuth surface states modified by the charge transfer and moiré potential formed by the FeCl₂ overlayer. Tight-binding calculation revealed that the interface states exhibit a characteristic trefoil shape.

These results demonstrate that proximity effects between 2D vdW magnets and nonmagnetic substrates strongly influence both the magnetic properties and interfacial electronic states. The findings provide an important step toward the future design of spintronic devices based on 2D vdW magnets and their heterostructures.