# Proximity Induced Magnetic Anisotropy and Trefoil Fermiology in Monolayer FeCl2/Bi(111)

## TL;DR

A 2D magnetic material's magnetic direction changes when placed on a non-magnetic bismuth surface, showing how substrates can influence magnetic properties.

## Contribution

Demonstrates non-magnetic substrates can reorient magnetic anisotropy in 2D magnets through proximity effects.

## Key findings

- Monolayer FeCl2 on Bi(111) shows in-plane magnetic anisotropy instead of its natural out-of-plane orientation.
- Moiré-induced interface states and a trefoil-shaped Fermi surface are observed in the FeCl2/Bi(111) heterostructure.
- Bilayer FeCl2/Bi(111) reverts to intrinsic out-of-plane magnetic anisotropy.

## Abstract

Interfaces between magnetic and non‐magnetic materials play a crucial role in various magnetic heterostructures. The emergence of 2D van der Waals (vdW) magnets has introduced new opportunities for exploring proximity effects in vdW heterostructures. While the influence of magnetic layers on nearby non‐magnetic materials has been widely studied, it remains unclear whether non‐magnetic substrates can similarly modulate the intrinsic magnetic properties of 2D magnets, particularly their magnetic anisotropy. In this work, by analyzing X‐ray magnetic circular dichroism spectra of an epitaxially grown FeCl2 monolayer on a Bi(111) surface, a reorientation of magnetic anisotropy is observed – from its natural out‐of‐plane to a predominantly in‐plane alignment. This effect vanishes in bilayer FeCl2/Bi(111), where the magnetic anisotropy reverts to its intrinsic out‐of‐plane orientation, consistent with the layered antiferromagnetic order of bulk FeCl2. Angle‐resolved photoelectron spectroscopy reveals the presence of metallic interface states derived from the Bi surface states, accompanied by charge transfer and emergence of a moiré potential that gives rise to a distinctive trefoil‐shaped Fermi surface. These results demonstrate that non‐magnetic substrates can exert strong proximity influence on the magnetic and electronic behavior of 2D vdW magnets, offering new strategies for engineering magnetic anisotropy and electronic structure in spintronic heterostructures.

Reorientation of magnetic anisotropy in a 2D van der Waals magnet is demonstrated in monolayer FeCl2 epitaxially grown on a bismuth substrate, from its natural out‐of‐plane direction to a predominantly in‐plane orientation. The FeCl2/bismuth heterostructure exhibits moiré‐induced interface states, suggesting strong coupling at the interface. These findings highlight new strategies for engineering magnetic anisotropy in spintronic heterostructures.