Air-Stable and Layer-Dependent Ferromagnetism in Atomically Thin van der Waals CrPS₄

TLDR

Two‑dimensional ferromagnets such as CrI₃ promise tunable spintronic functionality, yet many are unstable in air, limiting practical use. We discovered that the van der Waals crystal CrPS₄ exhibits air‑stable, layer‑dependent ferromagnetism, with monolayer out‑of‑plane FM and alternating AFM/FM behavior in even and odd layers that persists after a day of air exposure.

Abstract

Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI3, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. Here, we report the observation of an air-stable and layer-dependent ferromagnetic (FM) van der Waals crystal, CrPS4, using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in the monolayer crystal. Furthermore, alternating AFM and FM properties observed in even and odd layers suggest robust antiferromagnetic exchange interactions between layers. The observed ferromagnetism in these crystals remains resilient even after the air exposure of about a day, providing possibilities for the practical applications of van der Waals spintronics.