Coupling of Crystal Structure and Magnetism in the Layered, Ferromagnetic Insulator CrI₃

TLDR

CrI3 adopts a rhombohedral structure at low temperatures and transforms to a monoclinic structure near 210–220 K. CrI3 exhibits a first‑order crystallographic transition near 210–220 K with strong thermal hysteresis, coupled to its ferromagnetism, as evidenced by anomalies in interlayer spacing at the Curie temperature and in magnetic susceptibility at the structural transition, and first‑principles calculations predict that the bulk ferromagnetic order should survive in monolayers, making CrI3 a promising spintronic material.

Abstract

We have examined the crystallographic and magnetic properties of single crystals of CrI3, an easily cleavable, layered and insulating ferromagnet with a Curie temperature of 61 K. Our X-ray diffraction studies reveal a first-order crystallographic phase transition occurring near 210–220 K upon warming, with significant thermal hysteresis. The low-temperature structure is rhombohedral (R3̅, BiI3-type) and the high-temperature structure is monoclinic (C2/m, AlCl3-type). We find evidence for coupling between the crystallographic and magnetic degrees of freedom in CrI3, observing an anomaly in the interlayer spacing at the Curie temperature and an anomaly in the magnetic susceptibility at the structural transition. First-principles calculations reveal the importance of proper treatment of the long-ranged interlayer forces, and van der Waals density functional theory does an excellent job of predicting the crystal structures and their relative stability. Calculations also suggest that the ferromagnetic order found in the bulk material may persist into monolayer form, suggesting that CrI3 and other chromium trihalides may be promising materials for spintronic and magnetoelectronic research.