Coupling of crystal structure and magnetism in the layered, ferromagnetic insulator cri₃

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 (R3¯, 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.