Vapor Deposition of Magnetic Van der Waals NiI2 Crystals

The recent discovery of van der Waals magnetic materials has attracted great attention in materials science and spintronics. The preparation of ultrathin magnetic layers down to atomic thickness is challenging and is mostly by mechanical exfoliation. Here, we report vapor deposition of magnetic van der Waals NiI₂ crystals. Two-dimensional (2D) NiI₂ flakes are grown on SiO₂/Si substrates with a thickness of 5-40 nm and on hexagonal boron nitride (h-BN) down to monolayer thickness. Temperature-dependent Raman spectroscopy reveals robust magnetic phase transitions in the as-grown 2D NiI₂ crystals down to trilayer. Electrical measurements show a semiconducting transport behavior with a high on/off ratio of 10⁶ for the NiI₂ flakes. Lastly, density functional theory calculation shows an intralayer ferromagnetic and interlayer antiferromagnetic ordering in 2D NiI₂. This work provides a feasible approach to epitaxy 2D magnetic transition metal halides and also offers atomically thin materials for spintronic devices.