  Two-dimensional (2D) materials are promising candidates for next-generation
electronic devices. In this regime, insulating 2D ferromagnets, which remain
rare, are of special importance due to their potential for enabling new device
architectures. Here we report the discovery of ferromagnetism in a layered van
der Waals semiconductor, VI3, which is based on honeycomb vanadium layers
separated by an iodine-iodine van der Waals gap. It has a BiI3-type structure
(R-3, No.148) at room temperature, and our experimental evidence suggests that
it may undergo a subtle structural phase transition at 78 K. VI3 becomes
ferromagnetic at 49 K, below which magneto-optical Kerr effect imaging clearly
shows ferromagnetic domains, which can be manipulated by the applied external
magnetic field. The optical band gap determined by reflectance measurements is
0.6 eV, and the material is highly resistive.
