The surface degradation and its impact on the magnetic properties of bulk VI3

Despite belonging to a well-studied family of transition metal trihalides, VI3 has received significant attention only recently. As a hard ferromagnetic van der Waals compound with a large coercivity, it attracted much attention because of its potential use in atomically thin spintronic and optoelectronic devices. However, practical exploration of VI3 is challenging due to its instability under ambient conditions. Here, we present a comprehensive set of optical, x-ray diffraction, magnetization, and ellipsometric measurements of VI3 and demonstrate that, similarly to the related van der Waals ferromagnet CrI3, the degradation process is accelerated by the presence of moisture. The VI3 surface was covered by selected media commonly used in physical measurements to test its stability and lower the degradation rate three times or higher, providing practical information for experimentalists interested. The decomposition study at ambient conditions shows that the VI3 single crystal can be used for most of the bulk, magnetization, and optical measurements without any noticeable change of physical properties, as the significant degradation appears first after ∼2 h of exposition as illustrated, e.g., by the evolution of the ferromagnetic T1 and T2 transitions. The ellipsometric measurement demonstrates that even the surface remains optically stable for at least 5 min.