Surface oxidation and magnetic instability of two-dimensional chromium tellurides*

Two-dimensional (2D) chromium tellurides (Cr_1+δTe₂) have recently attracted growing interest owing to their versatile magnetic and electronic properties, including high-temperature perpendicular ferromagnetism, topologically nontrivial spin textures, and a large anomalous Hall effect, which offer great opportunities for 2D spintronic applications. To realize practical applications, one must also consider the stability of the 2D materials. While most 2D ferromagnets discovered thus far do not show robust stability in ambient conditions, various Cr_1+δTe₂ compounds have been reported to be air-stable, despite the known sensitivity of tellurides in air. In this work, we have performed detailed and systematic studies on the stability of a representative Cr_1+δTe₂ phase (Cr₂Te₃), where we evidenced its rapid oxidation in an ambient environment via dramatic changes in Raman vibrational modes, growth of oxides in X-ray photoelectron spectroscopy and transmission electron microscopy studies, as well as significant reductions of magnetization over time. Density functional theory calculations suggest that the surface oxidation of chromium tellurides is thermodynamically favorable.