Maciej Koperski
The magnetism of chromium has been investigated for almost a century now, providing substantial knowledge about its electronic configuration. Extensive research has been conducted regarding the physics of valence electrons from d-shell, which is fundamentally important for understanding the mechanisms of magnetic ordering. Interestingly, chromium atom, exhibiting a stable electronic configuration exempting from Hund’s rules, has half-filled 3d shell, which leads to manifestation of robust magnetic effects in a variety of structures. Recently, attention has been refocused on chromium trihalides (CrCl3, CrBr3 and CrI3), which constitute a group of electrically insulating layered materials displaying magnetic ordering at low temperatures, as established by inspection of bulk crystals carried out few decades ago. The progress of mechanical exfoliation techniques, performed in a controlled argon atmosphere, enables now isolation of thin layers (down to monolayers) and their incorporation in van der Waals heterostructures.
Initial reports demonstrated layer-dependent ferromagnetic and anti-ferromagnetic order below Curie temperature using Kerr rotation measurements as magnetization probe. These appealing findings motivate further study to uncover the underlying microscopic mechanisms. One possible path to learn about the electronic structure and characteristics of electronic states via optical methods involves investigations of emission and absorption processes. Here, we present detailed optical studies of exfoliated films of CrBr3 and CrI3 to demonstrate that the emergent interband luminescence has molecular-like character (most likely due to formation of Frenkel-type excitons) and the details of the structure of emission resonances can be explained by Franck-Condon principle involving multiple phonon modes. The photoluminescence studies unveil unambiguous signatures of coupling between the magnetic moments of Cr3+ ions with band carriers, offering insight into fundamental properties of these novel magnetic structures and opening up new routes for potential applications of 2D systems.