Imaging Domain Reversal in an Ultrathin Van der Waals Ferromagnet

• Published in: Advanced materials (Weinheim) 2020-10, Vol.32 (39), p.e2003314-n/a
• Main Authors: Broadway, David A., Scholten, Sam C., Tan, Cheng, Dontschuk, Nikolai, Lillie, Scott E., Johnson, Brett C., Zheng, Guolin, Wang, Zhenhai, Oganov, Artem R., Tian, Shangjie, Li, Chenghe, Lei, Hechang, Wang, Lan, Hollenberg, Lloyd C. L., Tetienne, Jean‐Philippe
• Format: Article
• Language: English
• Subjects: 2D magnetism; Anisotropy; Coercivity; Curie temperature; Domain walls; Ferromagnetic materials; Flakes; Hysteresis; magnetic domains; magnetic imaging; Magnetic materials; Magnetic properties; Magnetic semiconductors; Magnetism; Magnetization; nitrogen‐vacancy centers; Nucleation; Switching; Thickness; van der Waals materials; vanadium triiodide
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202003314

The recent isolation of 2D van der Waals magnetic materials has uncovered rich physics that often differs from the magnetic behavior of their bulk counterparts. However, the microscopic details of fundamental processes such as the initial magnetization or domain reversal, which govern the magnetic hysteresis, remain largely unknown in the ultrathin limit. Here a widefield nitrogen‐vacancy (NV) microscope is employed to directly image these processes in few‐layer flakes of the magnetic semiconductor vanadium triiodide (VI3). Complete and abrupt switching of most flakes is observed at fields Hc ≈ 0.5–1 T (at 5 K) independent of thickness. The coercive field decreases as the temperature approaches the Curie temperature (Tc ≈ 50 K); however, the switching remains abrupt. The initial magnetization process is then imaged, which reveals thickness‐dependent domain wall depinning fields well below Hc. These results point to ultrathin VI3 being a nucleation‐type hard ferromagnet, where the coercive field is set by the anisotropy‐limited domain wall nucleation field. This work illustrates the power of widefield NV microscopy to investigate magnetization processes in van der Waals ferromagnets, which can be used to elucidate the origin of the hard ferromagnetic properties of other materials and explore field‐ and current‐driven domain wall dynamics. A magnetic imaging technique based on nitrogen‐vacancy centers in diamond is used to investigate ultrathin flakes of vanadium triiodide (VI3). The results reveal that ultrathin VI3 is a nucleation‐type hard ferromagnet with a coercivity set by the anisotropy strength. The spontaneous magnetization determined for individual flakes is found to be lower than in bulk VI3.