Topological hall effect in bismuth/chromium telluride heterostructures and chromium telluride thin films with self-intercalation

Abstract

Chromium Telluride thin films attract the attention of numerous researchers because of their fancy magnetic properties and potential applications in spintronic devices. The crystal structure and magnetic properties, including magnetic easy axis and Curie temperature (TC) of Cr-Te compound thin films, are affected by the stoichiometric ratio of Cr and Te. Among them, Cr2Te3 thin film is widely researched because its strong perpendicular magnetic anisotropy (PMA) provides a promising component to generate novel properties with other topological materials. For example, the topological Hall effect (THE) was observed in the Cr2Te3 lattice with embedded black-phosphorus-like bismuth (BP-Bi) nanosheets. THE is induced by spatially varying magnetizations, including topological states such as magnetic skyrmions. Therefore, magnetic skyrmions might emerge in the Bi-Cr2Te3 system. Bi/Cr2Te3 heterostructure thin films with different thicknesses of Bi layers were fabricated in this thesis work. Bi (1 1 0) surface was grown on the Cr2Te3 (0 0 0 1) layer in islands from scanning transmission electron microscope (STEM) observation. THE signals confirmed in the method of minor loop were observed in the Bi/Cr2Te3 heterostructure thin films below 130 K and the intensity of THE signals would be affected by plenty of parameters including thicknesses of Bi layers, strains and the quality of Bi/Cr2Te3 interface. In addition, Cr-Te thin films with different Cr/Te flux ratios were investigated. The atomic-level STEM images revealed that some wider spacing bilayers appeared in these samples and their density is negatively proportional to the Cr/Te flux ratio. Such self-intercalation generated THE in Cr2Te3 thin films below 120 K and minor loop measurement ruled out the possibility of two-component AHE. Besides, the Bi layer was grown on the self-intercalated Cr-Te surface. The interface is uniform and continuous. THE signals in this sample persisted at 120 K. This sample is promising for future observation of magnetic skyrmions in the Lorentz transmission electron microscope (LTEM) at liquid nitrogen temperature.