Experiments on strongly correlated materials: magneto-transport properties of VO2 AND V2O3

Abstract

Vanadium oxides provide unusual electrical and magnetic phenomena emerging from strong electronic correlations, which include, among other things, a thermally induced metal-insulator transition (MIT). Investigation of the changes in carrier concentration and mobility across the MIT in vanadium oxides, such as vanadium dioxide (VO2) and vanadium sesquioxide (V2O3), carries great importance for understanding the micromechanisms behind suchfirst-order phase transitions. A well-known approach to measuring such parameters in semiconductor materials is Hall effect measurement. So far, magnetotransport studies have only been conducted on polycrystalline thinfilms of VO2/V2O3. As a result, reports on the Hall mobility of these materials often contradict with each other due to the non-uniform stress building on the crystal by adhesion to the substrate. Thus, a thorough investigation of Hall effect measurements on single-crystalline, stress-free VO2 nanobeams and V2O3 nanoplates is required. However, achieving this task is not a straightforward process. First of all, the relatively small size of nanobeams compared to the epitaxialfilms creates the necessity to utilize a bridge-type Hall-bar shaping of the crystal. Additionally, in order to produce a stress-free environment, the crystals must be detached from the substrate and transferred to an atomically at surface, such as hexagonal boron nitride (h-BN). Therefore, the device fabrication method demands many steps despite that VO2 is a very fragile material. In this work, we provide a new fabrication method for shaping VO2 and V2O3 into Hall-bar structure via Gallium and Argon-ion milling while inducing minimal damage on the crystal. We also investigate the strain level of shaped crystals and provide methods to prevent cracking in the devices upon structural phase As a second objective, we investigate the resistivity behavior and magnetic response of VO2 nanobeams at low temperature ranges. We show that the high magnetoresistance of VO2 creates demand for very high magneticfields in the Hall effect measurements. Finally, we demonstrate a Hall effect measurement on an as-grown V2O3 nanoplatelet across its phase transition.