Ferromagnetism at surface and step-edge boundaries in van der waals chromium ditelluride flakes at above room temperature
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Authors
Dhull, Neha
Issue Date
2025-12
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Physics
Alternative Title
Abstract
While emerging 2D magnetic layered materials hold exciting prospects for spintronics and nanoelectronics, several major challenges are limiting their use for practical applications. First, it is hard to achieve and sustain intrinsic long-range magnetic ordering at room temperature. Second, many conventional characterization techniques are only suitable for bulk continuous crystals making the accurate probing of near-surface magnetic properties a major limitation. Chromium ditelluride (CrTe2) with its van der Waals layered structure and sustained ferromagnetism with a Curie temperature 317 K in both bulk and a few layers, has emerged as a promising candidate to address these challenges. To overcome the second issue of probing ultrathin regimes (< 10 nm), the surface magneto-optical Kerr effect (SMOKE) with its near-surface detection due to several nm probing depth of laser light in metals offers exciting avenues to explore ultrathin continuous films as well as small size flakes in the ambient environment at room temperature. In this thesis, magnetic characterization using a home-built SMOKE setup at Rensselaer Physics and a commercial dual mode atomic force microscopy (AFM) and magnetic force microscopy (MFM) was employed to probe the near-surface magnetic properties of layered CrTe2. It is known the magnetic and structural properties are closely related. The structure and stoichiometry of CrTe2 single crystals grown using a K-deintercalation method were investigated using X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), electron backscatter diffraction (EBSD), and Raman spectroscopy. Raman signatures are crucial in distinguishing phase purity and tracking degradation and phase change in ambient environments. The SMOKE hysteresis loop and coercivity for CrTe2 in less than 10 nm thickness regimes show promising evidence of sustained long-range magnetic ordering at room temperature. For SMOKE, the incident laser angle changes with respect to surface normal accounted for changes in penetration depth and illumination area on the sample surface. These small variations provided insight into coercivity variation with the thickness and roughness of the crystal surface. The findings suggest that surface roughness dominates in influencing magnetic properties such as coercivity and Kerr intensity for CrTe2 crystal. This is very different from that of smoother ferromagnetic metals such as Nickel and Cobalt.
Magnetic force microscopy (MFM) measurements revealed strong in-plane magnetization and significantly enhanced magnetic phase contrast at the step edges of exfoliated CrTe2 flakes’ boundaries. The enhancement at the flake’s boundary depends on its shape. The enhanced magnetic phase contrast signal at step edges boundaries is about four times stronger than interior regions, indicating ~ 300% enhancement at flake’ boundaries. This is the first detailed observation of step edge-dominated enhanced ferromagnetism in CrTe2 flakes using MFM. The experimental results were further verified using magnetic stray field simulations. While MFM only provides magnetic phase shift contrast, it lacks magnetic field magnitude. To address this, scanning nitrogen vacancy magnetometry (SNVM) was employed. For a 75 nm step edge, magnetic fields around ~ 50 Gauss were detected, which is quite consistent with the observed strong enhanced magnetic stray fields at the step edges using MFM. This study demonstrates how the step-edge geometry in CrTe2 strikingly enhances the magnetic contrast. This edge-dominated ferromagnetism, verified by both experimental and simulation techniques, provides a pathway to engineer magnetic textures in van der Waals layered materials. These findings open new possibilities for designing structurally driven magnetic functionalities in CrTe2 and related layered magnets with high Curie temperatures.
Description
December2025
School of Science
School of Science
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY