Tunneling Transport and Spectroscopy of Two-Dimensional Magnetic Materials
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Date
2022-08-24
Authors
Yang, Bowen
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Publisher
University of Waterloo
Abstract
The recent discoveries of magnetism in the single atomic layer have opened up a new direction for two-dimensional (2D) materials research. In this thesis, two types of magnetic materials are investigated: chromium trihalides and α-RuCl3. Benefiting from the layer-dependent magnetism of few-layer CrI3, we realize a very large negative magnetoresistance in a van der Waals heterostructure incorporating few-layer CrI3, arising from spin flipping across the CrI3 atomic layers. At certain voltage bias, the value of magnetoresistance reaches nearly one million percent. This finding provides new opportunities for spintronics devices and elucidates the nature of the magnetic state in ultrathin CrI3.
Prompted by the large magnetoresistance in CrI3, we then conduct a comprehensive study for the entire family of magnetic chromium trihalides (CrX3, X=I, Br, Cl) by incorporating both few-layer and bilayer samples in van der Waals tunnel junctions. The tunneling measurements with magnetic field, combined with magnetic circular dichroism data, uncover interlayer magnetism, exchange gap and magnetic anisotropy of the three materials. Moreover, we demonstrate that ferromagnetism can persist down to monolayer in CrBr3. We then perform inelastic electron tunneling spectroscopy measurement for studying their magnon excitations. Their spin Hamiltonians are later determined by fitting with spin wave calculations.
Finally, we change our focus to α-RuCl3, which is predicted to realize spin liquid in the frame of Kitaev physics. We use a combination of tunneling spectroscopy, magnetotransport, electron diffraction, and ab initio calculations to study the layer-dependent magnons, anisotropy, structure, and exchange coupling in atomically thin samples. We find that the extremely large magnetic anisotropy in bulk crystals is reversed in monolayer. Given that the predicted magnetic field to make a spin-liquid phase transition is hardly accessible in bulk crystals, this observation shows that quantum spin liquid phase is possibly easier to induce in a pure 2D limit.