Abstract
Author(s): O'Hara, Dante Jamal | Advisor(s): Tom, Harry; Kawakami, Roland K | Abstract: Van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for next generation scientific and technological advances. While previously realized in 3D ultrathin films where the magnetism is stabilized via substrate-assisted magnetic anisotropy, recent reports have shown intrinsic ferromagnetism at low temperatures (l 60 K) in isolated µm-sized flakes mechanically exfoliated from a bulk single-crystal down to a single-atomic layer. This opens up the possibility to truly study magnetism in free-standing 2D layers without direct effects from the underlying substrate and being intrinsically susceptible to surface effects such as atomic adsorbates, electrostatic gating, and proximity-induced phenomena. This dissertation examines the molecular beam epitaxy (MBE) growth and characterization of new 2D magnets, monolayers of MnSe2 and VSe2, that show ferromagnetic ordering above room temperature. Growth on different substrates and varying the substrate temperature during growth affects the growth mode and morphology of the deposited 2D magnet and also affects the measured magnetization. Direct atomic and magnetic imaging via scanning transmission electron microscopy (STEM) and scanning tunneling microscopy (STM) show stable 2D magnetic layers.Due to the lack of dangling bonds at the surface of these 2D magnets, applying external epitaxial strain is a challenge. Later in this dissertation, the magnetic, electronic, and structural properties of vdW-layered, Fe-deficient Fe3−xGeTe2 are systematically investigated by he application of high pressure. Fe3GeTe2 is of particular interest due to its high Curie temperature, Tc, strong perpendicular magnetic anisotropy, and tunable magnetic properties depending on the concentration of Fe and its thickness. Electrical and magneto-transport measurements show a suppression in Tc with an increasing pressure up to 20 GPa. The decrease in Tc is due to the lattice shrinkage from pressurization which leads to a weakening of the exchange interaction. These observations showcase the tunability in vdW magnets via pressure which can complement other external stimuli such as chemical doping, making them candidates for future spintronic, electronic and photonic devices.
Highlights
Introduction to Magnetism and Magnetic MaterialsMagnetism and magnetic materials have become heavily involved in our everyday lives
We attribute the magnetic signal to intrinsic ferromagnetism of a van der Waals (vdW) manganese diselenide (MnSe2) monolayer, while for thicker films it could originate from a combination of vdW MnSe2 and/or interfacial magnetism of α-MnSe(111)
Our study further shows the importance of performing control experiments on the magnetic background signal for epitaxially grown van der Waals magnets on a case-by-case basis
Summary
Introduction to Magnetism and Magnetic MaterialsMagnetism and magnetic materials have become heavily involved in our everyday lives. Using the mechanical exfoliation method on van der Waals CrI3 and Cr2Ge2Te6, researchers thinned the crystals down to monolayers (bilayers for Cr2Ge2Te6) and obtained ferromagnetic signals from μm-sized flakes at cryogenic temperatures Since these discoveries, extensive studies have been performed on both materials, and exciting properties, such as gate tunable magnetism [241, 242, 243, 244, 245], strong magnetic proximity when coupled to a non-magnetic material [217], giant tunneling magnetoresistance and spin-filtering effects [97, 98, 99, 246] have been reported. Recent studies showing intrinsic ferromagnetism in the monolayer limit of van der Waals (vdW) materials has opened many opportunities to study two-dimensional (2D) magnetism and other scientific explorations [90, 93] Fascinating properties such as layer dependence, gate-tunable magnetism and giant magnetoresistances in tunneling junctions consisting of mechanically exfoliated CrI3 [97, 98, 100], and room temperature ferromagnetic ordering in large-area films of monolayer MnSe2 [107, 258] and VSe2 [106] grown by molecular beam epitaxy have been reported showing the potential integration for spin-based technological applications. To develop a better insight of their bulk counterparts, studies under varied environmental conditions will give a firmer foundation for going toward the 2D limit of these compounds
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