Van der Waals (vdW) magnets have been shown to retain their magnetic properties even when exfoliated down to single or few vdW layers. With such thin layers that are typically covalently bonded for high strength, these 2D magnets are distinguished by their extreme tunability by electrostatic gating and by strain. In addition, the highly anisotropic structure can generate strong perpendicular magnetic anisotropy, and the prevalence of hexagonal and honeycomb lattices can promote the formation of topological band structures for electrons and magnons.
Molecular Beam Epitaxy: While we are also investigating exfoliated 2D magnets, our group is focusing a lot of effort in synthesizing 2D magnets by molecular beam epitaxy (MBE). Beyond the benefits of epitaxial films over exfoliated flakes for technology development, the real benefit lies in the ability to create new artificial materials and to tune the properties by atomic level modification. Since the synthesis can be performed away from thermal equilibrium, MBE does more than simply replicate bulk crystals in a thin film form; it can produce new materials that cannot be achieved by equilibrium bulk crystal synthesis methods.
Highlights
Room temperature 2D magnet: MnSe2
Our first set of studies focused on the development of MnSe2 monolayers which were found to exhibit ferromagnetism at room temperature (O’Hara et al.). Shown to the right is a magnetic hysteresis loop taken at room temperature by SQUID magnetometry. This work was based on earlier work by the Furdyna group on Mn-doped SnSe2. In their study, weak ferromagnetism was observed at room temperature. However, the randomly placed Mn atoms led to a dominant antiferromagnetic interaction that led to a small net moment (~0.09 Bohr magnetons/Mn). By placing the Mn into a plane, we observed an enhancement of the net moment to 3-4 Bohr magnetons/Mn, which is the correct order of magnitude. While the room temperature ferromagnetism is an exciting outcome, some limitations emerged. First, growing thicker than a monolayer causes the material to revert to α-MnSe, an antiferromagnet with a rock-salt structure. Second, attempts to grow MnSe2 on topological insulator Bi2Se3 led to interfacial chemical reaction (Noesges et al.) and the interdiffusion led to the creation of magnetic topological insulator MnBi2Se4 (Zhu et al.), a vdW material that cannot be produced by bulk crystal synthesis (see Magnetic Topological Materials for more details).
Epitaxial Fe3GeTe2 on Ge(111) with sharp interfaces
For a robust 2D ferromagnet, we turned out attention to epitaxial Fe3GeTe2. These have strong perpendicular magnetic anisotropy with square hysteresis loops and remain ferromagnetic from monolayers to multilayers. Typical TC ranges from 150 – 200 K and room temperature ferromagnetism was observed under various conditions. With growth optimization, we achieved high-quality Fe3GeTe2 films grown on Ge(111) substrates (Zhou et al.). As shown on the right, optical magnetic circular dichroism (MCD) measurements with out-of-plane magnetic field show nice square hysteresis loops and a TC of ~250 K. Further, the cross-sectional TEM image shows a sharp interface between the Fe3GeTe2 films (top part) and the Ge(111) substrate (bottom part). Initially, we found substantial amounts of secondary phases (Fe3Ge2) at the interface and later realized that their formation could be prevented by kinetic control (i.e. faster growth rates). The next steps are to realize TC above room temperature (following other reports) and to understand the origin of the TC enhancement. We also hope to employ the epitaxial Fe3GeTe2 in spintronic devices.
For the future, we are interested in many other epitaxial 2D magnets including ferromagnetic semiconductors, antiferromagnets, and topological magnets.
Science | 2D Materials: Spintronics, Magnetism, and Photonics | 2D Magnets