Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chemical stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temperatures. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, Tc, is suppressed relative to the bulk Tc of 205 kelvin in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temperature, much higher than the bulk Tc. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7-11 based on atomically thin van der Waals crystals.

Gate-tunable Room-temperature Ferromagnetism in Two-dimensional Fe 3 GeTe 2

A novel crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2 the top layer of selenium atoms are substituted by sulfur atoms while the bottom selenium layer remains intact. The peculiar structure of this new material is systematically investigated by Raman, photoluminescence and X-ray photoelectron spectroscopy and confirmed by transmission-electron microscopy and time-of-flight secondary ion mass spectrometry. Density-functional theory calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction (HER) activity is discovered for the Janus monolayer and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Janus Monolayer Transition Metal Dichalcogenides

We report the connection between the stacking order and magnetic properties of bilayer CrI3 using first-principles calculations. We show that the stacking order defines the magnetic ground state. By changing the interlayer stacking order, one can tune the interlayer exchange interaction between antiferromagnetic and ferromagnetic. To measure the predicted stacking-dependent magnetism, we propose using linear magnetoelectric effect. Our results not only gives a possible explanation for the observed antiferromagnetism in bilayer CrI3 but also have direct implications in heterostructures made of two-dimensional magnets.

Stacking-dependent magnetism in bilayer CrI3

Proximity effects in two-dimensional (2D) van der Waals heterostructures offer controllable ways to tailor the electronic band structure of adjacent materials. Proximity exchange in particular is important for making materials magnetic without hosting magnetic ions. Such synthetic magnets could be used for studying magnetotransport in high-mobility 2D materials, or magneto-optics in highly absorptive nominally nonmagnetic semiconductors. Using first-principles calculations, we show that the proximity exchange in monolayer MoSe2 and WSe2 due to ferromagnetic monolayer CrI3 can be tuned (even qualitatively) by twisting and gating. Remarkably, the proximity exchange remains the same when using antiferromagnetic CrI3 bilayer, paving the way for optical and electrical detection of layered antiferromagnets. Interestingly, the proximity exchange is opposite to the exchange of the adjacent antiferromagnetic layer. Finally, we show that the proximity exchange is confined to the layer adjacent to CrI3, and that adding a separating hBN barrier drastically reduces the proximity effect. We complement our ab initio results with tight-binding modeling and solve the Bethe-Salpeter equation to provide experimentally verifiable optical signatures (in the exciton spectra) of the proximity exchange effects.

/pdf/proximity-exchange-effects-in-mose-2-and-wse-2-2117xfjzaz.pdf

Proximity exchange effects in MoSe 2 and WSe 2 heterostructures with CrI 3 : Twist angle, layer, and gate dependence

Remarkably enhanced Curie temperature in monolayer CrI3 by hydrogen and oxygen adsorption: A first-principles calculations

As MoSSe has been successfully synthesized, it is expected that Janus structures would be newly developed as 2D materials with intriguing physical properties. Here, monolayer Janus GdXY (with X/Y = Cl, Br, I), a ferromagnetic semiconductor with a high Curie temperature, is predicted by density functional theory combined with Monte Carlo simulations. The calculations show that the cleavage energies of monolayers 2 H - GdX 2 (X = Cl, Br, I) and GdXY are about 0.21 J / m 2, which are smaller than that of graphene. The Janus GdXY monolayers have semiconducting bandgaps and show in-plane ferromagnetic order with Curie temperatures of 167–181 K. The magnetic moment is as high as 8 μ B/Gd. In addition, GdXY monolayers also have strong Heisenberg exchange interactions in the range of 35–37 meV, which are larger than that of CrI 3. We also find that both of their magnetic anisotropy and Curie temperature can be largely altered by strain. High Curie temperatures and 100% spin polarization make Janus GdXY monolayers good platforms for the research studies of spintronics devices.

High Curie temperatures in Gd-dihalide Janus monolayers

First-principles calculations of magnetic edge states in zigzag CrI3 nanoribbons

The invention provides a chromium triiodide electric current spinning controller based on a bending mechanism. The chromium triiodide electric current spinning controller comprises a chromium triiodide magnetic film, a substrate, a piezoelectric crystal and a power supply; the chromium triiodide magnetic film is fixedly connected with the substrate in a film thickness direction, the chromium triiodide magnetic film and one side of the substrate in a first direction are respectively fixed, and the other side of the substrate in the first direction is fixedly connected with one side, perpendicular to a deformation direction, of the piezoelectric crystal; a side of the chromium triiodide magnetic film in a second direction is used for enabling an electric current about to be subjected to spinfiltering to pass through; and one end of the power supply is connected with one side, perpendicular to an electric field direction, of the piezoelectric crystal, and the other end of the power supply is connected with the other side, perpendicular to the electric field direction, of the piezoelectric crystal. The electric current spinning controller enables the substrate to be bent through deformation of the piezoelectric crystal, so that a CrI3 film is extruded or stretched, a non-spin-polarized electric current passing through a CrI3 thin film can be filtered into a single-spin electric current with a spin polarization direction controllable, and property of an electric current passing through the CrI3 thin film also can be not changed.

Chromium triiodide electric current spinning controller based on bending mechanism

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High Curie temperatures in Gd-dihalide Janus monolayers

First-principles calculations of magnetic edge states in zigzag CrI3 nanoribbons

Chromium triiodide electric current spinning controller based on bending mechanism