We investigated the evolution of ferromagnetism in layered Fe 3 GeTe 2 flakes under different pressures and temperatures using in situ magnetic circular dichroism (MCD) spectroscopy. We found that the rectangle shape of hysteretic loop under an out-of-plane magnetic field sweep can sustain below 7 GPa. Above that pressure, an intermediate state appears at low temperature region signaled by an 8-shaped skew hysteretic loop. Meanwhile, the coercive field and Curie temperature decrease with increasing pressures, implying the decrease of the exchange interaction and the magneto-crystalline anisotropy under pressures. The intemediate phase has a labyrinthine domain structure, which is attributed to the increase of ratio of exchange interaction to magneto-crystalline anisotropy based on Jagla's theory. Moreover, our calculation results reveal a weak structural transition around 6 GPa, which leads to a drop of the magnetic momentum of Fe ions. Introduction:Research on magnetic materials constructs the foundation of magnetic disk storages and spintronic devices. Since the intrinsic ferromagnetism in two-dimensional van der Waals crystals was discovered in 2017 1 , scientists paid much more attention to searching for novel 2D ferromagnets due to lots of advantages in fabricating nanoscale electronic devices, such as chemical stability 2 , effective modulation of ferromagnetism by external means [3][4][5] . The most important point is that ferromagnetism can maintain with decreasing the thickness of materials down to monolayer. So it can improve the density of data storage notably. Until now, it has been demonstrated experimentally that ferromagnetism can remain in Fe 3 GeTe 2 (FGT) and CrI 3 monolayers [6][7][8] . For 2D Ising ferromagnet in CrI 3 , it exhibits a layer-dependent transition between ferromagnetism and anti-ferromagnetism due to interlayer coupling and relative sliding between adjacent layers 9,10 , which can be further tuned by applying an out-of-plane electric field [11][12][13] and hydrostatic pressures in experiments 14,15 .
Two-dimensional (2D) transition metal dihalides (TMDHs) have been receiving extensive attention due to their diversified magnetic properties and promising applications in spintronics. However, controlled growth of 2D TMDHs remains challenging owing to their extreme sensitivity to atmospheric moisture. Herein, using a home-built nitrogen-filled interconnected glovebox system, a universal chemical vapor deposition synthesis route of high-quality 2D TMDH flakes (1T-FeCl 2 , FeBr 2 , VCl 2 , and VBr 2 ) by reduction of their trihalide counterparts is developed. Representatively, ultrathin (∼8.6 nm) FeCl 2 flakes are synthesized on SiO 2 /Si, while on graphene/Cu foil the thickness can be down to monolayer (1L). Reflective magnetic circular dichroism spectroscopy shows an interlayer antiferromagnetic ordering of FeCl 2 with a Neel temperature at ∼17 K. Scanning tunneling microscopy and spectroscopy further identify the atomic-scale structures and band features of 1L and bilayer FeCl 2 on graphene/Cu foil.
Ambipolar field-effect transistor (FET) devices based on two-dimensional (2D) materials have been attracted much attention due to potential applications in integrated circuits, flexible electronics and optical sensors. However, it is difficult to tune Fermi level between conduction and valence bands using a traditional SiO2 as dielectric layer. Here, we employed the lithium-ion conductive glass ceramic (LICGC) as the back-gate electrode in a monolayer WS2 FET. The effective accumulation and dissipation of Li+ ions in the interface induce a wide tune of Fermi level in the conducting channel by electron and hole doping, which show an ambipolar transport characteristics with threshold voltages at 0.9 V and −1.3 V, respectively. Our results provide an opportunity for fabricating ultra-thin ambipolar FET based on 2D materials.
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