The recent advent of atomically-thin ferromagnetic crystals has allowed experimental studies of two-dimensional (2D) magnetism 1-9 that not only exhibits novel behavior due to the reduced dimensionality but also often serves as a starting point for understanding of the magnetic properties of bulk materials 10-17 . Here we employ ballistic Hall micromagnetometry 18,19 to study magnetization of individual 2D ferromagnets. Our devices are multilayer van der Waals (vdW) heterostructures 20 comprising of an atomically-thin ferromagnetic crystal placed on top of a Hall bar made from encapsulated 21 graphene. 2D ferromagnets can be replaced repeatedly, making the graphene-based Hall magnetometers reusable and expanding a range of their possible applications. The technique is applied for the quantitative analysis of magnetization and its behavior in atomically thin CrBr 3 . The compound is found to remain ferromagnetic down to a monolayer thickness and exhibit high out-of-plane anisotropy. We report how the critical temperature changes with the number of layers and how domain walls propagate through the ultimately thin ferromagnets. The temperature dependence of magnetization varies little with thickness, in agreement with the strongly layered nature of CrBr 3 . The observed behavior is markedly different from that given by the simple 2D Ising model normally expected to describe 2D easy-axis ferromagnetism. Due to the increasingly common usage of vdW assembly, the reported approach offers vast possibilities for investigation of 2D magnetism and related phenomena.Research on magnetism in strongly layered (vdW) materials is only a couple of years old but has already revealed a number of interesting phenomena including, for example, unexpected changes in magnetic properties as a function of the number of layers 2,17 and the possibility to control magnetism by electric and chemical doping [12][13][14][15][16]22 . Of particular interest are ferromagnetic semiconductors such as Cr 2 Ge 2 Te 6 and CrI 3 , in which a magnetization-dependent optical response and switching of a magnetization direction by applied electric field have been reported [12][13][14][15][16] . A number of different techniques have been employed to study magnetic properties of the above compounds at a few-layer thickness, including magneto-optical Kerr effect 1,2,15 , circular dichroism

1 of 9) 1605928applications. [8][9][10][11][12][13][14] A spin quantum Hall state is also predicted in the distorted octahedral phase (1T′) of MX 2 in the monolayer limit, further extending applications of TMDs into spintronics and lowdissipation electronics. [13] As a part of the TMDs family, WTe 2 has recently attracted great interest due to its giant, nonsaturating magnetoresistance (MR) observed in bulk crystals, [15] and its predicted Weyl state. [16] Pressure-induced superconductivity and large spin-orbit coupling are also observed. [17,18] In addition, the lattice thermal conductivity of WTe 2 is predicted to be smaller than that of WSe 2 due to the heavier atom mass and the lower in-plane crystal symmetry. [19] Studies on WTe 2 have so far been carried out using bulk crystals or mechanically exfoliated flakes. Although mechanical exfoliation can produce high-quality flakes down to a monolayer, scaling it to obtain large-area thin films for practical applications is challenging. Thus, direct synthesis of WTe 2 thin films is desirable for potential electronic and thermal propertyrelated applications, but has yet to be realized due to the low bonding energy of W-Te. Synthesizing WTe 2 directly into largescale thin films is challenging due to its very small standard Gibbs free energy of reaction (−26.2 kJ mol −1 ) compared to that of WSe 2 (−135.0 kJ mol −1 ). [20,21] Additionally, the low melting point of the forming Te-W binary eutectic and high melting point of W (3422 °C) restrict the reaction efficiency between W and Te. Only recently, direct synthesis of MoTe 2 thin films, another interesting TMD [22] with a lower standard Gibbs free energy of reaction (−64.3 kJ mol −1 ) than WTe 2 , has been demonstrated via chemical vapor deposition synthesis (all values of standard Gibbs free energy of reaction are taken at 1100 K). [21,23,24] So far, no direct synthesis of large-area, highly crystalline WTe 2 thin films has been reported.Here, we demonstrate a large-area, facile synthesis of WTe 2 and MoTe 2 thin films by reacting sputtered metal films with H 2 Te, an intermediate vapor phase formed from Te vapor and H 2 carrier gas, through atmospheric pressure chemical vapor reaction. The synthesized films are polycrystalline whose grain size increases with increasing metal film thickness. Based on time-domain thermoreflectance (TDTR), [25,26] the in-plane thermal conductivity of our polycrystalline WTe 2 thin film is less than 2 W m −1 K −1 , at least 7.5 times smaller than that of single-crystalline exfoliated flakes (15 ± 3 W m −1 K −1 ) at room temperature. Through-plane thermal conductivity of our WTe 2 thin films was measured to be 0.8 W m −1 K −1 at room temperature, which is lower than that of the recently reported Large-scale, polycrystalline WTe 2 thin films are synthesized by atmospheric chemical vapor reaction of W metal films with Te vapor catalyzed by H 2 Te intermediates, paving a way to understanding the synthesis mechanism for low bonding energy tellurides and toward synthesis of single-crystallin...

We fabricate graphene-TiOx-Al tunnel junctions and characterize their radio frequency response.Below the superconducting critical temperature of Al and when biased within the superconducting gap, the devices show enhanced dynamic resistance which increases with decreasing temperature. Application of radio frequency radiation affects the dynamic resistance through electronic heating. The relation between the electron temperature rise and the absorbed radiation power is measured, from which the bolometric parameters, including heat conductance, noise equivalent power and responsivity, are characterized.

At very small twist angles of ∼0.1°, bilayer graphene exhibits a strain-accompanied lattice reconstruction that results in submicron-size triangular domains with the standard, Bernal stacking. If the interlayer bias is applied to open an energy gap inside the domain regions making them insulating, such marginally twisted bilayer graphene is expected to remain conductive due to a triangular network of chiral one-dimensional states hosted by domain boundaries. Here we study electron transport through this helical network and report giant Aharonov-Bohm oscillations that reach in amplitude up to 50% of resistivity and persist to temperatures above 100 K. At liquid helium temperatures, the network exhibits another kind of oscillations that appear as a function of carrier density and are accompanied by a sign-changing Hall effect. The latter are attributed to consecutive population of the narrow minibands formed by the network of one-dimensional states inside the gap.