Since the celebrated discovery of graphene 1,2 , the family of two-dimensional (2D) materials has grown to encompass a broad range of electronic properties. Recent additions include spin-valley coupled semiconductors 3 , Ising superconductors 4-6 that can be tuned into a quantum metal 7 , possible Mott insulators with tunable charge-density waves 8 , and topological semi-metals with edge transport 9,10 . Despite this progress, there is still no 2D crystal with intrinsic magnetism 11-16 , which would be useful for many technologies such as sensing, information, and data storage 17 . Theoretically, magnetic order is prohibited in the 2D isotropic Heisenberg model at finite temperatures by the Mermin-Wagner theorem 18 . However, magnetic anisotropy removes this restriction and enables, for instance, the occurrence of 2D Ising ferromagnetism. Here, we use magneto-optical Kerr effect (MOKE) microscopy to demonstrate that monolayer chromium triiodide (CrI3) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 K is only slightly lower than the 61 K of the bulk crystal, consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase transition, showcasing the hallmark thickness-dependent physical properties typical of van der Waals crystals 19-21 . Remarkably, bilayer CrI3 displays suppressed magnetization with a metamagnetic effect 22 , while in trilayer the interlayer ferromagnetism observed in the bulk crystal is restored. Our work creates opportunities for studying magnetism by harnessing the unique features of atomically-thin materials, such as electrical control for realizing magnetoelectronics 13,23 , and van der Waals engineering for novel interface phenomena 17 . Extended DataFigure 1 | Atomic force microscopy (AFM) and magneto-optic Kerr effect (MOKE) measurements of graphite-encapsulated few-layer CrI3. a, Optical microscope image of a bilayer CrI3 flake on 285 nm SiO2. b, AFM data for the CrI3 flake in a encapsulated in graphite, showing a line cut across the flake with a step height of 1.5 nm. c, Optical microscope image of a tri-layer CrI3 flake on 285 nm SiO2. d, AFM data for the CrI3 flake shown in c encapsulated in graphite. A line cut taken across the flake shows a step height of 2.2 nm. All scale bars are 2 µm in length. e and f show the MOKE signal as a function of applied magnetic field for the encapsulated bilayer in b and the encapsulated trilayer in d respectively. Extended Data Figure 2 | Magneto-optical Kerr effect experimental setup. Schematic of the optical setup used to measure Magneto-optical Kerr effect in CrI3 samples. 633 nm optical excitation is provided by a power-stabilized HeNe laser. A mechanical chopper and photoelastic modulator provide intensity and polarization modulation, respectively. The modulated beam is directed through a polarizing beam splitter to the sample, housed in a closed-cycle cryostat at 15 K. A magnetic field is applied at the sample using a 7 T solenoidal superconducting magnet in Farad...

Problems associated with large-scale pattern growth of graphene constitute one of the main obstacles to using this material in device applications. Recently, macroscopic-scale graphene films were prepared by two-dimensional assembly of graphene sheets chemically derived from graphite crystals and graphene oxides. However, the sheet resistance of these films was found to be much larger than theoretically expected values. Here we report the direct synthesis of large-scale graphene films using chemical vapour deposition on thin nickel layers, and present two different methods of patterning the films and transferring them to arbitrary substrates. The transferred graphene films show very low sheet resistance of approximately 280 Omega per square, with approximately 80 per cent optical transparency. At low temperatures, the monolayers transferred to silicon dioxide substrates show electron mobility greater than 3,700 cm(2) V(-1) s(-1) and exhibit the half-integer quantum Hall effect, implying that the quality of graphene grown by chemical vapour deposition is as high as mechanically cleaved graphene. Employing the outstanding mechanical properties of graphene, we also demonstrate the macroscopic use of these highly conducting and transparent electrodes in flexible, stretchable, foldable electronics.

Graphene devices on standard SiO(2) substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2). These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.

We have achieved mobilities in excess of 200,000 cm 2 V −1 s −1 at electron densities of ∼2×10 11 cm −2 by suspending single layer graphene. Suspension ∼150 nm above a Si/SiO2 gate electrode and electrical contacts to the graphene was achieved by a combination of electron beam lithography and etching. The specimens were cleaned in situ by employing current-induced heating, directly resulting in a significant improvement of electrical transport. Concomitant with large mobility enhancement, the widths of the characteristic Dirac peaks are reduced by a factor of 10 compared to traditional, non-suspended devices. This advance should allow for accessing the intrinsic transport properties of graphene. Graphene, the latest addition to the family of twodimensional (2D) materials, is distinguished from its cousins by its unusual band structure, rendering the quasiparticles in it formally identical to massless, chiral fermions. The experimental realization of graphene thus presents tantalizing opportunities to study phenomena ranging from the topological phase resulting in exotic quantum Hall states [1,2] to the famous Klein paradox -the anomalous tunneling of relativistic particles [3]. However, despite tremendous interest and concerted experimental efforts , the presence of strong impurity scattering -which limits the electron mean free path to less than a micron -has been a major barrier to progress. At the same time, there is strong evidence that graphene is a nearly perfect crystal free of the structural defects [4,5] that characterize most conductors. As a result, it has been put forth that the scattering of charge carriers stems from extrinsic sources [7,8,9,10].Although the exact nature of the scattering that limits the mobility of graphene devices remains unclear, evidence has mounted that interactions with the underlying substrate are largely responsible. Surface charge traps [6,7,8,9], interfacial phonons [11], substrate stabilized ripples [5,10,12], and fabrication residues on or under the graphene sheet may all contribute. Consequently, improving substrate quality or eliminating the substrate altogether by suspending graphene over a trench seems a promising strategy towards higher quality samples. While devices suspended over the substrate were achieved in the past [12,13], they lacked multiple electrical contacts thus precluding transport measurements.In this Letter we report the fabrication of electrically contacted suspended graphene and achieve a tenfold improvement in mobility as compared to the best values reported in the literature for traditional devices fabricated on a substrate. Besides opening new avenues for studying the intrinsic physics of Dirac fermions, this improvement demonstrates the dominant role played by extrinsic scattering in limiting the transport properties of unsuspended graphene samples.The fabrication of a suspended graphene device starts with optically locating a single-layer mechanically exfoliated graphene flake on top of a silicon substrate covered with 300 nm of SiO 2 . Singl...