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

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.

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A layered two-dimensional superconducting material 2H-NbSe is used to build a van der Waals heterostructure, where a proximity-coupled superconducting order can be induced in the interfacing materials. Vertically stacked NbSe-graphene-NbSe is fabricated using van der Waals interlayer coupling, producing defect-free contacts with a high interfacial transparency. The atomically thin graphene layer allows the formation of a highly coherent proximity Josephson coupling between the two NbSe flakes. The temperature dependence of the junction critical current (I) reveals short and ballistic Josephson coupling characteristics that agree with theoretical prediction. The strong Josephson coupling is confirmed by a large junction critical current density of 1.6 × 10 A/cm, multiple Andreev reflections in the subgap structure of the differential conductance, and a magnetic-field modulation of I. This is the first demonstration of strongly proximity-coupled Josephson junctions with extremely clean interfaces in a dry-transfer-stacked van der Waals heterostructure.

Electron-electron interactions play a critical role in many condensed matter phenomena, and it is tempting to find a way to control them by changing the interactions' strength. One possible approach is to place a studied system in proximity of a metal, which induces additional screening and hence suppresses electron interactions. Here, using devices with atomically-thin gate dielectrics and atomically-flat metallic gates, we measure the electronelectron scattering length in graphene and report qualitative deviations from the standard behavior. The changes induced by screening become important only at gate dielectric thicknesses of a few nm, much smaller than a typical separation between electrons. Our theoretical analysis agrees well with the scattering rates extracted from measurements of electron viscosity in monolayer graphene and of umklapp electron-electron scattering in graphene superlattices. The results provide a guidance for future attempts to achieve proximity screening of many-body phenomena in two-dimensional systems.