Magnetic layered van der Waals crystals are an emerging class of materials giving access to new physical phenomena, as illustrated by the recent observation of 2D ferromagnetism in Cr2Ge2Te6 and CrI3. Of particular interest in semiconductors is the interplay between magnetism and transport, which has remained unexplored. Here we report magneto-transport measurements on exfoliated CrI3 crystals. We find that tunneling conduction in the direction perpendicular to the crystalline planes exhibits a magnetoresistance as large as 10,000%. The evolution of the magnetoresistance with magnetic field and temperature reveals that the phenomenon originates from multiple transitions to different magnetic states, whose possible microscopic nature is discussed on the basis of all existing experimental observations. This observed dependence of the conductance of a tunnel barrier on its magnetic state is a phenomenon that demonstrates the presence of a strong coupling between transport and magnetism in magnetic van der Waals semiconductors.

The recent discovery of ferromagnetism in 2D van der Waals (vdW) crystals has generated widespread interest, owing to their potential for fundamental and applied research. Advancing the understanding and applications of vdW magnets requires methods to quantitatively probe their magnetic properties on the nanoscale. Here, we report the study of atomically thin crystals of the vdW magnet CrI 3 down to individual monolayers using scanning single-spin magnetometry, and demonstrate quantitative, nanoscale imaging of magnetisation, localised defects and magnetic domains. We determine the magnetisation of CrI 3 monolayers to be ≈ 16 µ B /nm 2 and find comparable values in samples with odd numbers of layers, whereas the magnetisation vanishes when the number of layers is even. We also establish that this inscrutable even-odd effect is intimately connected to the material structure, and that structural modifications can induce switching between ferro-and anti-ferromagnetic interlayer ordering. Besides revealing new aspects of magnetism in atomically thin CrI 3 crystals, these results demonstrate the power of single-spin scanning magnetometry for the study of magnetism in 2D vdW magnets.Magnetism in individual monolayers of vdW crystals has recently been observed in a range of materials, including semiconducting [3,4] and metallic [5][6][7] compounds. The discovery of such two dimensional magnetic order is per se non-trivial [8] and has triggered significant attention owing to emerging exotic phenomena including Kitaev spin liquids [9,10], or novel magneto-electric effects [11][12][13][14]. Remarkable efforts have led to the use of two-dimensional magnets as functional elements in spintronics, such as spin-filters [15, 16], spin-transistors [17], tunnelling magnetoresistance devices [18,19] or magnetoelectric switches [12][13][14]. Further advances hinge on methods for the quantitative study of the magnetic response of these atomically thin crystals at the nanoscale, but despite their central importance, the required experimental methods are still lacking. Indeed, transport ex-A z NV e NV z θ NV 3 µm 3 l a y e r s 2 l a y e r s B C, D 0.35 -0.35 0 B NV -B NV (mT) C 2 µm Magne�c stray-field map bias D 20 -20 0 σ (µ B /nm 2 ) 2 µm Magne�sa�on map FIG. 1.Nanoscale imaging of magnetism in twodimensional van der Waals magnets. A Schematic of the scanning single spin magnetometry technique employed in this work. A single Nitrogen-Vacancy (NV) electronic spin is scanned across few layer flakes of encapsulated CrI3 (encapsulation not shown). Stray magnetic fields from the sample are sensed by optically detected Zeeman shifts of the NV spin states, and imaged with nanoscale resolution (set by the sensor-sample separation zNV) by lateral scanning of the NV. The method detects magnetic fields along the NV spin quantisation axis eNV, at an angle θNV ∼ 54 • from the sample normal. B Optical micrograph of the CrI3 bi-and tri-layer flake of sample D1. C Magnetic field map of BNV across sample D1 recorded in a bias field B bias NV = 172.5...

Either in bulk form, or in atomically thin crystals, layered transition metal dichalcogenides continuously reveal new phenomena. The latest example is 1T'-WTe2, a semimetal found to exhibit the largest known magnetoresistance in the bulk, and predicted to become a topological insulator in strained monolayers. Here we show that reducing the thickness through exfoliation enables the electronic properties of WTe2 to be tuned, which allows us to identify the mechanisms responsible for the observed magnetotransport down to the atomic scale. The longitudinal resistance and the unconventional magnetic field dependence of the Hall resistance are reproduced quantitatively by a classical two-band model for crystals as thin as six monolayers, whereas a crossover to an Anderson insulator occurs for thinner crystals. Besides establishing the origin of the magnetoresistance of WTe2, our results represent a complete validation of the classical theory for two-band electron-hole transport, and indicate that atomically thin WTe2 layers remain gapless semimetals.

Van der Waals (vdW) interfaces based on two dimensional (2D) materials are promising for optoelectronics, as interlayer transitions between different compounds allow tailoring the spectral response over a broad range. However, issues such as lattice mismatch or a small misalignment of the constituent layers can drastically suppress electron-photon coupling for these interlayer transitions.Here, we engineer type-II interfaces by assembling atomically thin crystals that have the bottom of the conduction band and the top of the valence band at the Γ-point, thus avoiding any momentum mismatch. We find that these vdW interfaces exhibit radiative optical transitions irrespective of lattice constant, rotational/translational alignment of the two layers, or whether the constituent materials are direct or indirect gap semiconductors. Being robust and of general validity, our results broaden the scope of future optoelectronics device applications based on two-dimensional materials. Van der Waals interfaces of interest for optoelectronics consist of two distinct layered semiconductors with a suitable energetic alignment of their conduction and valence bands, such that electron and hole excitations reside in the two separate layers.[1-4] This allows the interfacial band gap to be controlled by material selection -as well as by application of an electrical bias or strain[5-9]-so that electron-hole recombination across the layers generates photons with frequency determined over a broad range at the design stage. Choosing the interface components among the vast gamut of 2D materials -including semiconducting transition metal dichalcogenides (TMDs, MoS 2 , MoSe 2 , MoTe 2 , WS 2 , WSe 2 , ReS 2 , ZrS 2 , etc.), III-VI compounds (InSe, GaSe), black phosphorous, and even magnetic semiconductors (CrI 3 , CrCl 3 , CrBr 3 , etc.)-enables, at least in principle, to cover a spectral range from the far infra-red to the violet. In practice, however, efficient light-emission from interlayer recombination requires the corresponding electron-hole transition to be direct in reciprocal (k-) space: the bottom of the conduction band in one layer has to be centered in k-space at the same position as the top of the valence band in the other layer.[10] This requirement poses severe constraints as concluded from heterostructures of monolayer semi-conducting TMDs, the systems that have been so far mostly used to realize light-emitting vdW interfaces. [7,[11][12][13][14] Indeed, in this case the minimum of the conduction band and top of valence band are at the K/K' points in the Brillouin zone and the presence of radiative

The semiconducting properties of most 2D magnets investigated so far, however, are strongly affected by the extremely narrow widths of their conduction and valence bands, typically a few tens of meV or less. [7][8][9][10][11][12][13] Such narrow bandwidths cause electron localization and prevent low-temperature conductivity measurements, which is why transport experiments probing the magnetic properties of 2D semiconductors have been so far limited to studies of tunneling through atomically thin multilayer barriers. [14][15][16][17][18][19][20][21] CrSBr [22] (see Figure 1a)-a recently introduced 2D magnetic semiconductor-appears to be an exception. [23,24] First-principles calculations (shown in Figure 1b) predict its conduction band to have a width of ≈1.5 eV. [24,25] Accordingly, low-temperature in-plane magnetoresistance measurements (see Figure 1c,d) could be performed successfully, and analyzed to determine the magnetic phase diagram. [23] The unique magnetic properties of this material have been further showcased by experiments on van der Waals (vdW) interfaces, in which CrSBr was found to imprint into an adjacent graphene layer a giant exchange interaction, much stronger than what has been reported in earlier work on analogous heterostructures. [26] Electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor of interest because of its magnetic properties, is investigated. An extremely pronounced anisotropy manifesting itself in qualitative and quantitative differences of all quantities measured along the in-plane a and b crystallographic directions is found. In particular, a qualitatively different dependence of the conductivities σ a and σ b on temperature and gate voltage, accompanied by orders of magnitude differences in their values (σ b /σ a ≈ 3 × 10 2 to 10 5 at low temperature and negative gate voltage) are observed, together with a different behavior of the longitudinal magnetoresistance in the two directions and the complete absence of the Hall effect in transverse resistance measurements. These observations appear not to be compatible with a description in terms of conventional band transport of a 2D doped semiconductor. The observed phenomenology-and unambiguous signatures of a 1D van Hove singularity detected in energy-resolved photocurrent measurements-indicate that electronic transport through CrSBr multilayers is better interpreted by considering the system as formed by weakly and incoherently coupled 1D wires, than by conventional 2D band transport. It is concluded that CrSBr is the first 2D semiconductor to show distinctly quasi-1D electronic transport properties.