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][12][13][14][15][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 . 2 Main Text:Magnetic anisotropy is an important requirement for realizing 2D magnetism. In ultrathin metallic films, an easy-axis can originate from symmetry reduction at the interface/surface, which hinges on substrate properties and interface quality [24][25][26] . In contrast, most van der Waals magnets have an intrinsic magnetocrystalline anisotropy due to the reduced crystal symmetry of their layered structures. This offers the coveted possibility to retain a magnetic ground state in the monolayer limit. In addition to studying magnetism in naturally formed crystals in the true 2D limit, layered magnets provide a platform for studying the thickness dependence of magnetism in isolated single crystals where the interaction with the underlying substrate is weak. Namely, the covalently bonded van der Waals layers prevent complex magnetization reorientations induced by epitaxial lattice reconstruction and strain 23 . For layered materials, these advantages come at a low fabrication cost, since the micromechanical exfoliation technique 27 is much simpler than conventional approaches requiring sputtering or sophisticated molecular beam epitaxy.A variety of layered magnetic compounds have recently drawn increased interest due to the possibility of re...

Discoveries of intrinsic two-dimensional (2D) ferromagnetism in van der Waals (vdW) crystals provide an interesting arena for studying fundamental 2D magnetism and devices that employ localized spins. However, an exfoliable vdW material that exhibits intrinsic 2D itinerant magnetism remains elusive. Here we demonstrate that FeGeTe (FGT), an exfoliable vdW magnet, exhibits robust 2D ferromagnetism with strong perpendicular anisotropy when thinned down to a monolayer. Layer-number-dependent studies reveal a crossover from 3D to 2D Ising ferromagnetism for thicknesses less than 4 nm (five layers), accompanied by a fast drop of the Curie temperature (T) from 207 K to 130 K in the monolayer. For FGT flakes thicker than ~15 nm, a distinct magnetic behaviour emerges in an intermediate temperature range, which we show is due to the formation of labyrinthine domain patterns. Our work introduces an atomically thin ferromagnetic metal that could be useful for the study of controllable 2D itinerant ferromagnetism and for engineering spintronic vdW heterostructures.

Magnetic multilayer devices that exploit magnetoresistance are the backbone of magnetic sensing and data storage technologies. Here, we report multiple-spin-filter magnetic tunnel junctions (sf-MTJs) based on van der Waals (vdW) heterostructures in which atomically thin chromium triiodide (CrI) acts as a spin-filter tunnel barrier sandwiched between graphene contacts. We demonstrate tunneling magnetoresistance that is drastically enhanced with increasing CrI layer thickness, reaching a record 19,000% for magnetic multilayer structures using four-layer sf-MTJs at low temperatures. Using magnetic circular dichroism measurements, we attribute these effects to the intrinsic layer-by-layer antiferromagnetic ordering of the atomically thin CrI Our work reveals the possibility to push magnetic information storage to the atomically thin limit and highlights CrI as a superlative magnetic tunnel barrier for vdW heterostructure spintronic devices.

Controlling magnetism via electric fields addresses fundamental questions of magnetic phenomena and phase transitions, and enables the development of electrically coupled spintronic devices, such as voltage-controlled magnetic memories with low operation energy. Previous studies on dilute magnetic semiconductors such as (Ga,Mn)As and (In,Mn)Sb have demonstrated large modulations of the Curie temperatures and coercive fields by altering the magnetic anisotropy and exchange interaction. Owing to their unique magnetic properties, the recently reported two-dimensional magnets provide a new system for studying these features. For instance, a bilayer of chromium triiodide (CrI) behaves as a layered antiferromagnet with a magnetic field-driven metamagnetic transition. Here, we demonstrate electrostatic gate control of magnetism in CrI bilayers, probed by magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic fields near the metamagnetic transition, we realize voltage-controlled switching between antiferromagnetic and ferromagnetic states. At zero magnetic field, we demonstrate a time-reversal pair of layered antiferromagnetic states that exhibit spin-layer locking, leading to a linear dependence of their MOKE signals on gate voltage with opposite slopes. Our results allow for the exploration of new magnetoelectric phenomena and van der Waals spintronics based on 2D materials.

A van der Waals heterostructure of monolayer WSe2 and ferromagnetic CrI3 enables exceptional control of valley pseudospin.

Two-dimensional materials with intrinsic functionality are becoming increasingly important in exploring fundamental condensed matter science and for developing advanced technologies. Bulk crystals that can be exfoliated are particularly relevant to these pursuits as they provide the opportunity to study the role of physical dimensionality and explore device physics in highly crystalline samples and designer heterostructures in a routine manner. Magnetism is a key element in these endeavors; however, relatively few cleavable materials are magnetic and none possess magnetic order at ambient conditions. Here, we introduce Fe 5−x GeTe 2 as a cleavable material with ferromagnetic behavior at room temperature. We established intrinsic magnetic order at room temperature in bulk crystals (T C = 310 K) through magnetization measurements and in exfoliated, thin flakes (T C ≈ 280 K) using the anomalous Hall effect. Our work reveals Fe 5 GeTe 2 as a prime candidate for incorporating intrinsic magnetism into functional van der Waals heterostructures and devices near room temperature.

Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. Recently, the van der Waals magnet, chromium triiodide (CrI 3 ), emerged as the first layered antiferromagnetic insulator in its few-layer form 1 , opening up ample opportunities for novel device functionalities 2-7 . Here, we discovered an emergent nonreciprocal second order nonlinear optical effect in bilayer CrI 3 . The observed second harmonic generation (SHG) is giant: several orders of magnitude larger than known magnetization induced SHG 8-11 and comparable to SHG in the best 2D nonlinear optical materials studied so far 12-15 (e.g. MoS 2 ). We showed that while the parent lattice of bilayer CrI 3 is centrosymmetric and thus does not contribute to the SHG signal, the observed nonreciprocal SHG originates purely from the layered antiferromagnetic order, which breaks both spatial inversion and time reversal symmetries. Furthermore, polarization-resolved measurements revealed the underlying C 2h symmetry, and thus monoclinic stacking order in CrI 3 bilayers, providing crucial structural information for the microscopic origin of layered antiferromagnetism 16-20 . Our results highlight SHG as a highly sensitive probe that can reveal subtle magnetic order and open novel nonlinear and nonreciprocal optical device possibilities based on 2D magnets.

Bulk chromium tri-iodide (CrI 3 ) has long been known as a layered van der Waals ferromagnet 1 . However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet 2 , providing a new platform for investigating light-matter interactions and magnetooptical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI 3 under linearly polarized excitation, with heli city determined by the monolayer magnetization direction. In contrast, the bilayer CrI 3 photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI 3 bilayers 2 . Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors 3 , our absorption and layer-dependent photoluminescence measurements reveal the importance of ligandfield and charge-transfer transitions to the optoelectronic response of atomically thin CrI 3 . We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr 3+ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.Van der Waals layered materials offer fascinating opportunities for studying light-matter interactions in the 2D limit. For instance, monolayer semiconducting transition metal dichalcogenides (for example, WSe 2 ) enable coupling between the helicity of light and the valley degree of freedom 4 . In all non-metallic 2D materials to date, it has been established that tightly bound Wannier-Mott excitons dominate the intrinsic optical response 3 , and there has been rapid progress in studying 2D excitonic interactions, dynamics and spin/valley physics 3,5 . However, none of these 2D materials possesses long-range magnetic order. A monolayer semiconductor or insulator with intrinsic magnetism would enable the study of novel photo-physical phenomena and the interplay with underlying magnetic order, possibly involving physics incompatible with the Wannier-Mott excitonic picture.On the other hand, the exploration of ferromagnetism in non-metallic bulk materials has a long history. Early studies examined the intrinsic ferromagnetic ordering of a variety of insulating and semiconducting materials, including, for example, the ferrites and ferrospinels 6 , Cr trihalides 7 , Eu chalcogenides 8 and Cr spinels 9 . Later, with the introduction of magnetic dopants into non-magnetic II-VI and III-V semiconductors, diluted magnetic semiconductors captured widespread attention 10 , boosted by the discovery of ferromagnetism in Mn-doped InAs (ref.11 ) and GaAs (ref.1 ) in the 1990s 12 . Central to progress in these fields, optical experiments have led to a deep understanding of electronic structure, magnetization dynamics and interactions between magnetism and light 8,[13][14][15] . While the fascinating physics in the quantum structures of diluted magnet...