We study the microscopical origin of anisotropic ferromagnetism in the van der Waals magnet CrI3. We conclude that the nearest neighbors exchange is well described by the Heisenberg-Kitaev-Gamma (HKGamma) model, and we also found a nonzero Dzyaloshinskii-Moriya interaction (DMI) on next nearest neighbors. Both Kitaev and DMI are known to generate a non-trivial topology of the magnons in the honeycomb lattice and have been used separately to describe the low energy regime of this material. We discuss that including one or the other leads to different signs of the Chern number. Furthermore, the topological gap at K-point seems to be mainly produced by DMI, despite it is one order of magnitude smaller than Kitaev. Finally, we show that by applying an external electric field perpendicular to the crystal plane, it is possible to induce DMI on nearest neighbors, and this could have consequences in non-collinear spin textures, such as domain walls and skyrmions.
Manipulating quantum state via electrostatic gating has been intriguing for many model systems in nanoelectronics. When it comes to the question of controlling the electron spins, more specifically, the magnetism of a system, tuning with electric field has been proven to be elusive. Recently, magnetic layered semiconductors have attracted much attention due to their emerging new physical phenomena. However, challenges still remain in the demonstration of a gate controllable magnetism based on them. Here, we show that, via ionic gating, strong field effect can be observed in few-layered semicon…
The discovery of an ever increasing family of atomic layered magnetic materials, together with the already established vast catalogue of strong spin-orbit coupling (SOC) and topological systems, calls for some guiding principles to tailor and optimize novel spin transport and optical properties at their interfaces. Here we focus on the latest developments in both fields that have brought them closer together and make them ripe for future fruitful synergy. After outlining fundamentals on van der Waals (vdW) magnetism and SOC effects, we discuss how their coexistence, manipulation and competitio…
Controlling the crystal structure is a powerful approach for manipulating the fundamental properties of solids. Unique to two-dimensional (2D) van der Waals materials, the control can be achieved by modifying the stacking order through rotation and translation between the layers. Here, we report the first observation of stacking dependent interlayer magnetism in the 2D magnetic semiconductor, chromium tribromide (CrBr3), enabled by the successful growth of its monolayer and bilayer through molecular beam epitaxy. Using in situ spin-polarized scanning tunneling microscopy and spectroscopy, we d…
Magnetic insulators are a key resource for next-generation spintronic and topological devices. The family of layered metal halides promises ultrathin insulating multiferroics, spin liquids, and ferromagnets, but new characterization methods are required to unlock their potential. Here, we report tunneling through the layered magnetic insulator CrI3 as a function of temperature and applied magnetic field. We electrically detect the magnetic ground state and inter-layer coupling and observe a field-induced metamagnetic transition. The metamagnetic transition results in magnetoresistances of 95%,…
The recent emergence of 2D van der Waals magnets down to atomic layer thickness provides an exciting platform for exploring quantum magnetism and spintronics applications. The van der Waals nature stabilizes the long-range ferromagnetic order as a result of magnetic anisotropy. Furthermore, giant tunneling magnetoresistance and electrical control of magnetism have been reported. However, the potential of 2D van der Waals magnets for magnonics, magnon-based spintronics, has not been explored yet. Here, we report the experimental observation of long-distance magnon transport in quasi-twodimensio…
Spin-waves have been studied for data storage, communication and logic circuits in the field of spintronics based on their potential to substitute electrons. Recent discovery of magnetism in two-dimensional (2D) systems such as monolayer CrI$_3$ and Cr$_2$Ge$_2$Te$_6$ has led to a renewed interest in such applications of magnetism in the 2D limit. Here we present direct evidence of standing spin-waves along with the uniform precessional resonance modes in van der Waals magnetic material, CrCl$_3$. Our experiment is the first direct observation of standing spin-wave modes, set up across a thick…
Controlling magnetism by purely electrical means is a key challenge to better information technology1. A variety of material systems, including ferromagnetic (FM) metals2,3,4, FM semiconductors5, multiferroics6,7,8 and magnetoelectric (ME) materials9,10, have been explored for the electric-field control of magnetism. The recent discovery of two-dimensional (2D) van der Waals magnets11,12 has opened a new door for the electrical control of magnetism at the nanometre scale through a van der Waals heterostructure device platform13. Here we demonstrate the control of magnetism in bilayer CrI3, an …
Van der Waals (vdW) magnets are an ideal platform for tailoring two-dimensional (2D) magnetism with immense potential for spintronics applications and are intensively investigated. However, little is known about the microscopic origin of magnetic order in these antiferromagnetic systems. We used X-ray photoemission electron microscopy to address the electronic and magnetic properties of the vdW antiferromagnet FePS3 down to the monolayer. Our experiments reveal a giant out-of-plane magnetic anisotropy of 22 meV per Fe ion, accompanied by unquenched magnetic orbital moments. Moreover, our calcu…
Two-dimensional (2D) magnetic van der Waals materials provide a powerful platform for studying fundamental physics of low-dimensional magnetism, engineering novel magnetic phases, and enabling ultrathin and highly tunable spintronic devices. To realize high quality and practical devices for such applications, there is a critical need for robust 2D magnets with ordering temperatures above room temperature that can be created via exfoliation. Here the study of exfoliated flakes of cobalt substituted Fe5GeTe2 (CFGT) exhibiting magnetism above room temperature is reported. Via quantum magnetic ima…
Comparing with the widely known transitional metal based van der Waals (vdW) materials, rare-earth based ones are rarely explored in the research of intrinsic two-dimensional (2D) magnetism. In this work, we report the physical properties of DyOCl, a rare-earth based vdW magnetic insulator with direct band gap of $\sim 5.72~eV$. The magnetic order of bulk DyOCl is determined by neutron scattering as the $A$-type antiferromagnetic structure below the Néel temperature $T_N=10~$K. The large magnetic moment near 10.1 $ μ_{B} $/Dy lies parallel to the $a$-axis with strong uniaxial magnetic anisotro…
The magnetic properties of the van der Waals magnetic topological insulators MnBi$_2$Te$_4$ and MnBi$_4$Te$_7$ are investigated by magneto-transport measurements. We evidence that the relative strength of the inter-layer exchange coupling J to the uniaxial anisotropy K controls a transition from an A-type antiferromagnetic order to a ferromagnetic-like metamagnetic state. A bi-layer Stoner-Wohlfarth model allows us to describe this evolution, as well as the typical angular dependence of specific signatures, such as the spin-flop transition of the uniaxial antiferromagnet and the switching fiel…
Precision magnetometry is fundamental to the development of novel magnetic materials and devices. Recently, the nitrogen-vacancy (NV) center in diamond has emerged as a promising probe for static magnetism in 2D van der Waals materials, capable of quantitative imaging with nanoscale spatial resolution. However, the dynamic character of magnetism, crucial for understanding the magnetic phase transition and achieving technological applications, has rarely been experimentally accessible in single 2D crystals. Here, we coherently control the NV center's spin precession to achieve ultra-sensitive, …
The intersection of topology and magnetism represents a new playground to discover novel quantum phenomena and device concepts. In this work, we show that a van der Waals compound MnSb$_{1.8}$Bi$_{0.2}$Te$_4$ exhibits a ferromagnetic ground state with a Curie temperature of 26 K, in contrast to the antiferromagnetic order previously found for other members of the Mn(Sb, Bi)$_2$Te$_4$ family. We employ magneto-transport, bulk magnetization and neutron scattering studies to illustrate the magnetic and electrical properties of MnSb$_{1.8}$Bi$_{0.2}$Te$_4$ and report on the observation of an unusu…
Topologically protected magnetic structures provide a robust platform for low power consumption devices for computation and data storage. Examples of these structures are skyrmions, chiral domain walls, and spin spirals. Here we use scanning electron microscopy with polarization analysis to unveil the presence of chiral counterclockwise Néel spin spirals at the surface of a bulk van der Waals ferromagnet Fe$_3$GeTe$_2$ (FGT), at zero magnetic field. These Néel spin spirals survive up to FGT's Curie temperature $T_\mathrm{C}= 220 \mathrm{~K}$, with little change in the periodicity $p=300 \mathr…
Stacking order can significantly influence the physical properties of two-dimensional (2D) van der Waals materials. The recent isolation of atomically thin magnetic materials opens the door for control and design of magnetism via stacking order. Here we apply hydrostatic pressure up to 2 GPa to modify the stacking order in a prototype van der Waals magnetic insulator CrI3. We observe an irreversible interlayer antiferromagnetic (AF) to ferromagnetic (FM) transition in atomically thin CrI3 by magnetic circular dichroism and electron tunneling measurements. The effect is accompanied by a monocli…
A van der Waals heterostructure combined with intrinsic magnetism and topological orders have recently paved attractive avenues to realize quantum anomalous Hall effects. In this work, using first-principles calculations and effective model analysis, we propose that the robust quantum anomalous Hall states with sizable band gaps emerge in the van der Waals heterostructure of germanene/Cr$_2$Ge$_2$Te$_6$. This heterostructure possesses high thermodynamic stability, thus facilitating its experimental fabrication. Furthermore, we uncover that the proximity effect enhances the coupling between the…
The discovery of magnetism in two-dimensional (2D) van der Waals (vdW) materials has flourished a new endeavour of fundamental problems in magnetism as well as potential applications in computing, sensing and storage technologies. Of particular interest are antiferromagnets, which due to their intrinsic antiferromagnetic exchange coupling show several advantages in relation to ferromagnets such as robustness against external magnetic perturbations. This property is one of the cornerstones of antiferromagnets and implies that information stored in antiferromagnetic domains is invisible to appli…
We present a phenomenological theory for coupled spin-charge dynamics in magnetic van der Waals heterostructures. The system studied consists of a layered antiferromagnet inserted into a capacitive vdW heterostructure. It has been recently demonstrated that charge doping in such layered antiferromagnets can modulate the strength, and even the sign, of exchange coupling between the layer magnetizations. This provides a mechanism for electrically generating magnetization dynamics. The central result we predict here is that the magnetization dynamics reciprocally results in inducing charge dynami…
A proximity effect at a van der Waals (vdW) interface enables creation of an emergent quantum electronic ground state. Here we demonstrate that an originally-superconducting two-dimensional (2D) NbSe2 forms a ferromagnetic ground state with spontaneous spin polarization at a vdW interface with a 2D ferromagnet V5Se8. We investigated the anomalous Hall effect (AHE) of the NbSe2/V5Se8 magnetic vdW heterostructures, and found that the sign of the AHE was reversed as the number of the V5Se8 layer was thinned down to the monolayer limit. Interestingly, the AHE signal of those samples was enhanced w…
Due to the atomically thin nature of monolayer and few-layer van der Waals magnets, the undesired background signal from the substrate can have significant contribution when characterizing their magnetic properties. This brings challenges in accurately determining the magnitude of the magnetic moment of the epitaxially grown van der Waals magnets on bulk substrates. In this paper, we discuss the impact of the background subtraction method for accurately determining the magnetic moments in such systems. Using the recently reported intrinsic two-dimensional (2D) van der Waals ferromagnet MnSe${_…
Magnetic van der Waals materials provide an ideal playground for exploring the fundamentals of low-dimensional magnetism and open new opportunities for ultrathin spin processing devices. The Mermin-Wagner theorem dictates that as in reduced dimensions isotropic spin interactions cannot retain long-range correlations; the order is stabilized by magnetic anisotropy. Here, using ultrashort pulses of light, we demonstrate all-optical control of magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS$_3$. Tuning the photon energy in resonance with an orbital transition between…
The discovery of two-dimensional (2D) van der Waals magnets opened unprecedented opportunities for the fundamental exploration of magnetism in quantum materials and the realization of next generation spintronic devices. Here, based on a multiscale modelling approach that combines first-principles calculations and a Heisenberg model supplied with ab-initio parameters, we report a strong magnetoelastic coupling in a free-standing monolayer of CrTe$_2$. We demonstrate that different crystal structures of a single CrTe$_2$ give rise to non-collinear magnetism through magnetic frustration and emerg…
There has been a huge increase of interests in two-dimensional van der Waals materials over the past ten years or so with the conspicuous absence of one particular class of materials: magnetic van der Waals systems. In this Viewpoint, we point it out and illustrate how we might be able to benefit from exploring these so-far neglected materials.
We study theoretically domain wall (DW) magnons-elementary collective excitations of magnetic DWs -- in easy-axis layered van der Waals (vdW) antiferromagnets, where they behave as normal modes of coupled spin superfluids. We uncover that, due to spin-charge coupling in vdW magnets, such DW magnons can be activated by voltage-induced torques, thereby providing a path for their low-dissipation and nanoscale excitation. Moreover, the electrical activation and the number of DW magnons at a frequency can be controlled by applying symmetry-breaking static magnetic field, adding tunability of signal…
Since the celebrated discovery of graphene, the family of two-dimensional (2D) materials has grown to encompass a broad range of electronic properties. Recent additions include spin-valley coupled semiconductors, Ising superconductors that can be tuned into a quantum metal, possible Mott insulators with tunable charge-density waves, and topological semi-metals with edge transport. Despite this progress, there is still no 2D crystal with intrinsic magnetism, which would be useful for many technologies such as sensing, information, and data storage. Theoretically, magnetic order is prohibited in…
Van der Waals materials are a new platform to study two-dimensional systems, including magnetic order. Since the number of spins is relatively small, measuring the magnetization is challenging. Here we report spin Hall magnetoresistance (SMR) up to room temperature caused by the magnetic surface texture of exfoliated flakes of magnetic van der Waals materials. For the antiferromagnet FePS3 the SMR amounts to 0.1 % for an applied magnetic field of 7 T at 5 K which implies a substantial canting of the magnetic moments relative to the colinear antiferromagnetic order. The canting is substantial e…
We evaluate the mean value of the van Vleck response of a two-level system with level spacing distribution and argue that it describes the orbital magnetism of small conducting particles.
The challenge of controlling magnetism using electric fields raises fundamental questions and addresses technological needs such as low-dissipation magnetic memory. The recently reported two-dimensional (2D) magnets provide a new system for studying this problem owing to their unique magnetic properties. For instance, bilayer chromium triiodide (CrI3) behaves as a layered antiferromagnet with a magnetic field-driven metamagnetic transition. Here, we demonstrate electrostatic gate control of magnetism in CrI3 bilayers, probed by magneto-optical Kerr effect (MOKE) microscopy. At fixed magnetic f…
Two-dimensional (2D) van der Waals heterostructures serve as a promising platform to exploit various physical phenomena in a diverse range of novel spintronic device applications. The efficient spin injection is the prerequisite for these devices. The recent discovery of magnetic 2D materials leads to the possibility of fully 2D van der Waals spintronics devices by implementing spin injection through magnetic proximity effect (MPE). Here, we report the investigation of magnetic proximity effect in 2D CrBr3/graphene van der Waals heterostructures, which is probed by Zeeman spin Hall effect thro…
The Mermin-Wagner theorem states that long-range magnetic order does not exist in one- or two-dimensional (2D) isotropic magnets with short-ranged interactions. The theorem has been a milestone in magnetism and has been driving the research of recently discovered 2D van der Waals (vdW) magnetic materials from fundamentals up to potential applications. In such systems, the existence of magnetic ordering is typically attributed to the presence of a significant magnetic anisotropy, which is known to introduce a spin-wave gap and circumvent the core assumption of the theorem. Here we show that in …
The effects of magnetic flux in statistical magnetisms, including Pauli paramagnetism, Landau diamagnetism, and De Hass-van Alphen oscillation, are discussed. It is shown that the diamagnetism could be much increased by the fractional magnetic flux, and the amplitude of the magnetic oscillation of De Hass-van Alphen can be amplified by the quantum effect of the flux.
The control of spin current is pivotal for spintronic applications, especially for spin-orbit torque devices. Spin Hall effect (SHE) is a prevalent method to generate spin current. However, it is difficult to manipulate its spin polarization in nonmagnet. Recently, the discovery of spin current in ferromagnet offers opportunity to realize the manipulation. In the present work, the spin current in van der Waals ferromagnet Fe3GeTe2 (FGT) with varying magnetization is theoretically investigated. It has been observed that the spin current in FGT presents the nonlinear behavior with respect to mag…
