Magnetic Van Der Waals Materials Research Articles

Orbital Complexity in Intrinsic Magnetic Topological Insulators MnBi_{4}Te_{7} and MnBi_{6}Te_{10}.

Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi_{4}Te_{7} and MnBi_{6}Te_{10}, the n=1 and 2 members of a modular (Bi_{2}Te_{3})_{n}(MnBi_{2}Te_{4}) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi_{2}Te_{3}-terminated surfaces but remains preserved for MnBi_{2}Te_{4}-terminated surfaces. Our results firmly establish the topologically nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination.

Noncollinear topological textures in two-dimensional van der Waals materials: From magnetic to polar systems

In recent years, noncollinear topological textures have long gained increasing research attentions for their high values of both fundamental researches and potential applications. The recent discovery of intrinsic orders in magnetic and polar two-dimensional (2D) van der Waals materials provides a new ideal platform for the investigation of noncollinear topological textures. Here, we review the theoretical and experimental progresses on noncollinear topological textures in 2D van der Waals materials in very recent years. During these years, magnetic skyrmions of both Bloch and Néel types have been observed experimentally in a few 2D van der Waals materials and related heterostructures. Concurrently, more theoretic predictions basing on various mechanisms have been reported about different noncollinear topological textures in 2D van der Waals materials, such as skyrmions, bimerons, anti-biskyrmions and skyrmionium, which are still waiting to be confirmed in experiments. Besides, noncollinear topological electric dipole orders have also been predicted in 2D van der Waals materials. Taking advantage of the intrinsic 2D nature and high integratability, the 2D van der Waals materials will play an important role in the investigation on noncollinear topological textures in both magnetic and polar systems.

Complete mapping of magnetic anisotropy for prototype Ising van der Waals FePS3

Several Ising-type magnetic van der Waals (vdW) materials exhibit stable magnetic ground states. Despite these clear experimental demonstrations, a complete theoretical and microscopic understanding of their magnetic anisotropy is still lacking. In particular, the validity limit of identifying their one-dimensional Ising nature has remained uninvestigated in a quantitative way. Here we performed the complete mapping of magnetic anisotropy for a prototypical Ising vdW magnet FePS3 for the first time. Combining torque magnetometry measurements with their magnetostatic model analysis and the relativistic density functional total energy calculations, we successfully constructed the three-dimensional mappings of the magnetic anisotropy in terms of magnetic torque and energy. The results not only quantitatively confirm that the easy axis is perpendicular to the ab plane, but also reveal the anisotropies within the ab, ac, and bc planes. Our approach can be applied to the detailed quantitative study of magnetism in vdW materials.

Revealing the Underlying Mechanisms of the Stacking Order and Interlayer Magnetism of Bilayer CrBr3

Aiming to clarify the mechanisms governing the interlayer magnetic coupling, we have investigated the stacking energy and interlayer magnetism of bilayer CrBr3 systemically. The magnetic ground states of bilayer CrBr3 with different R-type and H-type stacking orders are established, which are found to be in good agreement with recent experiments [ Science 2019, 366, 983]. Further analyses indicate that the stacking energy is mainly determined by the Coulomb interaction between the interlayer nearest-neighbor Br–Br atoms, while interlayer magnetism can be understood by a competition between super-superexchange interactions involving t2g–t2g and t2g–eg orbitals and semicovalent exchange interactions of eg–eg orbitals. Our studies give an insightful understanding of the stacking order and interlayer magnetism of bilayer CrBr3, which should be useful to understand the quantum confinement effect of other van der Waals magnetic materials.

Magnetic Exchange Field Modulation of Quantum Hall Ferromagnetism in 2D van der Waals CrCl3/Graphene Heterostructures.

The efforts toward the experimental realization of spin-polarized transports in ideal materials or platforms, such as the magnetized graphene or various quantum Hall states, is a research hotspot in spintronics. Magnetic van der Waals materials open the door for exploring various physical phenomena, technologies, and integrating novel spintronic devices seamlessly within the 2D limit. Here, we demonstrate magnetic proximity effect in chromium trichloride (CrCl3)/bilayer graphene (BLG) heterostructures by low-temperature transport measurements. An effective exchange field induced in BLG has been demonstrated by the Zeeman spin Hall effect via nonlocal measurements. Furthermore, the exchange field modulates the quantum Hall ground state of BLG and thus favors the formation of a canted antiferromagnetic (CAF) phase in an external perpendicular magnetic field (B⊥). Asymmetric nonlocal magneto-transport behaviors are also observed at opposite B⊥ directions, due to the asymmetric modulation on the exchange field by external B⊥ directions. Our work suggests that the 2D magnetic van der Waals materials and graphene hybrid systems offer a unique platform for quantum Hall ferromagnetism physics.

Magnetic ground state and electron-doping tuning of Curie temperature in Fe3GeTe2 : First-principles studies

Intrinsic magnetic van der Waals (vdW) materials have attracted much attention, especially Fe$_{3}$GeTe$_{2}$ (FGT), which exhibits highly tunable properties. However, despite vast efforts, there are still several challenging issues to be resolved. Here using a first-principles linear-response approach, we carry out comprehensive investigation of both bulk and monolayer FGT. We find that although the magnetic exchange interactions in FGT are frustrated, our Monte Carlo simulations agree with the total energy calculations, confirming that the ground state of bulk FGT is indeed ferromagnetic (FM). A tiny electron doping reduces the magnetic frustration, resulting in significant increasing of the Curie temperature. We also calculate the spin-wave dispersion, and find a small spin gap as well as a nearly flat band in the magnon spectra. These features can be used to compare with the future neutron scattering measurement and finally clarify the microscopic magnetic mechanism in this two-dimensional family materials.

Electron correlation effects on exchange interactions and spin excitations in 2D van der Waals materials

Despite serious effort, the nature of the magnetic interactions and the role of electron-correlation effects in magnetic two-dimensional (2D) van der Waals materials remains elusive. Using CrI3 as a model system, we show that the calculated electronic structure including nonlocal electron correlations yields spin excitations consistent with inelastic neutron-scattering measurements. Remarkably, this approach identifies an unreported correlation-enhanced interlayer super-superexchange, which rotates the magnon Dirac lines off, and introduces a gap along the high-symmetry Γ-K-M path. This discovery provides a different perspective on the gap-opening mechanism observed in CrI3, which was previously associated with spin–orbit coupling through the Dzyaloshinskii–Moriya interaction or Kitaev interaction. Our observation elucidates the critical role of electron correlations on the spin ordering and spin dynamics in magnetic van der Waals materials and demonstrates the necessity of explicit treatment of electron correlations in the broad family of 2D magnetic materials.

Synthesis of emerging 2D layered magnetic materials.

van der Waals atomically thin magnetic materials have been recently discovered. They have attracted enormous attention as they present unique magnetic properties, holding potential to tailor spin-based device properties and enable next generation data storage and communication devices. To fully understand the magnetism in two-dimensions, the synthesis of 2D materials over large areas with precise thickness control has to be accomplished. Here, we review the recent advancements in the synthesis of these materials spanning from metal halides, transition metal dichalcogenides, metal phosphosulphides, to ternary metal tellurides. We initially discuss the emerging device concepts based on magnetic van der Waals materials including what has been achieved with graphene. We then review the state of the art of the synthesis of these materials and we discuss the potential routes to achieve the synthesis of wafer-scale atomically thin magnetic materials. We discuss the synthetic achievements in relation to the structural characteristics of the materials and we scrutinise the physical properties of the precursors in relation to the synthesis conditions. We highlight the challenges related to the synthesis of 2D magnets and we provide a perspective for possible advancement of available synthesis methods to respond to the need for scalable production and high materials quality.

High‐Performance Spin Filters and Spin Field Effect Transistors Based on Bilayer VSe2

AbstractRecently, two‐dimensional (2D) magnetic van der Waals materials have drawn great attention as they are remarkably promising in numerous vital areas, such as data storage and information processing. Theoretically, bilayer (BL) 2H‐VSe2 has been predicted to be an A‐type 2D antiferromagnetic van der Waals crystal (intralayer ferromagnetism and interlayer antiferromagnetism) and to have electrically‐induced half‐metallicity. Herein, by using ab initio quantum transport simulations, BL 2H‐VSe2 spin devices are designed and spin‐resolved transport properties are investigated for the first time. The spin‐filter efficiency (SFE) of the dual‐gated BL 2H‐VSe2 spin filter reaches up to 99%, and the conductance on‐off ratio of this device is up to 106. Also, the conductance on‐off ratio of the quadruple‐gated BL 2H‐VSe2 spin field effect transistor can reach 4 × 103 after reversing the spin direction. This work shows the high performance of the BL 2H‐VSe2 in spintronic devices and will motivate further studies of the spin devices based on this kind of material.

Emergence of skyrmionium in a two-dimensional CrGe(Se,Te)3 Janus monolayer

Utilization of van der Waals magnetic materials as the host of topologically protected skyrmionic and other complex spin textures has been drawing increasing attention. Here, using first-principles-based calculations, we predict a new stable magnetic ${\mathrm{CrGe}(\mathrm{Se},\mathrm{Te})}_{3}$ Janus monolayer with a strong Dzyaloshinskii-Moriya interaction (DMI), due to the breaking of inversion symmetry. Consequently, nanometric skyrmions (that carry a topological charge of $\ifmmode\pm\else\textpm\fi{}1$) and skyrmionium states (with a zero topological charge) can spontaneously form in the absence of magnetic field. We further unveil a subtle competition between DMI and frustration as key for stabilizing skyrmioniums.

Gigantic Current Control of Coercive Field and Magnetic Memory Based on Nanometer-Thin Ferromagnetic van der Waals Fe3 GeTe2.

Controlling magnetic states by a small current is essential for the next-generation of energy-efficient spintronic devices. However, it invariably requires considerable energy to change a magnetic ground state of intrinsically quantum nature governed by fundamental Hamiltonian, once stabilized below a phase-transition temperature. Here, it is reported that, surprisingly, an in-plane current can tune the magnetic state of the nanometer-thin van der Waals ferromagnet Fe3 GeTe2 from a hard magnetic state to a soft magnetic state. It is a direct demonstration of the current-induced substantial reduction of the coercive field. This surprising finding is possible because the in-plane current produces a highly unusual type of gigantic spin-orbit torque for Fe3 GeTe2 . In addition, a working model of a new nonvolatile magnetic memory based on the principle of the discovery in Fe3 GeTe2 , controlled by a tiny current, is further demonstrated. The findings open up a new window of exciting opportunities for magnetic van der Waals materials with potentially huge impact on the future development of spintronic and magnetic memory.

Observation of Standing Spin Waves in a van der Waals Magnetic Material.

Spin waves are studied for data storage, communication, and logic circuits in the field of spintronics based on their potential to substitute electrons. The recent discovery of magnetism in 2D systems such as monolayer CrI3 and Cr2 Ge2 Te6 has led to a renewed interest in such applications of magnetism in the 2D limit. Here, direct evidence of standing spin waves is presented along with the uniform precessional resonance modes in the van der Waals magnetic material, CrCl3 . This is the first direct observation of standing spin-wave modes, set up along a thickness of 20mm, in a van der Waals material. Standing spin waves are detected in the vicinity of both branches, optical and acoustic, of the antiferromagnetic resonance. Magnon-magnon coupling and softening of resonance modes with temperature enable extraction of interlayer exchange field as a function of temperature.

Interlayer magnetism in Fe3−xGeTe2

Fe$_{3-x}$GeTe$_2$ is a layered van der Waals magnetic material with a relatively high ordering temperature and large anisotropy. While most studies have concluded the interlayer ordering to be ferromagnetic, there have also been reports of interlayer antiferromagnetism in Fe$_{3-x}$GeTe$_2$. Here, we investigate the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations and STM simulations. We conclude that the layers of Fe$_{3-x}$GeTe$_2$ are coupled ferromagnetically and that in order to capture the magnetic and electronic properties of Fe$_{3-x}$GeTe$_2$ within density functional theory, Hubbard U corrections need to be taken into account.

Imaging Domain Reversal in an Ultrathin Van der Waals Ferromagnet

The recent isolation of 2D van der Waals magnetic materials has uncovered rich physics that often differs from the magnetic behavior of their bulk counterparts. However, the microscopic details of fundamental processes such as the initial magnetization or domain reversal, which govern the magnetic hysteresis, remain largely unknown in the ultrathin limit. Here a widefield nitrogen-vacancy (NV) microscope is employed to directly image these processes in few-layer flakes of the magnetic semiconductor vanadium triiodide (VI3 ). Complete and abrupt switching of most flakes is observed at fields Hc ≈ 0.5-1T (at 5 K) independent of thickness. The coercive field decreases as the temperature approaches the Curie temperature (Tc ≈ 50K); however, the switching remains abrupt. The initial magnetization process is then imaged, which reveals thickness-dependent domain wall depinning fields well below Hc . These results point to ultrathin VI3 being a nucleation-type hard ferromagnet, where the coercive field is set by the anisotropy-limited domain wall nucleation field. This work illustrates the power of widefield NV microscopy to investigate magnetization processes in van der Waals ferromagnets, which can be used to elucidate the origin of the hard ferromagnetic properties of other materials and explore field- and current-driven domain wall dynamics.

Tunable interlayer magnetism and band topology in van der Waals heterostructures of MnBi2Te4 -family materials

Manipulating the interlayer magnetic coupling in van der Waals magnetic materials and heterostructures is the key to tailoring their magnetic and electronic properties for various electronic applications and fundamental studies in condensed matter physics. By utilizing the MnBi2Te4-family compounds and their heterostructures as a model system, we systematically studied the dependence of the sign and strength of interlayer magnetic coupling on constituent elements by using first-principles calculations. It was found that the coupling is a long-range superexchange interaction mediated by the chains of p orbitals between the magnetic atoms of neighboring septuple-layers. The interlayer exchange is always antiferromagnetic in the pure compounds, but can be tuned to ferromagnetic in some combinations of heterostructures, dictated by d orbital occupations. Strong interlayer magnetic coupling can be realized if the medial p electrons are delocalized and the d bands of magnetic atoms are near the Fermi level. The knowledge on the interlayer coupling mechanism enables us to engineer magnetic and topological properties of MnBi2Te4-family materials as well as many other insulating van der Waals magnetic materials and heterostructures.

Influence of stacking disorder on cross-plane thermal transport properties in TMPS3 (TM = Mn, Ni, Fe)

We investigated the thermal transport properties of magnetic van der Waals materials, TMPS3 (TM = Mn, Ni, and Fe), using the time-domain thermoreflectance technique. We determined the cross-plane thermal conductivity, which turns out to be relatively low, i.e., about 1 W m−1 K−1 for all TMPS3 investigated. When compared with previous results of graphite and transition metal dichalcogenides (TMDs), thermal conductivity becomes smaller as it goes from graphite to TMDs to TMPS3, and the difference is larger at low temperature, e.g., around 50 K. From the Callaway model analysis, we could attribute the large thermal conductivity reduction for TMPS3, particularly at low temperature, to the phonon scattering from the boundary. We actually confirmed the existence of the large population of the stacking faults with the cross-sectional transmission electron microscopy image of MnPS3. This suggests that intrinsic or extrinsic stacking faults formed in van der Waals materials and their heterostructures can play an important role in reducing the cross-plane thermal conductivity as a source of the boundary scattering.

Vapor Deposition of Magnetic Van der Waals NiI2 Crystals.

The recent discovery of van der Waals magnetic materials has attracted great attention in materials science and spintronics. The preparation of ultrathin magnetic layers down to atomic thickness is challenging and is mostly by mechanical exfoliation. Here, we report vapor deposition of magnetic van der Waals NiI2 crystals. Two-dimensional (2D) NiI2 flakes are grown on SiO2/Si substrates with a thickness of 5-40 nm and on hexagonal boron nitride (h-BN) down to monolayer thickness. Temperature-dependent Raman spectroscopy reveals robust magnetic phase transitions in the as-grown 2D NiI2 crystals down to trilayer. Electrical measurements show a semiconducting transport behavior with a high on/off ratio of 106 for the NiI2 flakes. Lastly, density functional theory calculation shows an intralayer ferromagnetic and interlayer antiferromagnetic ordering in 2D NiI2. This work provides a feasible approach to epitaxy 2D magnetic transition metal halides and also offers atomically thin materials for spintronic devices.

Recent breakthroughs in two-dimensional van der Waals magnetic materials and emerging applications

Abstract Two-dimensional (2D) magnetism is now the attention of central demands in fundamental condensed matter physics concerning about the understanding and control of new phases. The demonstration of ferromagnetism in an atomically thin layer develops the prospects for a variety of device applications of 2D van der Waals (vdW) materials. The long-range ferromagnetic ordering in 2D vdW crystals together with their fascinating electric and optical properties will lead to magnetic, magneto-electric, and magneto-optic applications. Low-power, high-speed, and ultra-compact spintronic devices, data storage, information recognition and processing, smart sensors, and quantum computing applications are highly necessary for future industrial applications. This review covers the fundamental chemical structures and synthesis methods of 2D magnetic materials, the techniques for characterizing magnetic properties, device applications and the challenges faced in this emerging field. The progress in both intrinsic and extrinsic magnetic 2D materials originated from external stimuli such as doping, defects, functionalization, and strain is emphasized. The comparison of fundamental physics, chemistry, and related issues of vdW 2D magnetic materials with other-dimensional counterparts concentrated on backgrounds is also emphasized. We focus on the design of chemical and crystal structures leading to 2D magnetism, detailed chemical and physical properties and the device applications of vdW 2D magnetism. Finally, challenges and outlooks in the realization of 2D magnetism are discussed and believed that this emerging field will excite more intensive research and provide exceptional breakthroughs in the field of spintronics.

In-Plane Magnetic Domains and Néel-like Domain Walls in Thin Flakes of the Room Temperature CrTe2 Van der Waals Ferromagnet.

The recent discovery of magnetic van der Waals (vdW) materials triggered a wealth of investigations in materials science and now offers genuinely new prospects for both fundamental and applied research. Although the catalog of vdW ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective X-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domain walls in micron-sized flakes of the CrTe2 vdW ferromagnet. Flux-closure magnetic patterns suggesting an in-plane six-fold symmetry are observed. Upon annealing the material above its Curie point (315 K), the magnetic domains disappear. By cooling back the sample, a different magnetic domain distribution is obtained, indicating material stability and lack of magnetic memory upon thermal cycling. The domain walls presumably have Néel texture, are preferentially oriented along directions separated by 120°, and have a width of several tens of nanometers. Besides microscopic mapping of magnetic domains and domain walls, the coercivity of the material is found to be of a few millitesla only, showing that the CrTe2 compound is magnetically soft. The coercivity is found to increase as the volume of the material decreases.

Role of nonlocality in exchange correlation for magnetic two-dimensional van der Waals materials

To obtain accurate independent-particle descriptions for ferromagnetic two-dimensional van der Waals materials, we apply the quasiparticle self-consistent $GW$ (QSGW) method to ${\mathrm{VI}}_{3}, {\mathrm{CrI}}_{3}, {\mathrm{CrGeTe}}_{3}$, and ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$. QSGW provides a description of the nonlocal exchange-correlation term in the one-particle Hamiltonian. The nonlocal term is important not only as the $U$ of density functional theory $(\mathrm{DFT})+U$ but also for differentiating occupied and unoccupied states in semiconductors. We show the limitations of $\mathrm{DFT}+U$ in mimicking QSGW.