Two-dimensional Magnets Research Articles

Artificial Intelligence Guided Studies of van der Waals Magnets (Adv. Theory Simul. 6/2023)

Searching for novel two-dimensional magnets using AI Intrinsic magnetic ordering in two dimensions was discovered in 2017 when a single layer of CrI3 and two layers of CrGeTe3 were shown to be ferromagnetic. In the research article, Rhone, Trevor David and coworkers harness AI to identify new two-dimensional magnetic materials and predict their magnetic and thermodynamic properties. Silver spheres represent transition metals. Yellow spheres represent the halogen atom: {Br, I}. Red spheres represent another halogen atom: {F, Cl}.

Plasmon-magnon interactions in two-dimensional honeycomb magnets

Two-dimensional honeycomb ferromagnets offer the unprecedented opportunity to study interactions between collective modes that in standard bulk ferromagnets do not cross paths. Indeed, they harbor an optical spin-wave branch, i.e. a spin wave which disperses weakly near the Brillouin zone center. When doped with free carriers, they also host the typical gapless plasmonic mode of 2D itinerant electron/hole systems. When the plasmon branch meets the optical spin-wave branch, energy and momentum matching occurs, paving the way for interactions between the charge and spin sector. In this Letter we present a microscopic theory of such plasmon-magnon interactions, which is based on a double random phase approximation. We comment on the possibility to unveil this physics in recently isolated 2D honeycomb magnets such as ${\rm Cr}_2{\rm Ge}_2{\rm Te}_6$.

Tunable second-order topological insulators in Chern insulators 2H-FeX2 (X = Cl and Br)

Engineering topological states in two-dimensional (2D) magnets is of pivotal importance to provide significantly rich physics and application potential. Here, we theoretically demonstrate that the second-order topological insulators (SOTIs) with robust nontrivial corner states can be realized in Chern insulators via the widely used strain engineering. The quantum anomalous Hall effect in Chern insulators of honeycomb 2H-FeX2 (X = Cl and Br) is revealed with a nonzero Chern number C=1 and the emergence of metallic chiral edge states. Remarkably, under compressive or tensile strains, topological phase transitions are proposed with the gap-closing in different valleys, giving birth to the 2D SOTIs or trivial insulating 2D magnets. Moreover, large valley polarizations are clearly shown. Our findings open up a promising way for exploring the first- and higher-order topology with intriguing effects.

A two-dimensional kagome magnet with tunable topological phases

Kagome magnets have garnered a lot of investigation due to their plentiful topological states and promising, strong correlated phenomena. However, there have been few reports of two-dimensional (2D) van der Waals kagome magnets. Here, we suggest a stable 2D van der Waals kagome material, V3Br8, which exhibits a ferromagnetic ground state with a Curie temperature of 89 K. Near the Fermi level, there is only a set of three kagome bands from one spin species, which possesses Van Hove singularities at the M point and a Weyl point at the K point. It is interesting that its ground state is gapless at the K point and maintains a 2D Weyl half semimetal state even in the presence of spin-orbit coupling, while when the magnetization rotates from the in-plane direction of the ground state to the out-of-plane direction, the spin-orbit coupling will open an energy gap at the K point, resulting in a topological phase transition to a quantum anomalous Hall effect state. We further demonstrate that the tensile strain can optimize the band structure and raise the Curie temperature to above 100 K. In addition, a Li atom intercalated V3Br8 (V3Br8Li) is revealed to be a stable structure that can successfully tune the Fermi level to the Weyl point while maintaining the ferromagnetic ground state. Moreover, V3Br8Li displays an out-of-plane magnetism, producing the quantum anomalous Hall effect . Our findings provide a solid foundation for future research into topological states and singular effects in 2D kagome magnets.

Non-Lifshitz invariants corrections to Dzyaloshinskii-Moriya interaction energy

We study the continuum limit of two-dimensional chiral magnets in which Dzyaloshinskii-Moriya interaction (DMI) is due to the interplay between a smooth magnetic texture and spin-orbit coupling. The resulting free-energy density of the system contains linear terms in the spatial gradient of the magnetic texture, which mark an instability of the system towards the formation of nontrivial magnetic orders such as skyrmions or chiral domain walls. We perform a microscopic analysis of DMI tensors responsible for this contribution to free energy based on a Berry phase formulation in the mixed space of momentum and position, and reveal that they exhibit non-Lifshitz invariants features. In particular, a perturbation theory shows in the case of Rashba spin-orbit interactions the presence of non-Lifshitz invariants to third order in the small spin-orbit interaction and fourth order in the small exchange coupling. The higher-order terms may even lead to an enhancement of DMI interaction at strong spin-orbit coupling due to divergences in the density of states at the bottom of the conduction band. Finally, we also study the DMI free energy generated from Rashba spin-orbit interaction in different symmetry groups.

Modeling and Optimization of a Large-Load Magnetic Levitation Gravity Compensator

This article presents the modeling and optimization design of a large load magnetic levitation gravity compensator. The proposed magnetic levitation gravity compensator comprises three layers of two-dimensional (2D) permanent magnet arrays. The middle mover layer can generate a large passive levitation force to compensate for the gravity of large loads in some magnetic levitation systems, such as the measurement framework in lithography machines or large space optical equipment to be tested on the ground. To accurately predict the passive levitation force, an improved magnetic charge model is proposed, in which the actual working points of each permanent magnet (PM) are considered. The accuracy of the improved model is verified via three-dimensional (3D) finite element simulation. Genetic algorithm (GA) is then adopted as the parameter optimization method to reduce the levitation force stiffness within the effective vertical displacement to the greatest extent possible. Compared with the levitation force performance without parameter optimization, the sensitivity of the levitation force with vertical displacement is significantly reduced. In addition, the mechanical structure is designed, and the mechanical strength is checked. Finally, a magnetic levitation gravity compensator prototype with a passive levitation force of 6200 N is manufactured and tested. The tested values of the levitation force match well with the analytical and simulation results. The tested levitation force stiffness within 4 mm is less than 50 N/mm and basically remains unchanged, which is a superior performance for magnetic levitation systems.

Force between magnetic nanoplates with dipolar interactions

This study considers the dependence of the force caused by the dipolar interaction between small low-dimensional magnets such as single-molecule magnets and two-dimensional magnets on the distance between them within the framework of the dipolar Ising model with nearest-neighbor exchange interactions and long-range dipolar interactions. In particular, we focus on the rapid change in the force between ferromagnetic and antiferromagnetic plates, which arise from the transition of the spin states and explain that this behavior originates from the spin frustrations between magnetic plates. Furthermore, the size and temperature dependence of the interaction energy are investigated using a Monte Carlo simulation.

Quantum Stoner–Wohlfarth model of two-dimensional single-domain magnets

The Stoner–Wohlfarth model is a classical model for magnetic hysteresis of single-domain magnets. For two-dimensional (2D) magnets at finite temperatures, the Stoner–Wohlfarth model must be extended to include intrinsic strong spin fluctuations. We predict several fundamentally different hysteresis properties between 2D and 3D magnets. The magnetization switching diagram known as the astroid figure in the conventional Stoner–Wohlfarth model becomes highly temperature dependent and asymmetric with respect to the transverse and longitudinal magnetic fields. Our results provide new insights into the spintronics applications based on 2D magnetic materials.

Physical insights into enhancing magnetic stability of 2D magnets

Recently, two-dimensional (2D) magnets have drawn substantial attention from researchers for their fascinating properties and great application potential in the fields of biomedicine, data storage, signal transfer, and energy conversion. However, the low Curie/Néel temperature of 2D magnets hinders their application. In this Perspective, we present some physical insights into enhancing the magnetic stability of 2D magnets. First, the microscope theoretical model of 2D magnets is introduced. Then, we review and analyze several effective and commonly used methods for enhancing the magnetic stability of 2D magnets. Finally, we present the perspective and summary. This Perspective presents the advanced understanding of magnetic stability in 2D materials, which can provide new opportunities for further advancement in a wide variety of applications.

Giant proximity exchange and flat Chern band in 2D magnet-semiconductor heterostructures

van der Waals (vdW) heterostructures formed by two-dimensional (2D) magnets and semiconductors have provided a fertile ground for fundamental science and spintronics. We present first-principles calculations finding a proximity exchange splitting of 14 meV (equivalent to an effective Zeeman field of 120 T) in the vdW magnet-semiconductor heterostructure MoS2/CrBr3, leading to a 2D spin-polarized half-metal with carrier densities ranging up to 1013 cm-2. We consequently explore the effect of large exchange coupling on the electronic band structure when the magnetic layer hosts chiral spin textures such as skyrmions. A flat Chern band is found at a "magic" value of magnetization [Formula: see text] for Schrödinger electrons, and it generally occurs for Dirac electrons. The magnetic proximity-induced anomalous Hall effect enables transport-based detection of chiral spin textures, and flat Chern bands provide an avenue for engineering various strongly correlated states.

Strong perpendicular anisotropic ferromagnet Fe3GeTe2/graphene van der Waals heterostructure

Two-dimensional magnets offer a new platform for exploring fundamental properties in van der Waals (vdW) heterostructures and their device applications. Here, we investigated heterostructure devices of itinerant metallic vdW ferromagnet Fe3GeTe2 (FGT) with monolayer chemical vapor deposited graphene. The anomalous Hall effect measurements of FGT Hall-bar devices exhibit robust ferromagnetism with strong perpendicular anisotropy at low temperatures. The electrical transport properties measured in FGT/graphene heterostructure devices exhibit a tunneling transport with weak temperature dependence. We assessed the suitability of such FGT/graphene heterostructures for spin injection and detection and investigated the presence of FGT on possible spin absorption and spin relaxation in the graphene channel. These findings will be useful for engineering spintronic devices based on vdW heterostructures.

Magnon Interference Tunneling Spectroscopy as a Probe of 2D Magnetism

Probing two-dimensional single-layer quantum magnets remains a significant challenge. In this Letter, we propose exploiting tunneling spectroscopy in the presence of magnetic impurities to obtain information about the magnon dispersion relations in analogy to quasiparticle interference in nonmagnetic materials. We show that this technique can be used to establish the dispersion relations even for frustrated magnets, where the presence of an impurity generally leads to a nontrivial spin texture. Finally, we consider the problem of establishing Chern magnon bands in 2D magnets showing how tunable impurities allow one to probe the nature of the surface states.

Atomically Unveiling an Atlas of Polytypes in Transition-Metal Trihalides.

Transition-metal trihalides MX3 (M = Cr, Ru; X = Cl, Br, and I) belong to a family of novel two-dimensional (2D) magnets that can exhibit topological magnons and electromagnetic properties, thus affording great promises in next-generation spintronic devices. Rich magnetic ground states observed in the MX3 family are believed to be strongly correlated to the signature Kagome lattice and interlayer van der Waals coupling raised from distinct stacking orders. However, the intrinsic air instability of MX3 makes their direct atomic-scale analysis challenging. Therefore, information on the stacking-registry-dependent magnetism for MX3 remains elusive, which greatly hinders the engineering of desired phases. Here, we report a nondestructive transfer method and successfully realize an intact transfer of bilayer MX3, as evidenced by scanning transmission electron microscopy (STEM). After surveying hundreds of MX3 thin flakes, we provide a full spectrum of stacking orders in MX3 with atomic precision and calculated their associated magnetic ground states, unveiled by combined STEM and density functional theory (DFT). In addition to well-documented phases, we discover a new monoclinic C2/c phase in the antiferromagnetic (AFM) structure widely existing in MX3. Rich stacking polytypes, including C2/c, C2/m, R3̅, P3112, etc., provide rich and distinct magnetic ground states in MX3. Besides, a high density of strain soliton boundaries is consistently found in all MX3, combined with likely inverted structures, allowing AFM to ferromagnetic (FM) transitions in most MX3. Therefore, our study sheds light on the structural basis of diverse magnetic orders in MX3, paving the way for modulating magnetic couplings via stacking engineering.

Quantum Anomalous Hall Effects Controlled by Chiral Domain Walls

We report the interplay between two different topological phases in condensed matter physics, the magnetic chiral domain wall (DW), and the quantum anomalous Hall (QAH) effect. It is shown that the chiral DW driven by Dzyaloshinskii–Moriya interaction can divide the uniform domain into several zones where the neighboring zone possesses opposite quantized Hall conductance. The separated domain with a chiral edge state (CES) can be continuously modified by external magnetic field-induced domain expansion and thermal fluctuation, which gives rise to the reconfigurable QAH effect. More interestingly, we show that the position of CES can be tuned by spin current driven chiral DW motion. Several two-dimensional magnets with high Curie temperature and large topological band gaps are proposed for realizing these phenomena. The present work thus reveals the possibility of chiral DW controllable QAH effects.

Spin-selective contact type and strong Fermi level pinning at a CrI3/metal interface

An interface in heterostructures formed by stacking two-dimensional magnets with metals could offer unprecedented electronic and magnetic properties. Here, in a heterostructure with monolayer CrI3 adsorbed on metals (Pb, Ag, Hf, and Au), we find that the nature of the CrI3-metal interface contact is spin-selective, i.e. Ohmic and Schottky contacts for the majority and minority spin channels, respectively. This spin-selective phenomenon is attributed to n-doping, and thus half metallicity of CrI3 in the heterostructure. For the Schottky contact, the slope parameter of a Schottky barrier height is found around 0.15, indicating a strong Fermi level pinning at the CrI3/metal interface. This strong Fermi level pinning is a result of two interfacial behaviors: interface gap states induced by a strong interaction between CrI3 and metals, and a interface dipole generated by charge transfer. These findings could pave the way for the free design of an electrical contact type by manipulating the spin channel in a heterostructure, and enable two-dimensionalmagnets-based versatile spintronics.

Characterizing random-singlet state in two-dimensional frustrated quantum magnets and implications for the double perovskite Sr2CuTe1−xWxO6

Understanding disorder-induced exotic states in two-dimensional magnets is one of the central topics in the study of frustrated magnetism. Here, the authors focus on the randomness-induced disordered phase in the spin-\textonehalf{} ${J}_{1}\ensuremath{-}{J}_{2}$ square-lattice Heisenberg model with a strong bond randomness, by using large-scale density matrix renormalization group (DMRG) simulations. The results not only reveal the absent spin-glass order, but also identify the power-law decay of the average spin correlation and the exponential decay of the typical spin correlation. These all agree with the corresponding behavior in the one-dimensional random-singlet state and characterize the random-singlet nature of this disordered phase. This shows that DMRG simulations provide opportunities for exploring the disorder effect in two-dimensional magnets.

Direct Optical Probe of Magnon Topology in Two-Dimensional Quantum Magnets.

Controlling edge states of topological magnon insulators is a promising route to stable spintronics devices. However, to experimentally ascertain the topology of magnon bands is a challenging task. Here we derive a fundamental relation between the light-matter coupling and the quantum geometry of magnon states. This allows us to establish the two-magnon Raman circular dichroism as an optical probe of magnon topology in honeycomb magnets, in particular of the Chern number and the topological gap. Our results pave the way for interfacing light and topological magnons in functional quantum devices.

Atomistic Modeling of Spin and Electron Dynamics in Two-Dimensional Magnets Switched by Two-Dimensional Topological Insulators

To design fast memory devices, we need material combinations that can facilitate fast read and write operations. We present a heterostructure comprising a two-dimensional (2D) magnet and a 2D topological insulator (TI) as a viable option for designing fast memory devices. We theoretically model the spin-charge dynamics between 2D magnets and 2D TIs. Using the adiabatic approximation, we combine the nonequilibrium Green's function method for spin-dependent electron transport and a time-quantified Monte Carlo method for simulating magnetization dynamics. We show that it is possible to switch a magnetic domain of a ferromagnet using the spin torque from spin-polarized edge states of a 2D TI. We show further that the switching of 2D magnets by TIs is strongly dependent on the interface exchange (${J}_{\mathrm{int}}$), and an optimal interface exchange, is required for efficient switching. Finally, we compare experimentally grown Cr compounds and show that Cr compounds with higher anisotropy (such as $\mathrm{Cr}{\mathrm{I}}_{3}$) result in a lower switching speed but a more stable magnetic order.

Generation of magnetic skyrmions in two-dimensional magnets via interfacial proximity

Two-dimensional (2D) magnetic van der Waals materials provide a fertile platform for the design and control of topological spin textures such as skyrmions. However, despite studies reporting skyrmions in many 2D magnetic systems, the hosting van der Waals materials are still limited. Here, via first-principles calculations and Monte Carlo simulations, we propose a series of 2D magnetic heterostructures as a different materials' family hosting skyrmions. Unlike previously studied van der Waals heterostructures, a strong interface coupling generates here a significant inversion symmetry breaking, which then results in an inherently large Dzyaloshinskii-Moriya interaction (DMI). Furthermore, upon applying a moderate magnetic field, isolated skyrmions and skyrmions lattices emerge in various types of these heterostructures, and are robust within a relatively wide temperature range. Our work thus suggests a general method to induce DMI and paves the way toward topological magnetism in 2D materials through interfacial engineering.

Bipolar ferrimagnetic semiconductor and doping concentration induced carrier spin flip in monolayer NiMnBr6.

Two-dimensional magnetic materials with tunable electronic properties have great potential application in spintronic devices. Here, based on first-principles calculations, we systematically study the electronic structures and magnetic properties of monolayer NiMnBr6. The magnetic ground state of monolayer NiMnBr6 is Néel ferrimagnetic (FIM-Néel) with a critical temperature (Tc) of 45 K. The magnetic properties of monolayer NiMnBr6 can be tuned effectively by strain. The magnetic phase transition from the FIM-Néel state to the ferromagnetic (FM) state can be triggered by applying a compressive strain greater than 4.5%. The Tc of the FIM-Néel state and FM state can be increased to 67 K and 95 K by applying 8.0% tensile and compressive strain, respectively. Monolayer NiMnBr6 in both the FIM-Néel state and FM state has large magnetic anisotropy energy. Remarkably, the monolayer NiMnBr6 in the FIM-Néel state acts as a bipolar ferrimagnetic semiconductor (BFIMS), while the compressive strained monolayer NiMnBr6 in the FM state acts as a half FM semiconductor (HFMS). The magnetic configuration of monolayer NiMnBr6 can also be tuned by carrier doping. Interestingly, for monolayer NiMnBr6 with the HFMS phase, the magnetic phase transition from the FM state to FIM-Néel state can be achieved with the increase of the hole doping concentration, which leads to the achievement of a doping concentration induced carrier spin flip. Our results show that monolayer NiMnBr6 is a promising candidate for exploring two-dimensional magnetism and spintronic devices.