Magnetic Van Der Waals Materials Research Articles

Field-Dependent Magnetic Domain Behavior in van der Waals Fe3GeTe2

Two-dimensional magnetic van der Waals (vdW) materials can show a variety of topological nontrivial spin textures, such as Bloch- or Néel-type stripe, skyrmion, or bubble domains under certain external stimuli. It is critical to understand the magnetic domain behavior in vdW materials in order to control their size and density in response to external stimuli, such as electric and magnetic fields. We examine the magnetic field dependence of topologically non-trivial magnetization spin textures in vdW Fe3GeTe2. Néel-type stripe domains and skyrmions are formed depending on the magnetic field-cooling protocol used during in situ Lorentz transmission electron microscopy (LTEM) experiments. Use of quantitative reconstruction of magnetic induction maps and micromagnetic simulations allow for the understanding of the LTEM results of Néel-type stripe domains as well as skyrmions. In addition, the deformation of skyrmion contrast is observed as a result of the introduction of an in-plane magnetic field. We demonstrate the stability of the stripe domains and skyrmions in response to an externally applied magnetic field due to an energy barrier for domain wall annihilation. Our results establish an understanding of the energy landscape that governs the behavior of the topologically non-trivial spin textures in vdW materials which can be harnessed for spintronic applications.

Multi-meron interactions and statistics in two-dimensional materials

As a fundamental type of topological spin textures in two-dimensional (2D) magnets, a magnetic meron carries half-integer topological charge and forms a pair with its antithesis to keep the stability in materials. However, it is challenging to quantitatively calculate merons and their dynamics by using the widely used continuum model because of the characteristic highly inhomogeneous spin textures. In this work, we develop a discrete method to address the concentrated spin structures around the core of merons. We reveal a logarithmic-scale interaction between merons when their distance is larger than twice their core size and obtain subsequent statistics of meron gas. The model also predicts how these properties of single and paired merons evolve with magnetic exchange interactions, and the results are in excellent agreement with the Monte Carlo simulations using the parameters of real 2D van der Waals magnetic materials. This discrete approach not only shows equilibrium static statistics of meron systems but also is useful to further explore the dynamic properties of merons through the quantified pairing interactions.

Abnormal Magnetoresistance Transport Properties of van der Waals Antiferromagnetic FeNbTe2

The emergence of two-dimensional (2D) van der Waals magnetic materials has attracted enormous attention due to their novel physical phenomena and potential application in the fields of spintronics and information storage technology. Here, we systematically study the magnetic and transport properties of a van der Waals antiferromagnetic material, FeNbTe2. The magnetic and magnetoresistance measurements verified its antiferromagnetic properties, spin glass state, and negative magnetoresistance effect at lower temperatures. In addition, the measurement results of transport also show the existence of angle-dependent anisotropic magnetoresistance in a wide temperature range and anisotropic magnetoresistance inversion in a certain temperature range.

Toward ferromagnetic semimetal ground state with multiple Weyl nodes in van der Waals crystal MnSb4Te7

The magnetic topological van der Waals materials family MnBi2Te4/(Bi2Te3) n have drawn markedly attention due to their novel multiple topological phases in different magnetic configurations. Recently, their close relative, the MnSb4Te7, was firstly synthesized in experiments (2021 Phys. Rev. Lett. 126 246601). To further explore the emergent properties of MnSb4Te7, we have systematically investigated the magnetic and topological characters under compressive strain and charge doping using first-principles calculations. We predict that MnSb4Te7 transits from an interlayer antiferromagnetic ground state to a ferromagnetic semimetal ground state with multiple Weyl points when compressive strained along c axis above 8% or charge doping before 0.1 hole/formula concentration. Notable anomalous Hall conductivity is also predicted. Meanwhile, the magnetic easy axis can be reoriented from out-of-plane to in-plane orientation when strain or electron doping is applied. The underlying magnetic exchange mechanism is also analyzed from our calculation results. Our work thus provides a feasible way to realize applications of the highly tunable magnetic-topological nature and a comprehensive theoretical understanding of this magnetic topological material.

Spin Hall Magnetoresistance Effect from a Disordered Interface.

The spin Hall magnetoresistance (SMR) emerged as a reference tool to investigate the magnetic properties of materials with an all-electrical setup. Its sensitivity to the magnetization of thin films and surfaces may turn it into a valuable technique to characterize van der Waals magnetic materials, which support long-range magnetic order in atomically thin layers. However, realistic surfaces can be affected by defects and disorder, which may result in unexpected artifacts in the SMR, rather than the sole appearance of electrical noise. Here, we study the SMR response of heterostructures combining a platinum (Pt) thin film with the van der Waals antiferromagnet MnPSe3 and observe a robust SMR-like signal, which turns out to originate from the presence of strong interfacial disorder in the system. We use transmission electron microscopy (TEM) to characterize the interface between MnPSe3 and Pt, revealing the formation of a few nanometer-thick platinum-chalcogen amorphous layer. The analysis of the transport and TEM measurements suggests that the signal arises from a disordered magnetic system formed at the Pt/MnPSe3 interface, washing out the interaction between the spins of the Pt electrons and the MnPSe3 magnetic lattice. Our results show that the damaged interfaces can yield an important contribution to SMR, questioning a widespread assumption on the role of disorder in such measurements.

Exploring the heavy transition metal trihalide family: Two-dimensional magnetic materials with tunable band gap, huge magnetic anisotropy, and high-temperature magnetic ordering

In light of the forecast of recent topics of magnetic van der Waals materials [Burch et al., Nature (London) 563, 47 (2018); Gong and Zhang, Science 363, 706 (2019)], it was encouraged to explore new magnetic two-dimensional (2D) materials that contain heavy atoms so as to elevate the magnetic transition temperature. Here, relativistic density functional theory calculations, combined with ab initio molecular dynamics simulations, lattice dynamical calculations, and classical Monte Carlo (MC) simulations, were performed on transition metal trihalide (${\mathrm{TMI}}_{3}$; TM = La, Hf, Ta, W, Re, Os, Ir, Pt) layers, for which heavy $5d$-block and $5p$-block elemental atoms were considered owing to their strong spin-orbit coupling. We identified that these newly designed layers are energetically, dynamically, thermally, and thermomagnetically stable at room temperature. Our investigations revealed that most of them possess the intrinsic semiconducting characteristic with tunable band gap, diverse magnetic properties (ferromagnetic, antiferromagnetic, and nonferromagnetic orders), and large magnetic anisotropy energy (generally larger than 1.9 meV/f.u. and up to 29 meV/f.u.). Based on MC simulations, we proposed that ${\mathrm{TaI}}_{3}$, ${\mathrm{OsI}}_{3}$, and ${\mathrm{PtI}}_{3}$ layers have high Curie temperatures of 575, 225, and 170 K, respectively, which are much larger than those of most magnetic 2D materials such as the notable ${\mathrm{CrI}}_{3}$ (45 K), ${\mathrm{CrGeTe}}_{3}$ (28 K), and ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ layers (130 K). Importantly, our calculations evidenced the high stability of the ${\mathrm{TaI}}_{3}$ layer under ambient conditions, large magnetic moment of $2\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$, large magnetic anisotropy energy of $\ensuremath{-}7.3$ meV/f.u., and high Curie temperature of 575 K, making it possible to experimentally study this layer by more ex situ measurements and even to utilize it in future electronic devices.

Relativistic domain-wall dynamics in van der Waals antiferromagnet MnPS3

The discovery of two-dimensional (2D) magnetic van der Waals (vdW) materials has flourished an endeavor for fundamental problems as well as potential applications in computing, sensing and storage technologies. Of particular interest are antiferromagnets, which due to their intrinsic exchange coupling show several advantages in relation to ferromagnets such as robustness against external magnetic perturbations. Here we show that, despite of this cornerstone, the magnetic domains of recently discovered 2D vdW MnPS3 antiferromagnet can be controlled via magnetic fields and electric currents. We achieve ultrafast domain-wall dynamics with velocities up to ~3000 m s−1 within a relativistic kinematic. Lorentz contraction and emission of spin-waves in the terahertz gap are observed with dependence on the edge termination of the layers. Our results indicate that the implementation of 2D antiferromagnets in real applications can be further controlled through edge engineering which sets functional characteristics for ultrathin device platforms with relativistic features.

Unconventional ferrimagnetism and enhanced magnetic ordering temperature in monolayer CrCl3 by introducing O impurities and Cl vacancies

Among chromium trihalides, a specific group of layered van der Waals magnetic materials, chromium trichloride (CrCl3) is the only system relatively stable under ambient conditions. This is also observed in reduced dimensionality where the emergence of extrinsic long-range ordered oxidized and Cl-vacancy-defective CrCl3 phases is experimentally reported. In this work, the magnetic properties of such two-dimensional (2D) systems are studied using density functional theory (DFT) calculations, including the electron-electron (U) repulsion interactions, and Monte Carlo (MC) simulations. Once the Cl vacancies are introduced, the results indicate that the monolayer CrCl3 has a magnetic moment that is enhanced linearly (up to 3.14 µ B /Cr) in the (1%–10%) vacancy concentration range. This determines a strengthening of the ferromagnetic state and a two-fold increase of the Curie temperature (up to 146 K) as valuated from MC simulations. More interestingly, once oxygen extrinsic impurities are considered, the monolayer CrCl3 structure is hybridized forming a stable ordered phase (O–CrCl3) with oxygen atoms allocated on the Cr atomic layer in the center of the honeycomb ring formed by Cr atoms. The magnetic moments of the O–CrCl3 system are localized on both Cr and O atoms, with oxygen antiferromagnetically coupled to chromium, resulting in a 2D ferrimagnetic hexagonal lattice system with an average magnetic moment of 2.14 µ B /Cr and a high magnetic ordering temperature (110 K) predicted with DFT in the mean field approach.

Giant coercivity in single crystal Ta<sub>3</sub>FeS<sub>6</sub> film

Van der Waals (vdW) layered ferromagnetic materials provide a unique platform for fundamental spintronic research, and have broad application prospects in the next-generation spintronic devices. In this study, we synthesize high-quality single crystals of vdW intrinsic ferromagnet Ta<sub>3</sub>FeS<sub>6</sub> by the chemical vapor transport method. We obtain thin layer samples of Ta<sub>3</sub>FeS<sub>6</sub> with thickness values ranging from 19 to 100 nm by the mechanical exfoliation method, and find that their corresponding Curie temperatures are between 176 and 133 K. The anomalous Hall measurement shows that the Ta<sub>3</sub>FeS<sub>6</sub> has out-of-plane ferromagnetism with the coercivity reaching 7.6 T at 1.5 K, which is the largest value in those of the layered vdW ferromagnetic materials reported so far. In addition, we observe that the reversal polarity of the hysteresis loop changes sign with temperature increasing. Our work provides an opportunity to construct stable and miniaturized spintronic devices and present a new platform for studying spintronics based on van der Waals magnetic materials.

High-temperature magnetic skyrmions in BiCrX3 (X = Se and Te) monolayers.

Two-dimensional 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, those of the hosting van der Waals materials remain limited. Here, via first-principles calculations and Monte Carlo simulations, we propose BiCrX3 as a new family of materials for hosting skyrmions. Due to the large SOC of the X atom and intrinsic inversion asymmetry, an inherent large DMI occurs in all systems, enabling intriguing Néel-type skyrmions. Furthermore, upon applying a moderate magnetic field, isolated skyrmions and skyrmion lattices emerge in our systems, and are robust within a relatively wide temperature range.

Optimized Liquid-Phase Exfoliation of Magnetic van der Waals Heterostructures: Towards the Single Layer and Deterministic Fabrication of Devices.

Van der Waals magnetic materials are promising candidates for spintronics and testbeds for exotic magnetic phenomena in low dimensions. The two-dimensional (2D) limit in these materials is typically reached by mechanically breaking the van der Waals interactions between layers. Alternative approaches to producing large amounts of flakes rely on wet methods such as liquid-phase exfoliation (LPE). Here, we report an optimized route for obtaining monolayers of magnetic cylindrite by LPE. We show that the selection of exfoliation times is the determining factor in producing a statistically significant amount of monolayers while keeping relatively big flake areas (~1 µm2). We show that the cylindrite lattice is preserved in the flakes after LPE. To study the electron transport properties, we have fabricated field-effect transistors based on LPE cylindrite. Flakes are deterministically positioned between nanoscale electrodes by dielectrophoresis. We show that dielectrophoresis can selectively move the larger flakes into the devices. Cylindrite nanoscale flakes present a p-doped semiconducting behaviour, in agreement with the mechanically exfoliated counterparts. Alternating current (AC) admittance spectroscopy sheds light on the role played by potential barriers between different flakes in terms of electron transport properties. The present large-scale exfoliation and device fabrication strategy can be extrapolated to other families of magnetic materials.

Probing 2D magnetism through electronic tunneling transport

Atomically thin van der Waals magnetic materials have gained intensive research interests in the past few years. The topic paves a way to understand the stabilization and dissipation mechanisms of long-range magnetic order in the 2D limit, which is crucial for both fundamental condensed matter physics research and technological applications. However, limited by the sample size, conventional experimental techniques such as neutron diffraction spectrum and magnetization measurements are difficult to be applied to these atomically thin materials. At the same time, electronic tunneling transport measurements have been proved to be a powerful technique in the study of 2D magnetism. Here we review the electronic tunneling transports in magnetic 2D crystals, especially the detection of the phase boundaries of 2D magnets, focusing on two-material systems, i.e. chromium halides and manganese phosphorus trisulfides. In addition, the magnetic van der Waals Josephson junctions and the spin valve devices based on the 2D magnetic metals are applied to probe 2D magnetism as well. Finally, we discuss the current challenges and perspectives of potential applications of 2D van der Waals magnetic materials.

Coexistence of structural and magnetic phases in van der Waals magnet CrI3

CrI3 has raised as an important system to the emergent field of two-dimensional van der Waals magnetic materials. However, it is still unclear why CrI3 which has a ferromagnetic rhombohedral structure in bulk, changed to anti-ferromagnetic monoclinic at thin layers. Here we show that this behaviour is due to the coexistence of both monoclinic and rhombohedral crystal phases followed by three magnetic transitions at TC1 = 61 K, TC2 = 50 K and TC3 = 25 K. Each transition corresponds to a certain fraction of the magnetically ordered volume as well as monoclinic and rhombohedral proportion. The different phases are continuously accessed as a function of the temperature over a broad range of magnitudes. Our findings suggest that the challenge of understanding the magnetic properties of thin layers CrI3 is in general a coexisting structural-phase problem mediated by the volume-wise competition between magnetic phases already present in bulk.

Inducing high- Tc ferromagnetism in the van der Waals crystal Mn(ReO4)2 via charge doping: A first-principles study

Magnetic van der Waals (vdW) materials, which can serve as ideal platforms to study low-dimensional magnetism and possess potential applications in next-generation spintronic devices, have attracted intensive attention recently. Here, based on density functional theory calculations we predict that the electron-doped $\mathrm{Mn}{({\mathrm{ReO}}_{4})}_{2}$ (MRO) with an undulating layered structure is a ferromagnetic (FM) vdW material. Our calculations show that the magnetic interaction between two nearest-neighboring Mn atoms in the undoped MRO is very weak due to the long distance between the Mn atoms separated by the nonmagnetic ${\mathrm{ReO}}_{4}$ tetrahedra. With moderate electron doping from charge injection or K intercalation, the Re atoms become spin polarized. Thus, an effective FM coupling between the Mn atoms can be induced via the Mn-O-Re antiferromagnetic superexchange, while the calculated ${T}_{c}$ can exceed room temperature. The electron-doped MRO not only takes the advantage of strong FM interaction as double perovskites, but also avoids the shortage of the antisite disorder. The calculated small cleavage energy and stable phonon spectra indicate that MRO can be exfoliated to thin films, which facilitates its surface modification. Our paper on MRO, thus, provides a template to realize ferromagnetism in vdW materials with multitype transition metals.

Gate-Controlled Magnetic Phase Transition in a van der Waals Magnet Fe5GeTe2.

Magnetic van der Waals (vdW) materials are poised to enable all-electrical control of magnetism in the two-dimensional limit. However, tuning the magnetic ground state in vdW itinerant ferromagnets by voltage-induced charge doping remains a significant challenge, due to the extremely large carrier densities in these materials. Here, by cleaving the vdW itinerant ferromagnet Fe5GeTe2 (F5GT) into 5.4 nm (around two unit cells), we find that the ferromagnetism (FM) in F5GT can be substantially tuned by the thickness. Moreover, by utilizing a solid protonic gate, an electron doping concentration of above 1021 cm-3 has been exhibited in F5GT nanosheets. Such a high carrier accumulation exceeds that possible in widely used electric double-layer transistors (EDLTs) and surpasses the intrinsic carrier density of F5GT. Importantly, it is accompanied by a magnetic phase transition from FM to antiferromagnetism (AFM). The realization of an antiferromagnetic phase in nanosheet F5GT suggests the promise of applications in high-temperature antiferromagnetic vdW devices and heterostructures.

Electronic structure of rhombohedral CrX3 (X=Br, Cl, I) van der Waals crystals

We perform a comprehensive analysis of both the chemical and correlated electronic structure reconstruction of rhombohedral $\mathrm{Cr}{X}_{3}$ $(X=\text{Br}, \mathrm{Cl}, \mathrm{I})$ van der Waals bulk crystals. Using the generalized gradient approximation (GGA) plus dynamical mean-field theory we explicitly demonstrate the importance of local dynamical correlations for a consistent understanding of emergent Kondo quasiparticles and Mott localized electronic states, showing the interplay between material-dependent one-electron GGA line-shape and multiorbital electronic interactions. To probe the correlated paramagnetic electronic state we performed x-ray absorption spectroscopy measurements for ${\mathrm{CrCl}}_{3}$ and ${\mathrm{CrBr}}_{3}$ bulk crystals. Our correlated many-body study is relevant to understanding the electronic structure reconstruction of paramagnetic Cr-trihalide crystals and should be widely applicable to other van der Waals magnetic materials.

Effect of biaxial strain and hydrostatic pressure on the magnetic properties of bilayer CrI3

Two-dimensional van der Waals magnetic materials are intriguing for applications in the future spintronics devices, so it is crucial to explore strategy to control the magnetic properties. Here, we carried out first-principles calculations and Monte Carlo simulations to investigate the effect of biaxial strain and hydrostatic pressure on the magnetic properties of the bilayer CrI3. We found that the magnetic anisotropy, intralayer and interlayer exchange interactions, and Curie temperature can be tuned by biaxial strain and hydrostatic pressure. Large compressive biaxial strain may induce a ferromagnetic-to-antiferromagnetic transition of both CrI3 layers. The hydrostatic pressure could enhance the intralayer exchange interaction significantly and hence largely boost the Curie temperature. The effect of the biaxial strain and hydrostatic pressure revealed in the bilayer CrI3 may be generalized to other two-dimensional magnetic materials.

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.