2D Magnetic Materials Research Articles

Recent research advances in two-dimensional magnetic materials

Two-dimensional (2D) magnetic materials with magnetic anisotropy can form magnetic order at finite temperature and monolayer limit. Their macroscopic magnetism is closely related to the number of layers and stacking forms, and their magnetic exchange coupling can be regulated by a variety of external fields. These novel properties endow 2D magnetic materials with rich physical connotation and potential application value, thus having attracted extensive attention. In this paper, the recent advances in the experiments and theoretical calculations of 2D magnets are reviewed. Firstly, the common magnetic exchange mechanisms in several 2D magnetic materials are introduced. Then, the geometric and electronic structures of some 2D magnets and their magnetic coupling mechanisms are introduced in detail according to their components. Furthermore, we discuss how to regulate the electronic structure and magnetism of 2D magnets by external (field modulation and interfacial effect) and internal (stacking and defect) methods. Then we discuss the potential applications of these materials in spintronics devices and magnetic storage. Finally, the encountered difficulties and challenges of 2D magnetic materials and the possible research directions in the future are summarized and prospected.

Two-dimensional magnetic transition metal halides: molecular beam epitaxy growth and physical property modulation

The spontaneous magnetization of two-dimensional (2D) magnetic materials can be maintained down to the monolayer limit, providing an ideal platform for understanding and manipulating magnetic-related properties on a 2D scale, and making it important for potential applications in optoelectronics and spintronics. Transition metal halides (TMHs) are suitable 2D magnetic candidates due to partially filled d orbitals and weak interlayer van der Waals interactions. As a sophisticated thin film growth technique, molecular beam epitaxy (MBE) can precisely tune the growth of 2D magnetic materials reaching the monolayer limit. Moreover, combining with the advanced experimental techniques such as scanning tunneling microscopy, the physical properties of 2D magnetic materials can be characterized and manipulated on an atomic scale. Herein, we introduce the crystalline and magnetic structures of 2D magnetic TMHs, and show the 2D magnetic TMHs grown by MBE and their electronic and magnetic characterizations. Then, the MBE-based methods for tuning the physical property of 2D magnetic TMHs, including tuning interlayer stacking, defect engineering, and constructing heterostructures, are discussed. Finally, the future development opportunities and challenges in the field of the research of 2D magnetic TMHs are summarized and prospected.

Effect of Mo doping on the electrical and magnetic properties of antiferromagnetic CrSe

Recently 2-dimentional (2D) magnetic materials have gained a lot of attention due to their peculiar magnetic properties, originated from the quantum confinement. Here, we report on the synthesis, magnetic and electrical transport properties of Mo doped Cr1-xMoxSe (x = 0.0, 0.01, and 0.03) polycrystalline systems. Room temperature powder X-ray diffraction (XRD) data confirm that all the samples are formed into the hexagonal crystal structure with a space group of P63/mmc. Electrical resistivity measurements suggest that all the samples are metallic in nature. From magnetic measurements, we observe a bifurcation between ZFC and FC curves in all the samples almost at the same temperature of 225 K, suggesting for an antiferromagnetic transition at this temperature that is not affected with Mo doping. Magnetization measurements suggest for a mixed magnetic phase at low temperatures, means, weak ferromagnetism is found in addition to the antiferromagnetic ordering for the parent system. Mo doping leads to a couple of additional antiferromagnetic magnetic transitions. We further noticed decreasing magnetic moment with increasing Mo doping.

Pivotal role of magnetic ordering and strain in lattice thermal conductivity of chromium-trihalide monolayers

Understanding the coupling between spin and phonons is critical for controlling the lattice thermal conductivity (κ l ) in magnetic materials, as we demonstrate here for CrX3 (X = Br and I) monolayers. We show that these compounds exhibit large spin-phonon coupling (SPC), dominated by out-of-plane vibrations of Cr atoms, resulting in significantly different phonon dispersions in ferromagnetic (FM) and paramagnetic (PM) phases. Lattice thermal conductivity calculations provide additional evidence for strong SPC, where particularly large κ l is found for the FM phase. Most strikingly, PM and FM phases exhibit radically different behavior with tensile strain, where κ l increases with strain for the PM phase, and strongly decreases for the FM phase—as we explain through analysis of phonon lifetimes and scattering rates. Taken all together, we uncover the high significance of SPC on the phonon transport in CrX3 monolayers, a result extendable to other 2D magnetic materials, that will be useful in further design of thermal spin devices.

2D growth of high-quality magnetic CuCr2Se4 crystals with a high Curie temperature

Recently, two-dimensional (2D) magnetic materials have attracted broad attention due to their fascinating magnetic behavior and potential application in spintronics. However, most of exfoliated magnetic materials usually show random thickness and size, and the stability in the air is still a challenge. Hence, the controllable synthesis of 2D magnetic materials with good quality should be addressed. Here we report the controllable growth of 2D CuCr2Se4 crystals on mica substrates with large domain size. Most importantly, the as-grown CuCr2Se4 crystals demonstrate obvious ferromagnetic properties, showing a high Curie temperature of up to 436 K. Our discovery of 2D room-temperature ferromagnet CuCr2Se4 provides paves the way for the epitaxial growth of many more 2D magnets and provides a new platform for the spintronic applications.

Surface Functionalization for Magnetic Property Tuning of Nonmagnetic 2D Materials

AbstractSurface functionalization with organic molecules has proven effective to modulate the electronic properties of 2D materials. More recently, surface functionalization has been shown to induce magnetic behaviors in nonmagnetic 2D materials. If magnetism can be imparted to 2D materials via surface functionalization by molecules with tailored magnetic properties, it would make available the whole spectrum of 2D materials as magnetic systems, which are currently limited to a small subset of 2D materials that are intrinsically magnetic. This perspective will present experimental results and theoretical predictions of surface functionalization of 2D materials to induce magnetism. Key experimental studies on graphene, layered topological insulators, and transition metal dichalcogenides functionalized with single‐molecule magnets, adatoms, and nonmagnetic molecules will be discussed to advocate for utilizing surface functionalization to induce magnetism in 2D materials. The modulation of magnetic properties of 2D materials via surface functionalization is still in its infancy, presenting exciting opportunities for new ideas and discoveries. The perspective will conclude with the future outlook and challenges for this topic.

Tunable electronic structure and magnetic characteristics of two-dimensional graphyne/VI3 van der Waals heterostructures

Due to the excellent properties of two-dimensional (2D) magnetic materials, 2D van der Waals magnetic heterostructures have obtained more research in nanoelectronics. In this work, the electronic and magnetic properties in 2D graphyne/VI3 (γ-GY/VI3) magnetic heterostructures have been elaborately examined on account of first-principles calculations. As shown by the research results, semiconductor or half-metallic characteristic of VI3 monolayer in γ-GY/VI3 heterostructures may be exhibited due to the difference in stacking pattern. Furthermore, the formation of heterostructures significantly enhances the perpendicular magnetic anisotropy (PMA) of VI3 monolayer. At compressive strains, VI3 monolayer of γ-GY/VI3 heterostructure realizes a shift from semiconductor to metal characteristic, and this significantly enhances the electrical conductivity of VI3 monolayer. Moreover, through shortening the interlayer distance in γ-GY/VI3 heterostructures, the spin-up half-metallic state appears in VI3 monolayer. Both compressive strains and interlayer distance can also enhance the interlayer interaction, charge transfer and PMA of γ-GY/VI3 heterostructure. These tunable electronic properties show that γ-GY/VI3 heterostructures can become potential alternatives applicable to nanoelectronic and spintronic device design.

Air-Stable 2D Cr5 Te8 Nanosheets with Thickness-Tunable Ferromagnetism.

2D magnetic materials have aroused widespread research interest owing to their promising application in spintronic devices. However, exploring new kinds of 2D magnetic materials with better stability and realizing their batch synthesis remain challenging. Herein, the synthesis of air-stable 2D Cr5 Te8 ultrathin crystals with tunable thickness via tube-in-tube chemical vapor deposition (CVD) growth technology is reported. The importance of tube-in-tube CVD growth, which can significantly suppress the equilibrium shift to the decomposition direction and facilitate that to the synthesis reaction direction, for the synthesis of high-quality Cr5 Te8 with accurate composition, is highlighted. By precisely adjusting the growth temperature, the thickness of Cr5 Te8 nanosheets is tuned from ≈1.2nm to tens of nanometers, with the morphology changing from triangles to hexagons. Furthermore, magneto-optical Kerr effect measurements reveal that the Cr5 Te8 nanosheet is ferromagnetic with strong out-of-plane spin polarization. The Curie temperature exhibits a monotonic increase from 100 to 160 K as the Cr5 Te8 thickness increases from 10 to 30nm and no apparent variation in surface roughness or magnetic properties after months of exposure to air. This study provides a robust method for the controllable synthesis of high-quality 2D ferromagnetic materials, which will facilitate research progress in spintronics.

Exotic Spintronic Properties of Transition‐Metal Monolayers on Graphyne

AbstractThe recent discovery of 2D magnetic materials, which are compounds of transition metals (TM) with other elements, has opened new avenues for basic research on low‐dimensional magnetism and potential applications in spintronics. To further explore new 2D magnets of pure TM is an interesting topic. Based on the first‐principles calculations, here a strategy of obtaining monolayer TM magnets, i.e., depositing TM atoms on graphyne (Gy), which has proper hexagonal hollow geometry, is proposed. It is found that a TM monolayer with perfect hexagonal geometry can be formed on Gy. The TM monolayer exhibits a wealth of physical properties in dependence of TM species, such as ferromagnetic and anti‐ferromagnetic ground states, as well as intriguing semimetal and half‐metal characteristics. It is also found that the half‐metal characteristics make the monolayer TM have great potential applications in horizontal magnetic tunnel junction (MTJ) devices, where the tunneling magnetoresistance can reach as high as 850 000%. These results provide a new framework for obtaining 2D magnets with outstanding spintronic properties.

Chemical Design and Magnetic Ordering in Thin Layers of 2D Metal-Organic Frameworks (MOFs).

Through rational chemical design, and thanks to the hybrid nature of metal–organic frameworks (MOFs), it is possible to prepare molecule-based 2D magnetic materials stable at ambient conditions. Here, we illustrate the versatility of this approach by changing both the metallic nodes and the ligands in a family of layered MOFs that allows the tuning of their magnetic properties. Specifically, the reaction of benzimidazole-type ligands with different metal centers (MII = Fe, Co, Mn, Zn) in a solvent-free synthesis produces a family of crystalline materials, denoted as MUV-1(M), which order antiferromagnetically with critical temperatures that depend on M. Furthermore, the incorporation of additional substituents in the ligand results in a novel system, denoted as MUV-8, formed by covalently bound magnetic double layers interconnected by van der Waals interactions, a topology that is very rare in the field of 2D materials and unprecedented for 2D magnets. These layered materials are robust enough to be mechanically exfoliated down to a few layers with large lateral dimensions. Finally, the robustness and crystallinity of these layered MOFs allow the fabrication of nanomechanical resonators that can be used to detect—through laser interferometry—the magnetic order in thin layers of these 2D molecule-based antiferromagnets.

Predicting MnB6 monolayer with room temperature ferromagnetism and high magnetic anisotropy

Searching for two-dimensional (2D) magnetic materials with high critical temperature and intrinsic magnetism have attracted significant research interests recently. By using swarm-intelligence structure search and first-principles calculations, we predict two low-energy MnB 6 monolayers, namely MnB 6 -I and MnB 6 -II, with high stabilities and intriguing electronic and magnetic properties . Specifically, the phonon spectrum and ab initio molecular dynamics simulations indicate the two sheets are dynamically and thermally stable. The MnB 6 -I and MnB 6 -II layers are both metallic with ferromagnetic (FM) and antiferromagnetic (AFM) ground state, respectively. When spin-orbital coupling is considered, the type-II Dirac points and a sizable magnetocrystalline anisotropy energy (938 μ eV per Mn atom) can be found in the MnB 6 -I sheet. The Monte Carlo simulation based on the Heisenberg model suggests the Curie temperature for MnB 6 -I sheet is up to 470 K, which is above room temperature. The superior electronic and magnetic properties of the MnB 6 monolayers render them promising candidates for spintronic applications. • Two stable phases of MnB 6 monolayers were predicted with ferromagnetic and antiferromagnetic ground states. • The calculated Curie temperature for MnB 6 -I sheet is up to 470 K, which is above room temperature. • The calculated Néel temperature for MnB 6 -II sheet is 180 K, higher than many reported antiferromagnetic materials. • When spin-orbital coupling is considered, MnB 6 -I sheet shows type-II Dirac points and a sizable magnetic anisotropy energy.

Switchable Interlayer Magnetic Coupling of Bilayer CrI3

Due to the weak van der Waals (vdW) interlayer interaction, interfacial geometry of two-dimensional (2D) magnetic vdW materials can be freely assembled, and the stacking order between layers can be readily controlled, such as laterally shifting or rotating, which may trigger the variation of magnetic order. We investigate the H-type bilayer CrI where the two layers are aligned in anti-parallel directions. Based on first-principles calculations, we propose two states with different interlayer magnetic couplings, i.e., ferromagnetic and antiferromagnetic, and analyze the superexchange mechanism inside. It is found that the two magnetic coupling states are stacking-dependent, and could be switched by applying out-of-plane axial strain and electron doping. Our findings show great application potential in the design of heterostructural and spintronic devices based on 2D magnetic vdW materials.

Structure Engineering of 2D Materials toward Magnetism Modulation

2D magnetic materials have shown great potentials in fundamental research and spintronic devices. However, the discovered 2D magnetic materials are still limited and their monotonous magnetic properties cannot meet the needs of practical applications. Recently, the modulation of magnetism in 2D materials attracted tremendous attention, which can enrich the 2D magnetic material family and extend their application prospects. Structure engineering is one of the most effective ways to realize tunable 2D magnetic properties. Herein, the recent progress on 2D magnetism modulation via structure engineering is summarized. The basic theory of 2D magnetism is introduced at first. Then, the magnetism modulation by structure engineering is classified into two ways: inducing long‐range magnetic ordering into nonmagnetic materials and regulating the native magnetism of intrinsic magnetic materials. Moreover, recent progress on magnetism modulation by intercalation, chemical doping, defect engineering, and heterostructure construction is discussed in detail. Finally, the challenges and prospects in the future of 2D magnetism are also provided.

Recent progress in the synthesis of novel two-dimensional van der Waals materials.

The last decade has witnessed the significant progress of physical fundamental research and great success of practical application in two-dimensional (2D) van der Waals (vdW) materials since the discovery of graphene in 2004. To date, vdW materials is still a vibrant and fast-expanding field, where tremendous reports have been published covering topics from cutting-edge quantum technology to urgent green energy, and so on. Here, we briefly review the emerging hot physical topics and intriguing materials, such as 2D topological materials, piezoelectric materials, ferroelectric materials, magnetic materials and twistronic heterostructures. Then, various vdW material synthetic strategies are discussed in detail, concerning the growth mechanisms, preparation conditions and typical examples. Finally, prospects and further opportunities in the booming field of 2D materials are addressed.

Strain-tunable phase transition and doping-induced magnetism in iodinene

Two-dimensional (2D) ferromagnetic materials with high Curie temperatures (TC) and tunable physical properties are crucial to the development of nanoscale spintronics devices. Here, we investigate the newly synthesized iodinene using first-principles calculations. Our results show that doping carriers in monolayer and bilayer iodinene can easily introduce itinerant ferromagnetism due to a flatband structure near the Fermi level, and that the associated TC is higher than room temperature. Moreover, we find that a structural phase transition can be achieved through the application of moderate tensile strain for both monolayer and bilayer iodinene. The magnetic moment, Curie temperature, band structure, carrier effective mass, and optical absorption can be changed significantly through this phase transition, and the transition can also lead to a magnetic phase transition with an appropriate doping concentration. Our work provides a feasible approach for designing 2D magnetic materials with potential for application in microelectronics devices.

3D Temporary-Magnetized Soft Robotic Structures for Enhanced Energy Harvesting.

The advent of functional materials offers tremendous potential in a broad variety of areas such as electronics, robotics, and energy devices. Magnetic materials are an attractive candidate that enable multifunctional devices with capabilities in both sensing and actuation. However, current magnetic devices, especially those with complex motion modalities, rely on permanently magnetized materials with complicated, non-uniform magnetization profiles. Here, based on magnetic materials with temporary-magnetization, a mechanically guided assembly process successfully converts laser-patterned 2D magnetic materials into judiciously engineered 3D structures, with dimensions and geometries ranging from mesoscale 3D filaments, to arrayed centimeter-scale 3D membranes. With tailorable mechanical properties and highly adjustable geometries, 3D soft structures can exhibit various tethered locomotions under the precise control of magnetic fields, including local deformation, unidirectional tilting, and omnidirectional rotation, and can serve as dynamic surfaces for further integration with other functional materials or devices. Examples demonstrated here focus on energy-harvesting systems, including 3D piezoelectric devices for noncontact conversion of mechanical energy and active motion sensing, as well as 3D solar tracking systems. The design strategy and resulting magnetic-controlled 3D soft structures hold great promise not only for enhanced energy harvesting, but also for multimodal sensing, robotic interfaces, and biomedical devices.

Large-area van der Waals epitaxy and magnetic characterization of Fe3GeTe2 films on graphene

Scalable fabrication of magnetic 2D materials and heterostructures constitutes a crucial step for scaling down current spintronic devices and the development of novel spintronic applications. Here, we report on van der Waals (vdW) epitaxy of the layered magnetic metal Fe3GeTe2 (FGT)—a 2D crystal with highly tunable properties and a high prospect for room temperature ferromagnetism (FM)—directly on graphene by employing molecular beam epitaxy. Morphological and structural characterization confirmed the realization of large-area, continuous FGT/graphene heterostructure films with stable interfaces and good crystalline quality. Furthermore, magneto-transport and x-ray magnetic circular dichroism investigations confirmed a robust out-of-plane FM in the layers, comparable to state-of-the-art exfoliated flakes from bulk crystals. These results are highly relevant for further research on wafer-scale growth of vdW heterostructures combining FGT with other layered crystals such as transition metal dichalcogenides for the realization of multifunctional, atomically thin devices.

Two-Dimensional Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) Monolayers: Half-Metallic Ferromagnets with Tunable Magnetic Properties under Strain.

Two-dimensional (2D) ferromagnetic materials with high spin polarization are highly desirable for spintronic devices. 2D Janus materials exhibit novel properties due to their broken symmetry. However, the electronic structure and magnetic properties of 2D Janus magnetic materials with high spin polarization are still unclear. Inspired by the successful synthesis of a ferromagnetic FeCl2 monolayer and 2D Janus MoSSe and WSSe, we systematically study the electronic structure and magnetic properties of Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) monolayers. Based on the Goodenough-Kanamori-Anderson theory, the ferromagnetism stems from the superexchange interaction mediated by Fe-X/Y-Fe bonds. The band gaps of spin-up channels are large enough (>4 eV) to prevent spin flipping, which is beneficial for spintronic devices. Additionally, the sizable magnetocrystalline anisotropy energy (MAE) indicates that Janus FeXY monolayers are suitable for information storage. More importantly, the half-metallic character is still kept in Janus FeXY monolayers, and their magnetic properties are enhanced by the biaxial compressive strain. The MAE of FeClI and FeBrI increases by 1 order of magnitude, and the Curie temperature of FeXY monolayers enhances by 100%. These results provide an example of the 2D Janus half-metallic materials and enrich the 2D magnetic material library.

Chiral Phonons and Giant Magneto-Optical Effect in CrBr3 2D Magnet.

Phonons with chirality determine the optical helicity of inelastic light scattering processes due to their nonzero angular momentum. Here it is shown that 2D magnetic CrBr3 hosts chiral phonons at the Brillouin-zone center. These chiral phonons are linear combinations of the doubly-degenerate Eg phonons, and the phonon eigenmodes exhibit clockwise and counterclockwise rotational vibrations corresponding to angular momenta of l = ± 1.Such Eg chiral phonons completely switch the polarization of incident circularly polarized light. On the other hand, the non-degenerate non-chiral Ag phonons display a giant magneto-optical effect under an external out-of-plane magnetic field, rotating the plane of polarization of the scattered linearly polarized light. The corresponding degree of polarization of the scattered light changes from 91% to -68% as the magnetic field strength increases from 0 to 5 T. In contrast, the chiral Eg modes display no field dependence. The results lay a foundation for the study of phonon chirality and magneto-optical phenomena in 2D magnetic materials, as well as their related applications, such as the phonon Hall effect, topological photonics, and Raman lasing.

Crystallographic and magnetic structures of the VI3 and LiVI3 van der Waals compounds

Two-dimensional (2D) layered magnetic materials are generating a great amount of interest for the next generation of electronic devices thanks to their remarkable properties associated with spin dynamics. The recently discovered layered $\mathrm{V}{\mathrm{I}}_{3}$ ferromagnetic phase belongs to this family, although a full understanding of its properties is limited by the incomplete understanding of its crystallographic structure. The motivation of this work is to address this issue. Here, we investigate the $\mathrm{V}{\mathrm{I}}_{3}$ crystal structures at low temperature using both synchrotron x-ray and neutron powder diffraction and provide structural models for the two structural transitions occurring at 76 and 32 K. Moreover, we confirm by magnetic measurements that $\mathrm{V}{\mathrm{I}}_{3}$ becomes ferromagnetic at 50 K and we question the establishment of a long-range magnetic structure by neutron diffraction. We equally determined the magnetic properties of our recently reported $\mathrm{LiV}{\mathrm{I}}_{3}$ phase, which is like the well-known $\mathrm{Cr}{\mathrm{I}}_{3}$ ferromagnetic phase in terms of electronic and crystallographic structures and found an antiferromagnetic behavior with a N\'eel temperature of 12 K. Such a finding provides extra clues for a better understanding of magnetism in these low-dimension compounds. Finally, the easiness of preparing Li-based 2D magnetic materials by chemical/electrochemical means opens wide the opportunity to design materials with exotic properties.