2D Magnetism Research Articles

Unlocking the charge doping effect in softly intercalated ultrathin ferromagnetic superlattice

The electrolyte-assisted exfoliation strategy is widely employed to synthesize ultrathin two-dimensional (2D) materials. Yet, spins in 2D magnets are susceptible to the electrolyte due to the underlying charge doping effect. Hence, it is crucial to understand and trace the doping process during the delamination of 2D magnets. Taking the prototype Fe3GeTe2, we utilized soft organic cations to exfoliate the bulk and obtain a freestanding organic–inorganic hybrid superlattice with a giant electron doping effect as high as 6.9 ​× ​1014/cm2 (∼1.15 electrons per formula unit). A remarkable ferromagnetic transition exceeding 385 ​K was revealed in these superlattices, together with unique anisotropic saturation magnetization. The doping enhanced the in-plane electron–phonon coupling and thus optimized originally poor indirect double-exchange scenario for spin electrons. The emerging vertical magnetization shift phenomenon served to evaluate the uniformity of charge doping. The above findings provide a new perspective for understanding the role of parasitic charge in 2D magnetism.

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.

One-dimensional magnetism and Rashba-like effects in zigzag bismuth nanoribbons

Discoveries of low-dimensional quantum materials have renewed interest in some of the fundamental phenomena, such as magnetism and Rashba-type spin orbit coupling. In particular, exploring these phenomena by themselves and/or in combination within one-dimensional (1D) systems is of interest for fields as disparate as spintronics and biology. For example, a better understanding of Rashba-type spin orbit coupling in 1D systems may be used to explain spin-selective electron transport in long helical molecules. In this paper, using first principle calculations, we show that each edge of a zigzag nanoribbon composed of a bismuth (Bi) bilayer is a truly 1D structure that naturally combines both 1D magnetism and Rashba-type spin orbit coupling in a single system. In particular, we study the combined effects of exchange and spin-orbit coupling in nanoribbons that are: (i) ideal freestanding, (ii) placed on a hexagonal boron nitride substrate, and (iii) decorated with N atoms. The edges of the Bi zigzag NRs can display ferromagnetic, antiferromagnetic, or noncollinear ordering, resulting in a broken quantum spin Hall state. The interplay of Rashba and exchange effects in different magnetic phases can result in different spin-dependent transport regimes.

Voltage-controlled magnetic anisotropy in heterostructures with a two-dimensional magnetic material

The advent of two-dimensional (2D) magnetic materials significantly expand the scope of voltage-controlled magnetization-switching scheme as their integration in diverse magnetic tunnel junctions (MTJ) offers a highly attractive perspective for designing future magnetoelectric random access memory (MeRAM). Here, we propose the utilization of 2D ferromagnetic ${\mathrm{Fe}}_{2}{\mathrm{I}}_{2}$ to substitute traditional magnetic thin films for assembling energy-efficient MTJs combined with Ir capping layers. We find these multilayers exhibit both giant perpendicular magnetic anisotropy (PMA) and voltage controlled magnetic anisotropy (VCMA) efficiency depending strongly on the Ir thickness and epitaxial strain effect. Furthermore, the spin reorientation has also been achieved with increased Ir thickness, and the magnitude and slope of VCMA behavior are dominated by the biaxial strain. We elucidate that the underlying mechanism is the electric-field-induced modifications of the spin-orbit coupling energies of the spin-polarized $\mathrm{Ir}\text{\ensuremath{-}}d$ orbitals. These findings not only reveal new approaches controlling 2D magnetism, but also pave the way of an alternative strategy for the design of nonvolatile and ultralow power spintronics and magnetic memory storage devices.

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.

Topological spin textures in 1T -phase Janus magnets: Interplay between Dzyaloshinskii-Moriya interaction, magnetic frustration, and isotropic higher-order interactions

The search for topological magnetism in two-dimensional (2D) magnetic materials is one of the hot topics in spintronics. We present comprehensive studies of magnetic phases in a series of Janus monolayers $\mathrm{Mn}XZ$ (MnAsBr, MnAsI, MnPBr, and MnPCl) and $\mathrm{Cr}YZ$ ($Y=\mathrm{Se}$, Te; and $Z=\mathrm{Cl}$, Br, I) via combining first-principles calculations and atomistic spin model simulations. Sizable Dzyaloshinskii-Moriya interaction can be realized in the $\mathrm{Mn}XZ$ and $\mathrm{Cr}YZ$ monolayers due to their intrinsic inversion symmetry breaking. More interestingly, the $\mathrm{Mn}XZ$ and $\mathrm{Cr}YZ$ monolayers exhibit different degrees of magnetic frustration and isotropic higher-order interactions. Lastly, our atomistic spin model simulations demonstrate that a variety of topological spin textures can be generated by the interplay among complex magnetic interactions. These results provide valuable information and fundamental understanding for topological magnetism in 2D magnets.

Seeding and Emergence of Composite Skyrmions in a van der Waals Magnet.

Topological charge plays a significant role in a range of physical systems. In particular, observations of real-space topological objects in magnetic materials have been largely limited to skyrmions - states with a unitary topological charge. Recently, more exotic states with varying topology, such as antiskyrmions, merons, or bimerons and 3D states such as skyrmion strings, chiral bobbers, and hopfions, have been experimentally reported. Along these lines, the realization of states with higher-order topology has the potential to open new avenues of research in topological magnetism and its spintronic applications. Here, real-space imaging of such spin textures, including skyrmion, skyrmionium, skyrmion bag, and skyrmion sack states, observed in exfoliated flakes of the van der Waals magnet Fe3-x GeTe2 (FGT) is reported. These composite skyrmions may emerge from seeded, loop-like states condensed into the stripe domain structure, demonstrating the possibility to realize spin textures with arbitrary integer topological charge within exfoliated flakes of 2D magnets. The general nature of the formation mechanism motivates the search for composite skyrmion states in both well-known and new magnetic materials, which may yet reveal an even richer spectrum of higher-order topologicalobjects.

Strain enhanced magnetism of V-implanted CrI3 monolayer

In recent years, one of the urgent issues for two dimensional (2D) magnetic materials is to find efficient ways in enhancing the magnetic ordering temperature Tc. It is believed that an in-plane (IP) compressive strain can greatly enhance the interatomic interactions by shortening the chemical bond length if at all possible, leading to the enlarged spin exchange and possibly higher Tc. However, a large compressive strain usually favors antiferromagnetic (AFM) ordering due to growing dominance of the Pauli exclusion principle, in contradiction with the common requirement of nonzero magnetization. In compromise, ferrimagnetic (FiM) ordering can be alternated by synthesizing artificial 2D compound with two magnetic sublattices. In this work, we propose a V-implanted CrI3 monolayer, short for V-(CrI3)2, and study its FiM ordering under a series of IP biaxial strains using the first-principles calculations and Monte Carlo simulations. It is found that the V-(CrI3)2 monolayer may evolve from the stripy-type AFM insulator toward the FiM half-metal with net magnetic moment of 5.0 μB/f.u. aligned in parallel to the ab-plane upon increasing the IP biaxial strain up to ∼−3% (compressive strain) and beyond. As the IP biaxial strain increases up to ∼−5%, the Tc of the FiM state may be raised to room temperature. This work suggests that the IP strain engineering combined with spin implantation can be an alternative strategy for enhancing 2D magnetism.

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.

Magnetic Amplification at Yb3+ "Designer Defects" in the van der Waals Ferromagnet CrI3.

The two-dimensional (2D) van der Waals ferromagnet CrI3 has been doped with the magnetic optical impurity Yb3+ to yield materials that display sharp multiline Yb3+ photoluminescence (PL) controlled by the magnetism of CrI3. Magneto-PL shows that Yb3+ magnetization is pinned to the magnetization of CrI3. An effective internal field of ∼10 T at Yb3+ is estimated, attributed to strong in-plane Yb3+-Cr3+ superexchange coupling. The anomalously low energy of Yb3+ PL in CrI3 reflects relatively high Yb3+-I- covalency, contributing to Yb3+-Cr3+ superexchange coupling. The Yb3+ PL energy and line width both reveal the effects of spontaneous zero-field CrI3 magnetic ordering within 2D layers below TC, despite the absence of net magnetization in multilayer samples. These results illustrate the use of optical impurities as "designer defects" to introduce unique functionality to 2D magnets.

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.

Research progress of two-dimensional magnetic materials

Because of their ultrathin thickness, two-dimensional (2D) materials exhibit unique properties in different fields, such as electronics and catalysis. As one of these materials, a 2D magnet is an ideal platform for fundamental physics research and magnetic device development, making it widely studied. This review provides the details of recent progress of 2D magnetic materials. First, the characterization methods of magnetism are summarized. Then, the categories of 2D magnets and the strategies for regulating 2D magnetism are discussed. Finally, the challenges and opportunities in 2D magnetic material development are pointed out.

Two-dimensional van der Waals ferromagnetic thin film CrTe2 with high Curie temperature and metallic conductivity

Two-dimensional van der Waals (2D vdW) materials have opened up an opportunity to explore an innovative spin-based magnetic nanodevice. However, controllable fabrication of 2D vdW ferromagnets with high Curie temperature remains challenging. In this paper, we reported the growth of 2D CrTe2 single-crystal films epitaxially on Al2O3 substrates using pulsed laser deposition. We find that it shows a typical paramagnetic–ferromagnetic (PM–FM) phase transition around 200 K. The precise Curie temperature and Weiss temperature are 189 and 206.7 K, respectively. The saturation magnetization reaches 73.64 emu/g for the film thickness of 30 nm. The critical exponent β = 0.329 indicates that the magnetic interactions obey the 3D-Ising model. Electronic transport measurement confirms that a CrTe2 film always remains a metallic behavior at 5 K ≤ T ≤ 320 K and the resistivity of room temperature is 1.5 mΩ/cm. The first-principles calculation uncovers that the FM ordering state mainly stems from an exchange coupling of the adjacent Cr-spin t2g polarized electrons and the metallic conductivity is due to p–d orbital hybridization between Cr and Te atoms. This work would shed new light on studying large-scale growth of 2D magnets and developing 2D magnet-based nanodevices of room temperature.

Probing Defects and Spin‐Phonon Coupling in CrSBr via Resonant Raman Scattering

AbstractUnderstanding the stability limitations and defect formation mechanisms in 2D magnets is essential for their utilization in spintronic and memory technologies. Here, defects in mono‐ to multilayer CrSBr are correlated with structural, vibrational, and magnetic properties. Resonant Raman scattering is used to reveal distinct vibrational defect signatures. In pristine CrSBr, it is shown that bromine atoms mediate vibrational interlayer coupling, allowing for distinguishing between surface and bulk defect modes. Environmental exposure is shown to cause drastic degradation in monolayers, with the formation of intralayer defects. This is in contrast to multilayers that predominantly show bromine surface defects. Through deliberate ion irradiation, the formation of defect modes is tuned: these are strongly polarized and resonantly enhanced, reflecting the quasi‐‐1D electronic character of CrSBr. Strikingly, pronounced signatures of spin‐phonon coupling of the intrinsic phonon modes and the ion beam‐induced defect modes are observed throughout the magnetic transition temperature. Overall, defect engineering of magnetic properties is possible, with resonant Raman spectroscopy serving as a direct fingerprint of magnetic phases and defects in CrSBr.

High‐Throughput Screening Assisted Discovery of a Stable Layered Anti‐Ferromagnetic Semiconductor: CdFeP2Se6

AbstractRecent advances in 2D magnetism have heightened interest in layered magnetic materials due to their potential for spintronics. In particular, layered semiconducting antiferromagnets exhibit intriguing low‐dimensional semiconducting behavior with both charge and spin as carrier controls. However, synthesis of these compounds is challenging and remains rare. Here, first‐principles based high‐throughput search is conducted to screen potentially stable mixed metal phosphorous trichalcogenides (MM′P2X6, where M and M′ are transition metals and X is a chalcogenide) that have a wide range of tunable bandgaps and interesting magnetic properties. Among the potential candidates, a stable semiconducting layered magnetic material, CdFeP2Se6, that exhibits a short‐range antiferromagnetic order at TN = 21 K with an indirect bandgap of 2.23 eV is successfully synthesized . This work suggests that high‐throughput screening assisted synthesis can be an effective method for layered magnetic materials discovery.

Above-Room-Temperature Ferromagnetism in Thin van der Waals Flakes of Cobalt-Substituted Fe5GeTe2

Two-dimensional (2D) magnetic van der Waals materials provide a powerful platform for studying the fundamental physics of low-dimensional magnetism, engineering novel magnetic phases, and enabling thin and highly tunable spintronic devices. To realize high-quality and practical devices for such applications, there is a critical need for robust 2D magnets with ordering temperatures above room temperature that can be created via exfoliation. Here, the study of exfoliated flakes of cobalt-substituted Fe5GeTe2 (CFGT) exhibiting magnetism above room temperature is reported. Via quantum magnetic imaging with nitrogen-vacancy centers in diamond, ferromagnetism at room temperature was observed in CFGT flakes as thin as 16 nm corresponding to 16 layers. This result expands the portfolio of thin room-temperature 2D magnet flakes exfoliated from robust single crystals that reach a thickness regime relevant to practical spintronic applications. The Curie temperature Tc of CFGT ranges from 310 K in the thinnest flake studied to 328 K in the bulk. To investigate the prospect of high-temperature monolayer ferromagnetism, Monte Carlo calculations were performed, which predicted a high value of Tc of ∼270 K in CFGT monolayers. Pathways toward further enhancing monolayer Tc are discussed. These results support CFGT as a promising platform for realizing high-quality room-temperature 2D magnet devices.

Controlling the spacing of the linked graphene oxide system with dithiol linkers under confinement.

2D nanoscale confined systems exhibit behavior that is markedly different from that observed at the macroscale. Confinement can be tuned by controlling the interlayer spacing between confining layers using organic dithiol linkers. Adjusting spacing and selective intercalation have important impacts for catalysis, superconductivity, spin engineering, sodium ion batteries, 2D magnets, optoelectronics, and many other applications. In this study, we report how reaction conditions and organic linkers can be used to create variable, reproducible spacings between graphene oxide to provide confinement systems. We determined the conditions under which the spacing can be variably adjusted by the type of linker used, the concentration of the linker, and the reaction conditions. Employing dithiol linkers of different lengths, such as three (TPDT) and four (QPDT) aromatic rings, we can adjust the spacing between graphene oxide layers under varied reaction conditions. Here, we show that by varying dithiol linker length and using different reaction conditions, we can reproducibly control the spacing between graphene oxide layers from 0.37 nm to over 0.50 nm.

Intrinsic exchange bias state in silicene and germanene materials EuX2.

2D magnets have recently emerged as a host for unconventional phases and related phenomena. The prominence of 2D magnetism stems from its high amenability to external stimuli and structural variations. The low dimensionality facilitates competition between magnetic orders which may give rise to exchange bias, in particular in magnetic heterostructures. Here, we propose a strategy for the search of exchange bias state in 2D individual compounds. We track the evolution of magnetic orders driven by the number of monolayers in a system exhibiting antiferromagnetism in the multilayer and ferromagnetism in the monolayer limit. The material, EuSi2, has the structure of multilayer silicene intercalated by Eu. A strong intrinsic exchange bias effect accompanies the dimensional crossover. Comparison with silicene-based GdSi2 and germanene-based EuGe2 suggests the competition between magnetic orders to be a common property of this class of materials that may be useful in spintronic applications.

Bilayer hexagonal structure MnN2 nanosheets with room-temperature ferromagnetic half-metal behavior and a tunable electronic structure.

Two-dimensional (2D) layered materials have atomically thin thickness and outstanding physical properties, attracting intensive research in the past year. As one of these materials, a 2D magnet is an ideal platform for fundamental physics research and magnetic device development. Recently, a non-MoS2-type geometry was found to be more favorable in 2D transition-metal dinitrides. In this work, driven by this new configuration, we perform a comprehensive first-principles study on the bilayer hexagonal structure of 2D manganese dinitrides. Our results show that 2D MnN2 is a ferromagnetic half-metal at its ground state with 100% spin-polarization ratio at the Fermi energy level. The phonon spectrum calculation and ab initio molecular dynamics simulation show that the 2D MnN2 crystal has a high thermodynamic stability and its 2D lattice can be retained at room-temperature. Monte Carlo simulations based on the Heisenberg model predict a Curie temperature of over 563 K and its electronic properties can be regulated by biaxial strain. The half-metallic states are mainly contributed by Mn d orbitals, and the magnetic exchange of the system mainly comes from the Mn-N-Mn super-exchange. The p-d orbital hybridization will provide a small antiparallel magnetic moment of N atoms, and the p-orbital dangling bond can be eliminated by oxidation to enhance the total magnetic moment of the system. The study of magnetic anisotropy energy indicates that the easy magnetization axis is in-plane and hybridization between Mn dyz and dz2 orbitals gives the largest magnetic anisotropy contribution. In view of these results, we consider that novel 2D MnN2 is one of the most promising two-dimensional materials for nano-spintronic applications.

Large vertical piezoelectricity and high-temperature ferromagnetism in 2D ferromagnetic semiconductors MnAsX (X = I,Br,Cl)

Recently, intrinsic two-dimensional (2D) magnetism has been found to demonstrate good physical and chemical properties, and gradually applied in spintronic devices. Based on first-principles calculations, this letter shows that the monolayers MnAsX (X = Cl, Br, I) are intrinsically ferromagnetic semiconductors. The large piezoelectric coefficients e 31 and d 31 of the monolayers MnAsX are larger than most TMD family materials because of the broken inversion symmetry along the vertical direction. We also discuss the causes of different bandgaps in MnAsX (X = Cl, Br, I) structure. The mechanism of super-exchange interactions explains the Curie temperature (T C ) change from 340 to 542 K as the halogen atoms change from Cl to I. These excellent magnetic properties show the potential of using the 2D family in basic research on spintronic applications. • We propose three emerging 2D intrinsic FM semiconductor materials: MnAsX (X = I, Br, Cl). • The large piezoelectric coefficients of MnAsX (X = I,Br,Cl) are larger than most TMD family materials because of the broken inversion symmetry. • The Curie temperature of MnAsI is as high as 542 K.