2D Ferromagnets Research Articles

Designing Two-Dimensional Versatile Room-Temperature Ferromagnets via Assembling Large-Scale Magnetic Quantum Dots.

Two-dimensional (2D) ferromagnets possess astonishing potential in new-concept spintronics. However, most of the reported intrinsic 2D ferromagnets show a low Curie temperature far below room temperature. Here, we propose a series of 2D magnetic covalent and metal organic frameworks (COFs/MOFs) by assembling triangular zigzag graphene quantum dots (TZGDs) with various linkages, involving small-sized TZGDs, nonmetal atoms, magnetic metal atoms, and molecules. Upon first-principles calculations, we demonstrate 2D magnetic semiconductors with an enhanced Curie temperature of up to 472 K can be realized through the strong p(d)-p direct exchange interaction between TZGDs and linkages. Particularly, the TZGD size hardly affects the Curie temperature, whereas linkages can modulate the Curie temperature significantly. The TZGD size and linkages can regulate the electronic and magnetic properties of TZGD-based 2D ferromagnets. Our results confirm the possibility of designing 2D ferromagnets based on TZGDs and motivate the research of 2D ferromagnets on magnetic quantum dots and molecular magnets.

Electric-Field-Driven Negative Differential Conductance in 2D van der Waals Ferromagnet Fe3GeTe2.

Understanding quantum tunneling principles over two-dimensional (2D) van der Waals (vdW) ferromagnets at the atomic level is essential and complementary to the fundamental study of low-dimensional strong correlated systems and is critical for the development of magnetic tunneling devices. Here, we demonstrate a local electric-field controlled negative differential conductance (NDC) in 2D vdW ferromagnet Fe3GeTe2 (FGT) by using scanning tunneling microscopy (STM). The STM reveals that NDC shows an atomic position dependence and can be precisely modulated by altering the tunneling junction. The band shift together with electric-field-driven 3d-orbital occupancy modulates the sensitive magnetic anisotropic energy (MAE) in 2D FGT and consequently leads to electric-field-tunable NDC, which is also verified by theoretical simulation. This work realizes the electric-field-driven NDC in 2D ferromagnet FGT, which paves a way to design and develop applications based on 2D vdW magnets.

Multiple magnetic phase transitions with different universality classes in bilayer La_{1.4}Sr_{1.6}Mn_{2}O_7 manganite

Here, we report three magnetic transitions at 101 K (T_{C1}), 246 K (T_{C2}) and 295 K (T_{C3}) in bilayer La_{1.4}Sr_{1.6}Mn_{2}O_7. The second order phase transitions have been identified at these transition points with the help of change in entropy analysis and modified Arrott plots (MAPs). The critical behavior around T_{C1}, T_{C2} and T_{C3} have been studied by MAPs and Kouvel–Fisher method. Based on these analyses four magnetic phases are: (1) 2D Ising ferromagnetic (FM) below T_{C1},(2) 2D Heisenberg canted antiferromagnetic (CAFM-I) and FM clusters in temperature range T_{C1} < T < T_{C2}, (3) 2D Heisenberg CAFM-II and FM clusters with non magnetically interacting planes in temperature range T_{C2} < T < T_{C3} and (4) paramagnetic for T > T_{C3}.

Structural, magnetic and magnetocaloric properties of double perovskite Ho2MMnO6 (M = Fe, Co, and Ni)

We report our systematic investigation of structural, magnetic, and magnetocaloric properties of double perovskite compounds Ho2FeMnO6, Ho2CoMnO6, and Ho2NiMnO6, which crystallize into monoclinic crystal structure with P21/n space group. The magnetic phase transition temperatures are determined to be 7.5 K for Ho2FeMnO6, 69 K for Ho2CoMnO6, and 71.5 K for Ho2NiMnO6, where Ho2FeMnO6 exhibits an antiferromagnetic to paramagnetic phase transition, while Ho2CoMnO6 and Ho2NiMnO6 exhibits ferromagnetic to paramagnetic phase transitions. The magnetic entropy change (−ΔSM), which are found to be 10.2, 3.4 and 4.7 J/kgK at ΔH = 7 T for Ho2FeMnO6, Ho2CoMnO6, and Ho2NiMnO6 samples, respectively. The magnetic and magnetocaloric characteristics of double perovskite compounds are well explained based on structural and electronic properties. For future applications, our quantitative method provides a guideline for magnetic property engineering of double perovskite systems containing ferromagnetic 3d transition metals.

Computing Curie temperature of two-dimensional ferromagnets in the presence of exchange anisotropy

We compare three first-principles methods of calculating the Curie temperature in two-dimensional (2D) ferromagnetic materials (FM), modeled using the Heisenberg model, and propose a simple formula for estimating the Curie temperature with high accuracy that works for all common 2D lattice types. First, we study the effect of exchange anisotropy on the Curie temperature calculated using the Monte Carlo (MC), the Green's function, and the renormalized spin-wave (RNSW) methods. We find that the Green's function method overestimates the Curie temperature in high-anisotropy regimes compared to the MC method, whereas the RNSW method underestimates the Curie temperature compared to the MC and the Green's function methods. Next, we propose a closed-form formula for calculating the Curie temperature of 2D FMs, which provides an estimate of the Curie temperature that is greatly improved over the mean-field expression for magnetic material screening. We apply the closed-form formula to predict the Curie temperature 2D magnets screened from the C2DB database and discover several high Curie temperature FMs, with ${\mathrm{Fe}}_{2}{\mathrm{F}}_{2}$ and ${\mathrm{MoI}}_{2}$ emerging as the most promising 2D ferromagnets. Finally, by comparing to experimental results for ${\mathrm{CrI}}_{3}$, ${\mathrm{CrCl}}_{3}$, and ${\mathrm{CrBr}}_{3}$, we conclude that for small effective anisotropies, the Green's-function-based equations are preferable, while for larger anisotropies, MC-based results are more predictive.

Ferroelectric and Room-Temperature Ferromagnetic Semiconductors in the 2D MIMIIGe2X6 Family: First-Principles and Machine Learning Investigations.

Inspired by experimentally discovering ferromagnetism and ferroelectricity in two-dimensional (2D) CrGeTe3 and CuInP2S6 with similar geometric structures, respectively, we systematically investigated ferroic properties in a large family of 2D MIMIIGe2X6 (MI and MII = metal elements, X = S/Se/Te) by combining high-throughput first-principles calculations and the machine learning method. We identified 12 stable 2D multiferroics containing simultaneously ferromagnetic (FM) and ferroelectric (FE) properties and 35 2D ferromagnets without FE polarization. Particularly, the predicted FM Curie temperatures (TC) of eight 2D FM+FE semiconductors are close to or above room temperature. The ferroelectricity originates from the spontaneous geometric symmetry breaking induced by the unexpected shift of Ge-Ge atomic pairs and the emergence of Ge lone pair electrons, which also strengthens the p-d orbital hybridization between X atoms and metal atoms, leading to enhanced super-super-exchange interactions and raising the FM TC. Our findings not only enrich the family of 2D ferroic materials and present room-temperature FM semiconductors but also disclose the mechanism of the emerging ferroelectricity and enhanced ferromagnetism, which sheds light on the realization of high temperature multiferroics as well as FM semiconductors.

Strain‐Induced Magnetism in MSi2N4 (M = V, Cr): A First‐Principles Study

AbstractThe discovery of ferromagnetic 2D van der Waals (vdW) materials has opened up opportunities for exploring and harnessing magnetism in low‐dimensional limit, and developing innovative paper‐like spintronic devices. Stress engineering demonstrates unique advantages in regulating material properties, especially for 2D materials. With the aid of first‐principles calculations, the effectiveness of strain engineering for modulating the electronic and magnetic properties of MSi2N4 (M = V, Cr) monolayers from a new family of layer‐structured 2D vdW materials is explored. Their magnetic configurations are effectively tuned via the uniaxial and biaxial tensile strain of in‐plane lattice vectors. VSi2N4 exhibits half‐metallic ferromagnetic character, whereas CrSi2N4 is prone to be antiferromagnetic (AFM), which is rationalized from the competition between the direct metal–metal AFM interaction and the 900 ferromagnetic superexchange. More intriguingly, VSi2N4 demonstrates a high magnetic ordering temperature of 285 K, which is close to room temperature. The predicted appealing properties of VSi2N4 under strain await experimental confirmation.

Ultra-long spin relaxation in two-dimensional ferromagnet Cr2Ge2Te6 flake

By utilizing time-resolved magneto-optical Kerr effect technique, we investigated both demagnetization and remagnetization processes of a typical two-dimensional (2D) van der Waals (vdW) ferromagnetic (FM) semiconductor Cr2Ge2Te6 (CGT) and two comparative Te-based crystals, i.e. three-dimensional (3D) FM metal Cr3Te4 (CT) and 2D vdW FM metal Fe3GeTe2 (FGT). Different from 3D CT showing one-step laser-induced demagnetization, two-steps demagnetization behavior is dominated in 2D vdW FGT and CGT, which indicates that the 2D vdW magnets have relatively weaker electron-spin coupling. Of particular interest, we discovered an ultra-long remagnetization behavior in the 2D vdW CGT flake. The laser pumping induced spin demagnetization would not recover even in the time scale of 3500 ps (experimental setup limitation). To the best of our knowledge, this is the longest remagnetization process observed so far and it might be one of the particular characterizations of 2D vdW ferromagnet semiconductors. Based on the modified three temperature model, the key roles played by the crystal dimensionality and thermal diffusion anisotropy were revealed in the spin dynamics of 2D vdW magnets. Our findings would not only open the door to the thoroughly understanding of the origin of the magnetism in 2D FM materials but also undoubtedly find suitable applications in spintronics and magnonics devices.

Ferromagnetism in 2D Vanadium Diselenide.

Two-dimensional (2D) van der Waals ferromagnets carry the promise of ultimately miniature spintronics and information storage devices. Among the 2D ferromagnets, all inherit the magnetic ordering from their bulk ancestors. Here we report a 2D ferromagnetic semiconductor at room temperature, 2H-phase vanadium diselenide (VSe2), which shows ferromagnetic at the 2D form only. This 2D ferromagnetic semiconductor manifests an enhanced magnetic ordering owing to structural anisotropy at the 2D limit.

Ac Susceptometry of 2D van der Waals Magnets Enabled by the Coherent Control of Quantum Sensors

Precision magnetometry is fundamental to the development of novel magnetic materials and devices. Recently, the nitrogen-vacancy (NV) center in diamond has emerged as a promising probe for static magnetism in two-dimensional (2D) van der Waals materials, capable of quantitative imaging with nanoscale spatial resolution. However, the dynamic character of magnetism, crucial for understanding the magnetic phase transition and achieving technological applications, has rarely been experimentally accessible in single 2D crystals. Here, we coherently control the NV center’s spin precession to achieve ultrasensitive, quantitative ac susceptometry of a 2D ferromagnet. Combining dc hysteresis with ac susceptibility measurements varying temperature, field, and frequency, we illuminate the formation, mobility, and consolidation of magnetic domain walls in few-layer CrBr3. We show that domain wall mobility is enhanced in ultrathin CrBr3, with minimal decrease for excitation frequencies exceeding hundreds of kilohertz, and is influenced by the domain morphology and local pinning of the flake. Our technique extends NV magnetometry to the multifunctional ac and dc magnetic characterization of wide-ranging spintronic materials at the nanoscale.10 MoreReceived 17 May 2021Accepted 23 August 2021DOI:https://doi.org/10.1103/PRXQuantum.2.030352Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum sensingPhysical Systems2-dimensional systemsNitrogen vacancy centers in diamondTechniquesAC susceptibility measurementsCondensed Matter, Materials & Applied Physics

Theoretical study of extrinsic spin current generation in ferromagnets induced by anisotropic spin-flip scattering

The spin Hall effect (SHE) and the magnetic spin Hall effect (MSHE) are responsible for electrical spin current generation, which is a key concept of modern spintronics. We theoretically investigated the spin conductivity induced by spin-dependent s-d scattering in a ferromagnetic 3d alloy model by employing microscopic transport theory based on the Kubo formula. We derived a novel extrinsic mechanism that contributes to both the SHE and MSHE. This mechanism can be understood as the contribution from anisotropic (spatial-dependent) spin-flip scattering due to the combination of the orbital-dependent anisotropic shape of s-d hybridization and spin flipping, with the orbital shift caused by spin-orbit interaction with the d-orbitals. We also show that this mechanism is valid under crystal-field splitting among the d-orbitals in either the cubic or tetragonal symmetry.

Giant Topological Hall Effect in van der Waals Heterostructures of CrTe2/Bi2Te3.

Discoveries of the interfacial topological Hall effect (THE) provide an ideal platform for exploring the physics arising from the interplay between topology and magnetism. The interfacial topological Hall effect is closely related to the Dzyaloshinskii-Moriya interaction (DMI) at an interface and topological spin textures. However, it is difficult to achieve a sizable THE in heterostructures due to the stringent constraints on the constituents of THE heterostructures, such as strong spin-orbit coupling (SOC). Here, we report the observation of a giant THE signal of 1.39 μΩ·cm in the van der Waals heterostructures of CrTe2/Bi2Te3 fabricated by molecular beam epitaxy, a prototype of two-dimensional (2D) ferromagnet (FM)/topological insulator (TI). This large magnitude of THE is attributed to an optimized combination of 2D ferromagnetism in CrTe2, strong SOC in Bi2Te3, and an atomically sharp interface. Our work reveals CrTe2/Bi2Te3 as a convenient platform for achieving large interfacial THE in hybrid systems, which could be utilized to develop quantum science and high-density information storage devices.

Conversion of non-van der Waals VO2 solid to 2D ferromagnet by CO2-induced phase engineering

Two-dimensional (2D) ferromagnetic semiconductors that combine ferromagnetic order with desirable physical attributes could find transformative applications in atomically-thin magneto-optical and magnetoelectric devices. The mainstream strategies of creating magnetic moments in 2D materials are introducing charge carriers. Here we introduce a CO2-induced phase engineering strategy that achieves 2D ferromagnet via the transformation of non-van der Waals (non-vdW) VO2 solid to 2D defective structure with identified metastable phases. Our approach requires only exposing the structure to supercritical CO2 liquid that is able to first infiltrate and swell the material at the molecular scale, and then “plasticize” VO2 solid at the architectural scale to form a 2D defective network that ‘lock’ the metastable phases into a new topological structure, which would lead to a significantly enhanced ferromagnetic response. We attribute the phase transformation to the CO2 pressure-induced selective cleavage of covalent bond.

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.

Box scaling as a proxy of finite size correlations

The scaling of correlations as a function of size provides important hints to understand critical phenomena on a variety of systems. Its study in biological structures offers two challenges: usually they are not of infinite size, and, in the majority of cases, dimensions can not be varied at will. Here we discuss how finite-size scaling can be approximated in an experimental system of fixed and relatively small extent, by computing correlations inside of a reduced field of view of various widths (we will refer to this procedure as “box-scaling”). A relation among the size of the field of view, and measured correlation length, is derived at, and away from, the critical regime. Numerical simulations of a neuronal network, as well as the ferromagnetic 2D Ising model, are used to verify such approximations. Numerical results support the validity of the heuristic approach, which should be useful to characterize relevant aspects of critical phenomena in biological systems.

Monte Carlo simulation study of the influence of defects on two-dimensional ferromagnetic order

We study the magnetic properties of two-dimensional (2D) ferromagnetic lattices with random disorders or ordered antidots through Monte Carlo simulations of the anisotropic Heisenberg model. We find that the Curie temperature (Tc) decreases linearly with the concentrations of the random vacancy of the magnetic moments (ρv) or the broken exchange interaction (ρb), and only the disorders with a concentration up to 30% can lead to a dropping of Tc by about half. The systems with a random broken exchange interaction have a much lower Tc than those with random vacancies at the same effective ρb, which suggests that the more randomness of the disorders in 2D ferromagnets induces a larger impact on Tc. When the magnetic anisotropy and exchange interaction become extremely small, Tc of the pristine lattice becomes much lower and the random disorders have a greater influence on Tc. The disorders also have an impact on the behavior of the correlation function. The correlation length decreases slower with temperature (T) just above Tc at a higher ρb, and its behavior is consistent with the disorder effect on the variation of heat capacity with T around Tc. For ordered antidot lattices, Tc can maintain a rather large value at a significantly low concentration of the remaining sites for systems with more than two chains of sites in one supercell.

Graphene-mediated ferromagnetic coupling in the nickel nano-islands/graphene hybrid

Nanoscale magnetic structures are fundamental to the design and fabrication of spintronic devices and have exhibited tremendous potential superior to the conventional semiconductor devices. However, most of the magnetic moments in nanostructures are unstable due to size effect, and the possible solution based on exchange coupling between nanomagnetism is still not clear. Here, graphene-mediated exchange coupling between nanomagnets is demonstrated by depositing discrete superparamagnetic Ni nano-islands on single-crystal graphene. The heterostructure exhibits ideal two-dimensional (2D) ferromagnetism with clear hysteresis loops and Curie temperature up to 80 K. The intrinsic ferromagnetism in graphene and antiferromagnetic exchange coupling between graphene and Ni nano-islands are revealed by x-ray magnetic circular dichroism and density functional theory calculations. The artificial 2D ferromagnets constitute a platform to study the coupling mechanism between complex correlated electronic systems and magnetism on the nanoscale, and the results and concept provide insights into the realization of spin manipulation in quantum computing.

Thickness Dependent Magnetic Transition in Few Layer 1T Phase CrTe2.

Room temperature two-dimensional (2D) ferromagnetism is highly desired in practical spintronics applications. Recently, 1T phase CrTe2 (1T-CrTe2) nanosheets with 5 and thicker layers have been successfully synthesized, which all exhibit the properties of ferromagnetic (FM) metals with Curie temperatures around 305 K. However, whether the ferromagnetism therein can be maintained when continuously reducing the nanosheet's thickness to monolayer limit remains unknown. Here, through first-principles calculations, we explore the evolution of magnetic properties of 1 to 6 layer CrTe2 nanosheets and several interesting points are found: First, unexpectedly, monolayer CrTe2 prefers a zigzag antiferromagnetic (AFM) state with its energy much lower than that of FM state. Second, in 2 to 4 layer CrTe2, both the intralayer and interlayer magnetic coupling are AFM. Last, when the number of layers is equal to or greater than 5, the intralayer and interlayer magnetic coupling become FM. Such highly thickness dependent magnetism provides a new perspective to control the magnetic properties of 2D materials.

Monolayer CrCl3 as an Ideal Test Bed for the Universality Classes of 2D Magnetism

The monolayer halides CrX$_3$ (X=Cl, Br, I) attract significant attention for realizing 2D magnets with genuine long-range order (LRO), challenging the Mermin-Wagner theorem. Here, we show that monolayer CrCl$_3$ has the unique benefit of exhibiting tunable magnetic anisotropy upon applying a compressive strain. This opens the possibility to use CrCl$_3$ for producing and studying both ferromagnetic and antiferromagnetic 2D Ising-type LRO as well as the Berezinskii-Kosterlitz-Thouless (BKT) regime of 2D magnetism with quasi-LRO. Using state-of-the-art density functional theory, we explain how realistic compressive strain could be used to tune the monolayer's magnetic properties so that it could exhibit any of these phases. Building on large-scale quantum Monte Carlo simulations, we compute the phase diagram of strained CrCl$_3$, as well as the magnon spectrum with spin-wave theory. Our results highlight the eminent suitability of monolayer CrCl$_3$ to achieve very high BKT transition temperatures, around 50 K, due to their singular dependence on the weak easy-plane anisotropy of the material.

Two-dimensional ferromagnetism in Eu-intercalated few-layer graphene

2D magnets hold high promise for unconventional quantum phases, exploration of fundamental physics of magnetism, ultra-compact devices based on van der Waals heterostructures. Graphene is a constant inspiration for design of 2D materials; stoichiometric graphene-based compounds with magnetic atoms give rise to 2D ferromagnets. Unfortunately, addition of magnetic species induces high chemical reactivity. Here, we encapsulate a monolayer Eu lattice between graphene monolayers to form a chemically inert van der Waals 2D ferromagnet. The material is produced by Eu intercalation into few-layer graphene, as confirmed by a combination of diffraction and microscopy techniques. The emergence of ferromagnetism is witnessed by magnetization and electron transport measurements. The effective Curie temperature is controlled by weak magnetic fields manifesting the 2D nature of the magnetic state. The results expand the family of 2D magnets and establish a versatile platform for synthesis of its new members by metal intercalation.