Monolayer CrI3 Research Articles

Prediction of semiconducting ferromagnetic CrVI6 monolayer

In this work, the effects of V atoms on the atomic structures, electronic and magnetic properties of monolayer CrI3 have been investigated and clarified using first-principles calculations and Monte Carlo simulations. We find that the most stable configuration is the evenly mixed CrVI6 for CrV8x I24 by calculating the formation energy. Through substituting one Cr atom with a V atom, the evenly mixed CrVI exhibits a ferromagnetic (FM) semiconducting character with a direct band gap energy of 0.779 eV, a large magnetic anisotropy energy (MAE) of 1.959 meV per magnetic atom and a high Curie temperature up to 88 K. Our results indicate that CrVI6 monolayer may be a promising material for spintronics applications and substituting Cr with transition metal V atom is a viable strategy in optimizing the performance of CrI3.

Curved Magnetism in CrI_{3}.

Curved magnets attract considerable interest for their unusually rich phase diagram, often encompassing exotic (e.g., topological or chiral) spin states. Micromagnetic simulations are playing a central role in the theoretical understanding of such phenomena; their predictive power, however, rests on the availability of reliable model parameters to describe a given material or nanostructure. Here we demonstrate how noncollinear-spin polarized density-functional theory can be used to determine the flexomagnetic coupling coefficients in real systems. By focusing on monolayer CrI_{3}, we find a crossover as a function of curvature between a magnetization normal to the surface to a cycloidal state, which we rationalize in terms of effective anisotropy and Dzyaloshinskii-Moriya contributions to the magnetic energy. Our results reveal an unexpectedly large impact of spin-orbit interactions on the curvature-induced anisotropy, which we discuss in the context of existing phenomenological models.

Dynamical mean-field theory study of a ferromagnetic CrI3 monolayer

We have employed one of the well-known many-body techniques, density functional theory plus dynamical mean-field theory (DFT + DMFT), to investigate the electronic structure of ferromagnetic monolayer CrI3 as a function of temperature and hole-doping concentration. The computed magnetic susceptibility follows the Curie's law, indicating that the ferromagnetism of monolayer CrI3 originates from localized magnetic moments of Cr atoms rather than Stoner-type itinerant ones. The DFT + DMFT calculations show a different coherent temperature for each spin component, demonstrating apparent strong spin-dependent electronic correlation effects in monolayer CrI3. Furthermore, we have explored the doping-dependent electronic structure of monolayer CrI3 and found that its electronic and magnetic properties are easily tunable by the hole-doping.

Tuning electronic properties and ferromagnetism of CrI3 monolayers with doped transition-metal atoms

Chromium triiodide (CrI3) monolayers have attracted much attention among the first two-dimensional materials discovered experimentally in both electronics and spintronics due to their potential applications. By means of density functional theory, we perform investigations of the electronic structures and magnetic properties of CrI3 monolayer doped with 3 d transition-metal (TM) atoms, which is also called CrXI6 monolayer with X changed from Sc to Fe. It is shown that the electron properties of the CrXI6 system can be tuned from semiconductor to metal/half-metal, which depend on the types of TM atoms. In addition, the CrXI6 system improves ferromagnetic (FM) stabilities, enhancement of magnetic moments, and FM-to-antiferromagnetic transition. These findings enrich the potential application perspectives of CrI3 monolayer in spintronics.

Tailoring high-frequency magnonics in monolayer chromium trihalides

Monolayer chromium-trihalides, the archetypal two-dimensional (2D) magnetic materials, are readily suggested as a promising platform for high-frequency magnonics. Here we detail the spin-wave properties of monolayer CrBr3 and CrI3, using spin-dynamics simulations parametrized from the first principles. We reveal that spin-wave dispersion can be tuned in a broad range of frequencies by strain, paving the way towards flexo-magnonic applications. We further show that ever-present halide vacancies in these monolayers host sufficiently strong Dzyaloshinskii-Moriya interaction to scatter spin-waves, which promotes design of spin-wave guides by defect engineering. Finally we discuss the spectra of spin-waves propagating across a moiré-periodic modulation of magnetic parameters in a van der Waals heterobilayer, and show that the nanoscale moiré periodicities in such samples are ideal for realization of a magnonic crystal in the terahertz frequency range. Recalling the additional tunability of magnetic 2D materials by electronic gating, our results situate these systems among the front-runners for prospective high-frequency magnonic applications.

CrI3 monolayer: magnetic and electronic behaviors under biaxial strain and external electric field—a first principle study

CrI3 monolayer: magnetic and electronic behaviors under biaxial strain and external electric field—a first principle study

Element-dependent unique properties of Janus Cr2I3X3 (X = F, Cl, Br) monolayer: Insight from first-principles calculations

Two-dimensional (2D) intrinsic ferromagnetic (FM) semiconductors are important to develop low-dimensional spintronic devices. The recent discovery of FM CrI3 monolayer provides ample opportunities for the spintronic applications of atomically thin magnets. Breaking the structural inversion symmetry of CrI3 would result in some extraordinary physical characteristics. Through first-principles calculations, here we systematically studied the structural, electronic, magnetic and mechanical properties of 2D Janus Cr2I3X3 (X = F, Cl, Br) monolayers, and compared them with those of the pure CrX3 monolayers. The results show that the properties of the Janus Cr2I3X3 (X = F, Cl, Br) monolayers are not within the range between CrI3 and CrX3, but go beyond this range. With the decrease of the atomic number of X elements, from Br to F, the properties of Cr2I3X3 increasingly deviate from that of CrI3 or CrX3. All the element-dependent properties of Cr2I3X3 are closely related with the charge redistribution and structural distortion, which is originated from the substitution of Iodine atoms with X atoms (atomic layer substitution) in the Janus structures. Our study enriches the diversity of Janus 2D materials, and provides the theoretical guidance for the application in spintronic devices.

Atomically Thin 2D van der Waals Magnetic Materials: Fabrications, Structure, Magnetic Properties and Applications

Two-dimensional (2D) van der Waals (vdW) magnetic materials are considered to be ideal candidates for the fabrication of spintronic devices because of their low dimensionality, allowing the quantization of electronic states and more degrees of freedom for device modulation. With the discovery of few-layer Cr2Ge2Te6 and monolayer CrI3 ferromagnets, the magnetism of 2D vdW materials is becoming a research focus in the fields of material science and physics. In theory, taking the Heisenberg model with finite-range exchange interactions as an example, low dimensionality and ferromagnetism are in competition. In other words, it is difficult for 2D materials to maintain their magnetism. However, the introduction of anisotropy in 2D magnetic materials enables the realization of long-range ferromagnetic order in atomically layered materials, which may offer new effective means for the design of 2D ferromagnets with high Curie temperature. Herein, current advances in the field of 2D vdW magnetic crystals, as well as intrinsic and induced ferromagnetism or antiferromagnetism, physical properties, device fabrication, and potential applications, are briefly summarized and discussed.

A combined first principles study of the structural, magnetic, and phonon properties of monolayer CrI3.

The first magnetic 2D material discovered, monolayer (ML) CrI3, is particularly fascinating due to its ground state ferromagnetism. However, because ML materials are difficult to probe experimentally, much remains unresolved about ML CrI3's structural, electronic, and magnetic properties. Here, we leverage Density Functional Theory (DFT) and high-accuracy Diffusion Monte Carlo (DMC) simulations to predict lattice parameters, magnetic moments, and spin-phonon and spin-lattice coupling of ML CrI3. We exploit a recently developed surrogate Hessian DMC line search technique to determine CrI3's ML geometry with DMC accuracy, yielding lattice parameters in good agreement with recently published STM measurements-an accomplishment given the ∼10% variability in previous DFT-derived estimates depending upon the functional. Strikingly, we find that previous DFT predictions of ML CrI3's magnetic spin moments are correct on average across a unit cell but miss critical local spatial fluctuations in the spin density revealed by more accurate DMC. DMC predicts that magnetic moments in ML CrI3 are 3.62 μB per chromium and -0.145 μB per iodine, both larger than previous DFT predictions. The large disparate moments together with the large spin-orbit coupling of CrI3's I-p orbital suggest a ligand superexchange-dominated magnetic anisotropy in ML CrI3, corroborating recent observations of magnons in its 2D limit. We also find that ML CrI3 exhibits a substantial spin-phonon coupling of ∼3.32 cm-1. Our work, thus, establishes many of ML CrI3's key properties, while also continuing to demonstrate the pivotal role that DMC can assume in the study of magnetic and other 2D materials.

First-principles study of strain-tunable charge carrier transport properties and optical properties of CrI<sub>3</sub> monolayer

Because the single-layer CrI<sub>3</sub> is a half semiconductor with indirect band gap and magnetic anisotropy, it has received much attention in the spintronic, magneto-electronic and magnetic storage applications. However, the knowledge of the dependence of carrier mobility and optical property on strain is still rather limited. The uniaxial and biaxial strain dependence of electronic, transport, optical and magnetic properties of single-layer CrI<sub>3</sub> are systematically investigated by using first-principles calculations, and the results are compared with experimental results. The electronic structures under different strains are first calculated by using the accurate HSE06 functional, then the carrier mobility is estimated by the deformation potential theory and the dielectric function is obtained to estimate the optical absorption especially in the visible light range. Finally, the magnetic anisotropy energy used to estimate the magneto-electronic properties is studied by the Perdew-Bueke-Ernzerhof functional including the spin-orbit coupling. It is found that the ferromagnetic CrI<sub>3</sub> is an indirect and half semiconductor with band gap 2.024 eV,<inline-formula><tex-math id="M1">\begin{document}$ \Delta {\text{CBM}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20221019_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20221019_M1.png"/></alternatives></inline-formula>= 1.592 eV, <inline-formula><tex-math id="M2">\begin{document}$ \Delta {\text{VBM}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20221019_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20221019_M2.png"/></alternatives></inline-formula>= 0.238 eV and can be driven into AF-Néel antiferromagnetic phase by applying –6% to –8% (compressive) biaxial stain, exhibiting excellent agreement with the results from the literature. It is found that of single-layer CrI<sub>3</sub> has very low carrier mobility with a value within 10 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> due to the large effective mass and small in-plane stiffness can be remarkably increased by increasing biaxial compression strain attributed to the reduced effective mass. A high electron mobility 174 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> is obtained in the zigzag direction by applying a –8% biaxial strain reaching the level of monolayer MoS<sub>2</sub>. The calculated imaginary component of dielectric function along the <i>x </i>(<i>y</i>) direction having two peaks (I, II) in the visible light range is obviously different from that along the <i>z</i> direction, indicating that the single-layer CrI<sub>3</sub> has optical anisotropy, demonstrating the good agreement with results from the literature. It is found that the imaginary part of dielectric function shows that an obvious redshift and peak (I, II) values strongly increase with the increase of compressive strain (biaxial), showing good agreement with the calculated electronic structures and indicating that monolayer CrI<sub>3</sub> possesses high optical adsorption of visible light under a compressive biaxial strain. Furthermore, it is found that the magnetic anisotropy energy of monolayer CrI<sub>3</sub> mainly stemming from the orbital magnetic moment of Cr ions remarkably increases from 0.7365 to 1.08 meV/Cr with g compressive strain increasing. These results indicate that the optoelectronic property of single-layer CrI<sub>3</sub> can be greatly improved by applying biaxial compressive strain and the single-layer CrI<sub>3</sub> is a promising material for applications in microelectronic, optoelectronic and magnetic storage.

Comment on “Distinct spin–lattice and spin–phonon interactions in monolayer magnetic CrI” by L. Webster, L. Liang and J.-A. Yan, Phys. Chem. Chem. Phys., 2018, 20, 23546

We point out four mistakes in the symmetry classification of vibrational modes of the CrI3 monolayer at the Γ point in Webster et al.'s paper [PCCP, 2018, 20, 23546]. The discrepancy of Raman activity of A2g mode as well as the misidentification of two optical modes A1u and A2u have been clarified.

Stability, electronic and magnetic properties of the penta-CoAsSe monolayer: a first-principles and Monte Carlo study.

Using density functional theory (DFT) and ab initio molecular dynamics (AIMD) we predict the existence of a new 2D monolayer, namely Penta-CoAsSe with robust mechanical, thermal and dynamical stabilities. The electronic and magnetic properties of this monolayer are investigated within the generalized gradient approximation including the Hubbard interaction (U) on the localized d orbitals of Co. We show that this material is an antiferromagnetic (AFM) narrow-gap semiconductor and exhibits in-plane magnetic anisotropy energy with a sizable magnetocrystalline anisotropy (MCA) of -1.08 erg cm-2. Furthermore, this system is found to have a substantial intrinsic piezoelectric response with an out-of-plane coefficient d36 of 0.34 pm V-1, surpassing other previously reported Penta-2D piezoelectric materials. By combining our DFT calculations with the Monte Carlo simulations, we find that CoAsSe has a transition temperature four times higher than that of Penta-CoS2. The effects of biaxial strain and electron-electron correlation on the magnetic properties and electronic structure are also examined. These fascinating properties make the Penta-CoAsSe monolayer a promising candidate for a wide range of technological applications.

Light helicity detector based on 2D magnetic semiconductor CrI3

Two-dimensional magnetic semiconductors provide a platform for studying physical phenomena at atomically thin limit, and promise magneto-optoelectronic devices application. Here, we report light helicity detectors based on graphene-CrI3-graphene vdW heterostructures. We investigate the circularly polarized light excited current and reflective magnetic circular dichroism (RMCD) under various magnetic fields in both monolayer and multilayer CrI3 devices. The devices exhibit clear helicity-selective photoresponse behavior determined by the magnetic state of CrI3. We also find abnormal negative photocurrents at higher bias in both monolayer and multilayer CrI3. A possible explanation is proposed for this phenomenon. Our work reveals the interplay between magnetic and optoelectronic properties in CrI3 and paves the way to developing spin-optoelectronic devices.

Half-metallicity and Curie temperature enhancement of CrI3 through boron atoms adsorption

Monolayer chromium iodide (CrI 3 ) as a two-dimensional material with intrinsic ferromagnetism has been successfully prepared experimentally, expanding the applications of two-dimensional materials in the field of spintronics . In this paper, we have investigated the effect of boron atoms adsorption on the electronic structure, magnetic properties of monolayer CrI 3 through first-principles calculation based on density functional theory . Our results show that each boron atom adsorption enhance the magnetic moment of CrI 3 by 0.25 μ B . When the adsorption concentration increases from 1B@CrI 3 to 2B@CrI 3 , a semiconductor to half-metallicity transition occurs; when the concentration increases up to 4B@CrI 3 , CrI 3 tunes to semiconductor. The Monte Carlo simulation based on the Ising model predicts the Curie temperature are 81.5 K, 117.7 K, 129.4 K and 143.9 K for 1B@CrI 3 , 2B@CrI 3 , 3B@CrI 3 and 4B@CrI 3 , much higher than that of pristine CrI 3 . Our calculation results are theoretically instructive for the application of CrI 3 in spintronic devices. • The electronic structures and magnetic properties of boron adsorption CrI 3 have been investigated. • The semiconductor-half metallicity-semiconductor transition of adsorption systems depended on boron concentration. • The Curie temperature of CrI 3 can be largely enhanced through boron atoms adsorption. Firstly, we calculated the band structure of the 2 × 2 × 1 supercell CrI 3 (shown in Fig. 1(c)), and the results show that the monolayer CrI 3 is an indirect bandgap semiconductor with a bandgap of 1.16 eV. The intrinsic magnetic moment is 24 μ B for CrI 3 supercell, it means that the magnetic moment is 6 μ B for CrI 3 unit cell. The magnetic moments mainly come from the contribution of Cr atoms, and the magnetic moment of each Cr atom is 3 μ B , which is consistent with the results of previous studies [26,35]. The properties of CrI 3 with different concentrations of boron atoms adsorption are shown in Table 1, each adsorbed B atom increases the magnetic moment average to each CrI 3 primitive cell by about 0.25 μ B . This indicates that the magnetic properties of CrI 3 can be effectively tuned by boron atoms adsorption. The Bader charge analysis shows that each B atom transfers about 0.13 electrons to CrI 3 . The magnetic moments of top position Cr ( M ІCr ) do not much change, while the magnetic moments for other Cr atoms ( M ⅡCr ) increased with adsorption concentration. Since the B atoms are adsorbed on the surface of the CrI 3 layer, the nearly equal electrons per B atom transfer from the B atoms to CrI 3 increased almost equal magnetic moment of Cr atoms, which may be the reason for the increased total magnetic moments of 0.25 μ B of the monolayer CrI 3 .

Environmental screening and ligand-field effects to magnetism in CrI3 monolayer

We study the microscopic origin of magnetism in suspended and dielectrically embedded CrI3 monolayer by down-folding minimal generalized Hubbard models from ab initio calculations using the constrained random phase approximation. These models are capable of describing the formation of localized magnetic moments in CrI3 and of reproducing electronic properties of direct ab initio calculations. Utilizing the magnet force theorem, we find a multi-orbital super-exchange mechanism as the origin of magnetism in CrI3 resulting from an interplay between ferro- and anti-ferromagnetic Cr-Cr d coupling channels, which is decisively affected by the ligand p orbitals. We show how environmental screening, such as resulting from encapsulation with hexagonal boron nitride, affects the Coulomb interaction in the film and how this controls its magnetic properties. Driven by a non-monotonic interplay between nearest and next-nearest neighbor exchange interactions we find the magnon dispersion and the Curie temperature to be non-trivially affected by the environmental screening.

First-principles study on the electronic properties and enhanced ferromagnetism of alkali metals adsorbed monolayer CrI3

As the first two-dimensional (2D) semiconductor material with intrinsic ferromagnetism, the application prospects of monolayer chromium triiodide (CrI3) materials have been of great interest in recent years. In this paper, the electronic and magnetic property of alkali metals, H, and O adsorbed monolayer CrI3 are investigated by first-principles calculations. The results show that adsorption of alkali metals as a way to introduce carriers can transform monolayer CrI3 from a semiconductor to a half-metal. The tunability of the magnetic anisotropy energy (MAE) of monolayer CrI3 was achieved by changing the orientation of the easy magnetization axis in the lower adsorption coverage range. Adsorption leads to a significant increase in MAE, with K being the most significant, and the MAE at 12.50% is 6.8 times higher than that without adsorption. The monolayer CrI3 remains as a semiconductor after adsorption of H and O. The adsorption has a significant effect on increasing the Curie temperature (TC) of monolayer CrI3, and the TC of H–CrI3 at 12.50% is estimated to be around 195 K. The results of multiple calculations indicate a stable improvement of the ferromagnetic stability of the system, which provides a direction for the development of magnetic material storage functions and applications in spintronic devices.

Apparent Ferromagnetism in Exfoliated Ultrathin Pyrite Sheets

Experimental evidence for ferromagnetic ordering in isotropic atomically thin two-dimensional crystals has been missing until a bilayer Cr2Ge2Te6, and a three-atom thick monolayer CrI3 are shown to retain ferromagnetic ordering at finite temperatures. Here, we demonstrate successful isolation of a non-van der Waals type ultra-thin nanosheet of FeS2 derived from naturally occurring pyrite mineral (FeS2) by means of liquid-phase exfoliation. Structural characterizations imply that (111) oriented sheets are predominant and is supported theoretically by means of density functional theory surface energy calculations. Spin-polarized density theory calculations further predicted that (111) oriented three-atom thick pyrite sheet has a stable ferromagnetic ground state different from its diamagnetic bulk counterpart. This theoretical finding is evaluated experimentally employing low temperature superconducting quantum interference device measurements and observed an anomalous ferromagnetic kind of behavior.

Defect stability and intriguing magnetic properties in Janus chromium trihalides monolayer

Monolayer CrI3 with robust ferromagnetism has been extensively studied. Its Janus structures could further enhance its applications in spintronic devices. Here we investigate the intrinsic defects and their induced magnetic properties in Janus structures. We constructed the point defects in Janus CrX1.5Y1.5 (X, Y = Cl, Br, I; X ≠ Y) monolayer, and studied the structural stability and migration barriers by first-principles calculations. The single vacancy with one halogen atom missing, is the most stable defect in Cr-rich condition, while the Cr vacancy is preferred to form in halogen-rich condition. Our results revealed that the surface defects not only enhance the intrinsic ferromagnetism of the Janus structures, but also increase the Curie temperatures effectively. The intriguing properties induced by the surface defects make the Janus chromium trihalides monolayer promising for multifunctional nanodevices.

Systematic Study of Monolayer to Trilayer CrI3: Stacking Sequence Dependence of Electronic Structure and Magnetism

In recent years, two-dimensional magnetic materials have attracted a lot of attention due to their unique magnetic properties, which can be manipulated by various degrees of freedom in a van der Waals stacked heterostructure, where different interlayer stackings affect the properties in different ways by exploiting weak interlayer interactions. An interesting example is CrI3, where the interlayer magnetic coupling can be determined by the stacking sequence. Here we present a systematic study of the magnetic properties of monolayer, bilayer, and trilayer CrI3 by using density functional theory and Monte Carlo simulations. The effects of different stacking sequences and the number of layers on the magnetic interaction in multilayer CrI3 have been critically analyzed. We have found that the antiferromagnetic interlayer coupling occurs with specific interlayer shifting, while intralayer exchange interactions increase with the number of layers. It is noteworthy that interlayer interactions are independent of the number of layers, whereas the magnetic ordering temperature rises with the increasing number of layers. Moreover, we present the Curie temperatures obtained from Monte Carlo simulations and adiabatic magnon spectra for all the stacking sequences considered here.

Remarkably improved Curie temperature for two-dimensional CrI3 by gas molecular adsorption: a DFT study

Possessing long-range FM order and tunable interlayer magnetic coupling, extensive investigations of monolayered ferromagnetic chromium triiodide (CrI3) have been developed. In this work, by means of first-principles simulation, we systematically calculated the electronic and magnetic characteristic of two-dimensional (2D) CrI3 adsorbed by gas molecules, CO, H2, H2O and N2, which can provide theoretical guidance to modulate the desired properties of this layered material. The results show that the adsorption of those gas molecules can significantly enhance FM couplings of monolayer CrI3. In particular, the N2 adsorption can remarkably improve the Curie temperatures of the pristine CrI3 monolayer by about 2.6 times. We also hope that our findings can provide more interest around the recently studied 2D FM semiconductors.