Ferromagnetic Monolayer Research Articles

Two-dimensional ferromagnetic semiconductors of rare-earth monolayer GdX2 (X = Cl, Br, I) with large perpendicular magnetic anisotropy and high Curie temperature

Two-dimensional (2D) ferromagnetic semiconductors are considered to be one of the most promising candidates for spintronics. Nonetheless, the 2D ferromagnetic semiconductors with large perpendicular magnetic anisotropy and high Curie temperature (Tc) are rarely reported. Here, through first-principles calculations, we predict a series of ferromagnetic semiconductors monolayer GdX2 (X = Cl, Br, I) by utilizing the rare-earth Gd with large spin-orbit coupling effect. Monte Carlo simulations manifest that the monolayer GdX2 show high Tc beyond 220 K, showing the promising applications in spintronics devices. Most interesting, three monolayer GdX2 show different magnetic anisotropy, among which the easy axis of monolayer GdCl2 is along the [001] direction, while monolayer GdBr2 and GdI2 possess in-plane magnetic anisotropy. Based on the analysis of the contributions from different atomic orbitals to magnetic anisotropy energy (MAE), it is found that the competition between the contributions of Gd-p orbitals, Gd-d orbitals, and p orbitals of halogen atoms to MAE transfers easy axis from out-of-plane to in-plane from Cl to I. The lattice mismatch between 2D materials and substrate definitely affects the properties of 2D materials, −5 % ∼ +5 % strain have been applied on the monolayer GdX2. The MAE of monolayer GdCl2 can increase from 111 to 190 μeV/f.u. under −5 % compressive strain. Besides, the easy axis of monolayer GdBr2 transfers from in-plane to out-of-plane over −3 % compressive strain. Furthermore, all the monolayer GdX2 have excellent thermal and dynamic stabilities. The present study is useful to design functional 2D materials with large perpendicular magnetic anisotropy and high Tc for the next generation spintronics.

Layer dependent magnetism and topology in monolayer and bilayers ReX 3 (X = Br, I)

The realization of robust intrinsic ferromagnetism in two-dimensional materials with the possibility to support topologically non-trivial states has provided the fertile ground for novel physics and next-generation spintronics and quantum computing applications. In this contribution, we investigated the formation of topological states and magnetism in monolayer and bilayer systems of ReX 3 (X = Br, I), with PBE, ACBN0 (self-consistent Hubbard-U), excluding/including van der Waals (vdW) corrections and/or spin–orbit coupling. Bulk ReX 3 (X = Br, I) is predicted to crystallize in space group (#148), similar to CrI3, with monolayer exfoliation energies that are comparable or less than that of graphite. The topological character of the monolayer and bilayer systems of ReX 3 (X = Br, I) is derived from anomalous Hall conductivity computations. Topologically non-trivial states in ReX 3 (X = Br, I) are absent in the Hubbard-U computations if vdW interactions are included, a prediction that is attributed to the large Hubbard-U difference between the chemical constituents, ΔU ∼ 1.5–1.6 eV, and a significant ∼2.0%–3.6% compressive in-plane strain introduced by vdW interactions. In contrast to the fragile and likely absent topological states in ReX 3 (X = Br, I), magnetic properties are robust and independent of the level of theory: ferromagnetic monolayers are coupled antiferromagnetically to bilayers, with an energy separation between ferromagnetic and antiferromagnetic bilayer spin configurations that could be as low as 0.02 meV/Re (f = 4.8 GHz), well within the microwave range. This suggests that layer dependent magnetism in ReX 3 (X = Br, I) may support a microwave controllable magnetic qubit, consisting of a superposition of antiferromagnetic and ferromagnetic bilayer states.

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.

Modeling the half-metallicity of the CrN/GaN (1 1 1) heterostructure

Spin-polarized total-energy calculations are employed to investigate the structural, electronic and magnetic properties of the CrN epitaxial growth on the GaN (1 1 1) surface. Cr adsorption and incorporation were considered as initial stages. The adsorption takes place at high symmetry sites, and in the incorporation, Cr replaces Ga. The structures stability is studied with the surface formation energy (SFE) formalism. Results show that in the range from Ga-rich to Ga-poor conditions, and from Cr-rich to Cr-intermediate conditions, the ferromagnetic CrN monolayer is the most stable structure. The interface shows half-metal characteristics, where the minority spins displays a large band-gap. When depositing extra CrN layers, the structure displays the CrN rock-salt phase as the most stable configuration. The epitaxial growth of 2 and 3 CrN layers depicts antiferromagnetic (AFM) characteristics; moreover the interface remains with half-metal properties and Dirac-cone-like features. However, the band-gap of the minority spins is reduced, therefore, it is expected that a large number of CrN bilayers could drive the system to a half-metal to metallic transition. Calculations establish a strategy to generate 100% polarized carriers injection to GaN, which poses this CrN/GaN (1 1 1)||(1 1 1) heterostructure as a component to construct spintronic devices.

Strain-Controllable High Curie Temperature and Magnetic Crystal Anisotropy in a 2D Ferromagnetic Semiconductive FeI3 Monolayer

Strain-Controllable High Curie Temperature and Magnetic Crystal Anisotropy in a 2D Ferromagnetic Semiconductive FeI₃ Monolayer

Intrinsic piezoelectric ferromagnetism with large out-of-plane piezoelectric response in Janus monolayer CrBr1.5I1.5

A two-dimensional (2D) material system with both piezoelectric and ferromagnetic (FM) orders, referred to as a 2D piezoelectric ferromagnetism (PFM), may open up unprecedented opportunities for intriguing physics. Inspired by experimentally synthesized Janus monolayer MoSSe from MoS2, in this work, the Janus monolayer CrBr1.5I1.5 with dynamic, mechanical, and thermal stabilities is predicted, which is constructed from synthesized ferromagnetic CrI3 monolayer by replacing the top I atomic layer with Br atoms. Calculated results show that monolayer CrBr1.5I1.5 is an intrinsic FM half semiconductor with valence and conduction bands being fully spin-polarized in the same spin direction. Furthermore, monolayer CrBr1.5I1.5 possesses a sizable magnetic anisotropy energy. By symmetry analysis, it is found that both in-plane and out-of-plane piezoelectric polarizations can be induced by a uniaxial strain in the basal plane. The calculated in-plane d22 value of 0.557 pm/V is small. However, more excitingly, the out-of-plane d31 is as high as 1.138 pm/V, which is obviously higher compared with that of other 2D known materials. The strong out-of-plane piezoelectricity is highly desirable for ultrathin piezoelectric devices. Moreover, strain engineering is used to tune piezoelectricity of monolayer CrBr1.5I1.5. It is found that compressive strain can improve d22 and tensile strain can enhance d31. A FM order to antiferromagnetic order phase transition can be induced by compressive strain, and the critical point is about 0.95 strain. That is to say that 2D piezoelectric antiferromagnetism can be achieved by compressive strain, and the corresponding d22 and d31 are 0.677 and 0.999 pm/V at 0.94 strain, respectively. It is also found that magnetic order has important effects on piezoelectricity of monolayer CrBr1.5I1.5. Finally, similar to CrBr1.5I1.5, the PFM can also be realized in the monolayers CrF1.5I1.5 and CrCl1.5I1.5. Amazingly, their d31 can reach up to 2.578 and 1.804 pm/V for monolayers CrF1.5I1.5 and CrCl1.5I1.5. Our paper proposes a realistic way to achieve PFM with large d31, making these systems very promising for multifunctional semiconductor spintronic applications.

High-temperature ferromagnetism in monolayers MnGaX3 (X = Te, Se)

Two-dimensional (2D) ferromagnetic monolayers are highly desirable in the spintronic field owing to their atomic-thickness and controllable spin degree of freedom. We investigated the 2D transition-metal trichalcogenides MnGaX3 (X = Te, Se, S) as the promising candidate monolayer ferromagnets via first-principles calculations. Our calculations show that monolayer MnGaTe3 and MnGaSe3 are ferromagnetic (FM) metal and half-metal with large magnetic moments and sizeable magneto-crystal anisotropy energy. Both the monolayers are mechanical and dynamical stable, and can be exfoliated from the corresponding layered crystals. More importantly, Monte Carlo simulations predict high Curie temperature for MnGaTe3 (720 K) and MnGaSe3 (910 K). Plus, their ferromagnetic configurations become more stable under the increasing biaxial tensile strain from 0 to 5%. Metallic MnGaTe3 converts into half-metal under biaxial tensile strain, and the band gap of semiconducting spin-channel of half-metallic MnGaSe3 increases under the increasing strain. The distinct half-metallic and robust intrinsic ferromagnetism at high temperature render the two monolayers attractive in spintronics.

A first-principles study on the electronic property and magnetic anisotropy of ferromagnetic CrOF and CrOCl monolayers

Two-dimensional ferromagnetic materials with large perpendicular magnetic anisotropy (PMA) hold great potential in realizing low critical switching current, high thermal stability and high density nonvolatile storage in magnetic random-access memories. Our first-principles calculations reveal that CrOF and CrOCl monolayers (MLs) are two-dimensional (2D) ferromagnetic semiconductors with out-of-plane magnetic easy axis, and PMAs of CrOF and CrOCl MLs are mainly contributed by Cr atoms. The magnetic anisotropy of CrOF and CrOCl MLs can be controlled and enhanced by applying biaxial strain. Tensile strain can further enhance PMAs of CrOF and CrOCl MLs by 82.9% and 161.0% higher than those of unstrained systems, respectively. In addition, appropriate compressive strain can switch the magnetic easy axis of CrOF and CrOCl MLs from out-of-plane direction to in-plane direction. The semiconductor natures of CrOF and CrOCl MLs robust against biaxial strain, the band gaps of these systems under biaxial strain are in the range of 1.26 eV to 2.40 eV. By applying biaxial strain, the Curie temperatures of CrOF and CrOCl MLs increase up to 282 K and 163 K, respectively. These tunable properties suggest that CrOF and CrOCl MLs have great application potentials for magnetic data storage.

Magnetic Order and Symmetry in the 2D Semiconductor CrSBr

The advent of two-dimensional (2D) magnets offers unprecedented control over electrons and spins. A key factor in determining exchange coupling and magnetic order is symmetry. Here, we apply second harmonic generation (SHG) to probe a 2D magnetic semiconductor CrSBr. We find that monolayers are ferromagnetically ordered below 146 K, an observation enabled by the discovery of a large magnetic dipole SHG effect in the centrosymmetric structure. In multilayers, the ferromagnetic monolayers are coupled antiferromagnetically, and in contrast to other 2D magnets, the Néel temperature of CrSBr increases with decreasing layer number. We identify magnetic dipole and magnetic toroidal moments as order parameters of the ferromagnetic monolayer and antiferromagnetic bilayer, respectively. These findings establish CrSBr as an exciting 2D magnetic semiconductor and extend the SHG probe of magnetic symmetry to the monolayer limit, opening the door to exploring the applications of magnetic-electronic coupling and the magnetic toroidal moment.

CrI3/Y2CH2 Heterointerface-Induced Stable Half-Metallicity of Two-Dimensional CrI3 Monolayer Ferromagnets.

Two-dimensional (2D) CrI3 monolayer ferromagnets are key to the development of future miniature spintronic devices and modulating them into a half-metal will greatly expand the application scenarios of CrI3 in nanospintronics. Nevertheless, existing strategies to induce half-metallicity of a CrI3 monolayer remain experimentally challenging and have unstable issues. In this work, the introduction of a 2D electride [Y2C]2+·2e- as an auxiliary layer is shown to be an effective way to achieve the generation of stable half-metallicity in the CrI3 monolayer. When the fully hydrogenated Y2CH2 and ferromagnetic CrI3 monolayer combine to form a heterostructure, surprisingly the appropriate amount of charge injection (0.72 e) turns CrI3 into a half-metal. Hetero-interfacial half-metallicity in CrI3 is an intrinsic one and does not require any chemical functionalization or external physical modification. Therefore, it is advantageous for practical applications of CrI3 in miniature spintronic devices, such as magnetic tunnel junctions, spin valves or spin field-effect transistors. A new strategy of the stable CrI3/Y2CH2 heterostructure was successfully developed to induce the half-metallicity of 2D CrI3 ferromagnets, which is experimentally feasible and half-metallic stable enough. This work paves the way for the application of the CrI3 monolayer in half-metallic-based spintronics.

Intrinsic room-temperature ferromagnetic semiconductor InCrTe3 monolayers with large magnetic anisotropy and large piezoelectricity

A tremendous amount of research is currently focused on two-dimensional (2D) magnetic semiconductors because of their remarkable physical properties and diverse applications. However, their applications are highly limited by the low Curie temperature (TC). Based on first-principles calculations and Monte Carlo simulations, we demonstrate that 2D InCrTe3 monolayers are ideal candidates for next-generation spintronics applications. We show that 2D InCrTe3 monolayers have thermodynamical and mechanical stability. The magnetic couplings between Cr atoms are strong ferromagnetic (FM) interactions. 2D InCrTe3 monolayers have robust FM semiconducting behavior with a bandgap of ∼0.67 eV, adequate TC (higher than ∼300 K), and a giant out-of-plane magnetic anisotropy energy (MAE) of ∼1.4 meV/Cr. Moreover, the low symmetry C3v point group leads to large in-plane piezoelectric coefficients d11 (larger than 4.8 pm/V) and out-of-plane piezoelectric coefficients d31 (larger than 0.39 pm/V), which are higher than many Janus transition metal dichalcogenides. The theoretical predictions of high TC, large MAE, and piezoelectricity in 2D InCrTe3 monolayers suggest that they have great potential for applications in spintronics, nano-sized sensors, and electromechanics.

Two-Dimensional Cr2X3S3 (X = Br, I) Janus Semiconductor with Intrinsic Room-Temperature Magnetism.

The exploration of two-dimensional (2D) semiconductors with intrinsic room-temperature magnetism for use in nanoscale spintronic devices is of particular interest. Recently, the ferromagnetic CrX3 monolayer (X = Br, I) has received growing attention, but low critical temperature hinders its practical applications in spintronics. Here, using first-principles calculations, we report 2D Cr2X3S3 (X = Br, I) Janus semiconductors with room-temperature magnetism by replacing one layer of halogon atoms with sulfur atoms in CrX3 monolayer. Our results demonstrate that Cr2Br3S3 and Cr2I3S3 Janus crystals are ferrimagnetic semiconductors, that maintain their magnetic order, with a direct bandgap of 1.19 and 0.61 eV and high critical temperature of 387 and 447 K, respectively. The residual unpaired p electrons on the S anions lead to a strong direct-exchange interaction between the Cr and S atoms. Moreover, their room-temperature magnetism is robust under biaxial strain, while the bandgap can be remarkably modulated with strain. The novel magnetic properties in 2D Cr2X3S3 Janus magnetic semiconductors give them promising applications in spintronics.

Single-Atom-Layer Catalysis in a MoS2 Monolayer Activated by Long-Range Ferromagnetism for the Hydrogen Evolution Reaction: Beyond Single-Atom Catalysis.

Single-atom-layer catalysts with fully activated basal-atoms will provide a solution to the low loading-density bottleneck of single-atom catalysts. Herein, we activate the majority of the basal sites of monolayer MoS2 , by doping Co ions to induce long-range ferromagnetic order. This strategy, as revealed by in situ synchrotron radiation microscopic infrared spectroscopy and electrochemical measurements, could activate more than 50 % of the originally inert basal-plane S atoms in the ferromagnetic monolayer for the hydrogen evolution reaction (HER). Consequently, on a single monolayer of ferromagnetic MoS2 measured by on-chip micro-cell, a current density of 10 mA cm-2 could be achieved at the overpotential of 137 mV, corresponding to a mass activity of 28, 571 Ag-1 , which is two orders of magnitude higher than the multilayer counterpart. Its exchange current density of 75 μA cm-2 also surpasses most other MoS2 -based catalysts. Experimental results and theoretical calculations show the activation of basal plane S atoms arises from an increase of electronic density around the Fermi level, promoting the H adsorption ability of basal-plane S atoms.

Single‐Atom‐Layer Catalysis in a MoS2 Monolayer Activated by Long‐Range Ferromagnetism for the Hydrogen Evolution Reaction: Beyond Single‐Atom Catalysis

AbstractSingle‐atom‐layer catalysts with fully activated basal‐atoms will provide a solution to the low loading‐density bottleneck of single‐atom catalysts. Herein, we activate the majority of the basal sites of monolayer MoS2, by doping Co ions to induce long‐range ferromagnetic order. This strategy, as revealed by in situ synchrotron radiation microscopic infrared spectroscopy and electrochemical measurements, could activate more than 50 % of the originally inert basal‐plane S atoms in the ferromagnetic monolayer for the hydrogen evolution reaction (HER). Consequently, on a single monolayer of ferromagnetic MoS2 measured by on‐chip micro‐cell, a current density of 10 mA cm−2 could be achieved at the overpotential of 137 mV, corresponding to a mass activity of 28, 571 Ag−1, which is two orders of magnitude higher than the multilayer counterpart. Its exchange current density of 75 μA cm−2 also surpasses most other MoS2‐based catalysts. Experimental results and theoretical calculations show the activation of basal plane S atoms arises from an increase of electronic density around the Fermi level, promoting the H adsorption ability of basal‐plane S atoms.

Intrinsic Polarization-Induced Enhanced Ferromagnetism and Self-Doped p-n Junctions in CrBr3/GaN van der Waals Heterostructures.

Two-dimensional (2D) ferromagnetic (FM) semiconductors with a high Curie temperature and tunable electronic properties are a long-term pursuing target for the development of high-performance spin-dependent optoelectronic devices. Herein, on the basis of density functional theory calculations, we report a new strategy to tune the Curie temperature and electronic structures of a ferromagnetic CrBr3 monolayer through the formation of CrBr3/GaN van der Waals heterostructures. Our calculated results demonstrate that the Curie temperature and band alignment of CrBr3/GaN heterostructures strongly depend on the thickness and polarization direction of the GaN layer. The combination of the CrBr3 monolayer with N-terminated GaN nanosheets leads to enhanced FM coupling via superexchange interactions between the Cr-t2g and Cr-eg orbitals, consequently resulting in a Curie temperature of CrBr3 of up to 67 K. Moreover, self-doped p-n junctions can be naturally formed in the heterostructures without additional modulation of external fields. The enhanced FM coupling and self-doping effect in the heterostructures are associated with the intrinsic polarization of the GaN layer that drives interfacial electron transfers from GaN to CrBr3. Therefore, this work not only offers an efficient scheme to boost the Curie temperature of the CrBr3 monolayer but also opens up a new route to realize nonvolatile van der Waals p-n junctions.

Magnon relaxation time in ferromagnetic Cr2Ge2Te6 monolayer governed by magnon-phonon interaction

Recently, bulk and multi-layer Cr2Ge2Te6 (CGT) has garnered much attention because of its interesting physical properties and promising technological aspects, including information storage and spintronic application. Here, using density functional theory calculations, we investigate the spin-wave spectrum and magnon-phonon interaction (MPI) in the CGT monolayer. The strength of MPI and the magnon relaxation time are estimated by the broadening in the spin-wave spectrum. We find that the MPI in the CGT monolayer exhibits weak in-plane isotropy and a strong wave vector dependence. The broadening of optical magnons is constant near the center of the Brillouin zone, while that for acoustic magnons shows a quadratic wave-vector dependence. Moreover, this wave-vector dependence is independent of temperature. As the temperature increases from 5 K to 55 K, the magnon relaxation time decreases 4 times, revealing the importance of MPI in spin dynamics of the ferromagnetic CGT monolayer. Our work provides an in-depth understanding of the magnon-phonon interaction in the ferromagnetic Cr2Ge2Te6 monolayer.

Kagome quantum anomalous Hall effect with high Chern number and large band gap

Due to the potential applications in the low-power-consumption spintronic devices, the quantum anomalous Hall effect (QAHE) has attracted tremendous attention in past decades. However, up to now, QAHE was only observed experimentally in topological insulators with Chern numbers C= 1 and 2 at very low temperatures. Here, we propose three novel two-dimensional stable kagome ferromagnets Co3Pb3S2, Co3Pb3Se2and Co3Sn3Se2that can realize QAHE with high Chern number of |C|=3. Monolayers Co3Pb3S2, Co3Pb3Se2 and Co3Sn3Se2 possess the large band gap of 70, 77 and 63 meV with Curie temperature TC of 51, 42 and 46 K, respectively. By constructing a heterostructure Co3Sn3Se2/MoS2, its TC is enhanced to 60 K and the band gap keeps about 60 meV due to the tensile strain of 2% at the interface. For the bilayer compound Co6Sn5Se4, it becomes a half-metal, with a relatively flat plateau in its anomalous Hall conductivity corresponding to |C| = 3 near the Fermi level. Our results provide new topological nontrivial systems of kagome ferromagnetic monolayers and heterostructrues possessing QAHE with high Chern number |C| = 3 and large band gaps.

Epitaxial growth of large-grain-size ferromagnetic monolayer CrI3 for valley Zeeman splitting enhancement.

Two-dimensional (2D) magnetic CrI3 has received considerable research attention because of its intrinsic features, including insulation, Ising ferromagnetism, and stacking-order-dependent magnetism, as well as potential in spintronic applications. However, the current strategy for the production of ambient-unstable CrI3 thin layer is limited to mechanical exfoliation, which normally suffers from uncontrollable layer thickness, small size, and low yet unpredictable yield. Here, via a confined vapor epitaxy (CVE) method, we demonstrate the mass production of flower-like CrI3 monolayers on mica. Interestingly, we discovered the crucial role of K ions on the mica surface in determining the morphology of monolayer CrI3, reacting with precursors to form a KIx buffer layer. Meanwhile, the transport agent affects the thickness and size of the as-grown CrI3. Moreover, the Curie temperature of CrI3 is greatly affected by the interaction between CrI3 and the substrate. The monolayer CrI3 on mica could act as a magnetic substrate for valley Zeeman splitting enhancement of WSe2. We reckon our work represents a major advancement in the mass production of monolayer 2D CrI3 and anticipate that our growth strategy may be extended to other transition metal halides.

Magnon-magnon interaction and magnon relaxation time in a ferromagnetic Cr2Ge2Te6 monolayer

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Magnetism and stability of all primitive stacking patterns in bilayer chromium trihalides

Chromium trihalides, CrX3 (with X = Cl, Br, I), are a family of layered magnetic materials that can be easily exfoliated to provide ferromagnetic monolayers. When two layers are stacked together to form a bilayer the interlayer exchange coupling can be either ferromagnetic or antiferromagnetic depending on the stacking sequence. Here we combine crystallographic arguments based on the close-packing condition with first-principles simulations to enumerate all possible stacking patterns in CrX3 bilayers that preserve the spatial periodicity of each layer. We recover all configurations observed in bulk crystals and disclose stacking sequences with no bulk counterpart where the two layers have opposite chirality. Stacking sequences are ranked according to their relative stability and a preferential interlayer magnetic ordering is assigned to each of them. Simulations provide a consistent picture to frame all current experimental observations on bulk and exfoliated CrX3 crystals, with interesting implications for future measurements, including synthetic bilayers with non-standard stacking patterns.