Intrinsic Ferromagnetic Semiconductors Research Articles

M3XSe4 (M = V, Cr; X = S, Te) monolayers: Intrinsic high-temperature ferromagnetic semiconductors and half metals

Creating low dimensional ferromagnetic (FM) semiconductors or half metals with strong FM orders is promising to meet the requirement for next-generation spintronics. However, most of the demonstrated FM semiconductors or half metals suffer from low Curie temperatures (TCs). Here, by first-principles calculations, we predict that the two-dimensional (2D) M3XSe4 (M = V, Cr; X = S, Te) monolayers are a type of intrinsic 2D ferromagnets with thermodynamical stability. Our results show that V3XSe4 (X = S, Te) monolayers are FM semiconductors with indirect bandgaps of 0.60 and 0.50 eV, respectively. Particularly, both structures are revealed to have high TCs of 387 and 770 K and suppress the application limit of room-temperature. In addition, Cr3XSe4 (X = S, Te) monolayers are FM half metals with 100% spin-polarized currents. Moreover, the electronic and magnetic properties of these M3XSe4 monolayers can be modulated by biaxial strains. V3TeSe4 monolayer can be tuned to be room temperature direct bandgap semiconductor under biaxial 1% tensile strain, and TC of V3SSe4 can be largely enhanced under compressive strains. Our results suggest that M3XSe4 monolayers are promising candidates for spintronic devices.

Two-dimensional Cr2Cl3S3 Janus magnetic semiconductor with large magnetic exchange interaction and high-TC

Abstract The two-dimensional (2D) Janus monolayers are promising in spintronic device application due to their enhanced magnetic couplings and Curie temperatures. Van der Waals CrCl3 monolayer has been experimentally proved to have an in-plane magnetic easy axis and a low Curie temperature of 17 K, which will limit its application in spintronic devices. In this work, we propose a new Janus monolayer Cr2Cl3S3 based on the first principles calculations. The phonon dispersion and elastic constants confirm that Janus monolayer Cr2Cl3S3 is dynamically and mechanically stable. Our Monte Carlo simulation results based on magnetic exchange constants reveal that Janus monolayer Cr2Cl3S3 is an intrinsic ferromagnetic semiconductor with T C of 180 K, which is much higher than that of CrCl3 due to the enhanced ferromagnetic coupling caused by S substitution. Moreover, the magnetic easy axis of Janus Cr2Cl3S3 can be tuned to the perpendicular direction with a large magnetic anisotropy energy (MAE) of 142 μeV/Cr. Furthermore, the effect of biaxial strain on the magnetic property of Janus monolayer Cr2Cl3S3 is evaluated. It is found that the Curie temperature is more robust under tensile strain. This work indicates that the Janus monolayer Cr2Cl3S3 presents increased Curie temperature and out-of-plane magnetic easy axis, suggesting greater application potential in 2D spintronic devices.

Unraveling the microscopic origin of out of plane magnetic anisotropy in VI3

Intrinsic two-dimensional (2D) ferromagnetic (FM) semiconductors have attracted extensive attentions for their potential applications in next-generation spintronics devices. In recent years, the van der Waals material VI3 has been experimentally found to be an intrinsic FM semiconductor. However, the electronic structure of the VI3 is not fully understood. To reveal why the VI3 is a ferromagnetic semiconductor with strong out-of-plane anisotropy, we systematically studied the electronic structure of the monolayer VI3. Our results confirm that the monolayer VI3 is a Mott insulator, and d2 electrons occupy ag and egπ+ orbitals. The half-metallic state is a metastable state with a total energy 0.7 eV higher than the ferromagnetic Mott insulating state. Furthermore, our study confirmed that the VI3 exhibits the out-of-plane magnetic anisotropy, which originates from d2 electrons occupying low-lying ag and egπ+ orbitals. Since the orbital angular momentum of the egπ+ state is not completely quenched, the VI3 has the out-of-plane anisotropy under interplay between the spin-orbit coupling and crystal field. Our study provides valuable guidance for the design of 2D magnetic materials with pronounced out-of-plane anisotropy.

Strain tunable optical and transport properties of intrinsic ferromagnetic semiconductor CrOCl monolayer

Based on density functional theory, the electronic structure, transport, and optical properties of monolayer CrOCl are studied theoretically. The electronic structure analysis shows that monolayer CrOCl is a ferromagnetic semiconductor with an indirect band gap of 2.45 eV, and the electronic properties of monolayer CrOCl can be changed significantly by applying external strain. When 12 % uniaxial tensile strain is applied along the a-axis, the band gap of monolayer CrOCl decreases to 1.94 eV, the hole mobility can reach 1520.11 cm2 V−1·s−1, and the mobility difference between holes and electrons increase to 1471.53 cm2 V−1·s−1. Furthermore, the applied strain, regardless of tensile or compressive, can cause the redshift of the optical absorption peak of monolayer CrOCl, thereby improving the absorption performance in the visible light region. In addition, it was found that the in-plane absorption of bilayer CrOCl increased to nearly twice as compared with the monolayer one. These results indicate that single and few layers of CrOCl have the potential application prospect in the field of optoelectronic devices.

First-principles study of bilayer hexagonal structure CrN2 nanosheets: A ferromagnetic semiconductor with high Curie temperature and tunable electronic properties

Two-dimensional magnetic materials have been increasingly studied and discussed in the field of spintronics due to their unique electronic properties, high spin polarizability, and a variety of magnetic properties. In this paper, we report a new two-dimensional bilayer hexagonal monolayer material bilayer hexagonal structure (BHS)-CrN2 by first-principles calculations. The BHS-CrN2 nanosheet is an intrinsic ferromagnetic semiconductor material, and the Curie temperature obtained by Monte Carlo simulation is 343 K. The absence of a significant imaginary frequency in the phonon spectrum indicates the dynamic stability of BHS-CrN2. After ab initio molecular dynamics simulation, the supercell of BHS-CrN2 remains a complete structure, indicating its thermal stability. The calculated elastic moduli satisfy the Born–Huang criterion, indicating that the BHS-CrN2 system has good mechanical stability. Interestingly, the compressive strain and O atom doping can transform the electronic structure of BHS-CrN2 from a semiconductor to a half-metal, and the Curie temperature of BHS-CrN2 can be further increased to 1059 K when a 5% tensile strain is applied. Furthermore, the BHS-CrN2 in the ferromagnetic state shows a significant in-plane magnetic anisotropy energy of 0.01 meV per Cr, and the CrP2 and CrAs2 show a large out-of-plane magnetic anisotropy energy of 0.207 and 0.988 meV per Cr, respectively. The results show that the intrinsic ferromagnetic semiconductor BHS-CrN2 has good stability, high Curie temperature, and tunable magnetic properties, which is a promising material for room-temperature spintronic devices.

Two-dimensional HfCr2N4 semiconductor with intrinsic room-temperature ferromagnetism and enhanced conductivity via electrostatic doping

Two-dimensional semiconductors with room-temperature intrinsic ferromagnetism and superior conductivity are crucial components for low-dimensional semiconductor spintronic devices. Based on density functional theory and Boltzmann transport theory, we investigate the electromagnetic and transport properties of the HfCr2N4 monolayer and modulate these properties via electrostatic doping. The HfCr2N4 monolayer is an intrinsic ferromagnetic semiconductor with a narrow band gap of 0.18 eV. The notably high Curie temperature of 719 K is highly desired for the practical applications of two-dimensional ferromagnetic materials. Under electrostatic doping, the HfCr2N4 monolayer maintains its ferromagnetic ground state, easy magnetization xy-plane, and high Curie temperature. Interestingly, owing to the narrow band gap of the HfCr2N4 monolayer, a significant enhancement in conductivity is observed at lower carrier doping concentrations, leading to a transition from a semiconductor to a half-metal. These unique features can be utilized to represent binary states ‘0’ and ‘1’. Our results indicate that the HfCr2N4 monolayer emerges as an ideal candidate for dual-gate field effect transistors.

Janus Single‐Layer CoClBr: A Direct Ferromagnetic Semiconductor with Controllable BandGap and Enhanced Magnetic Anisotropy Under Strain

AbstractThe 2D materials with both ferromagnetism and semiconducting properties are desirable for spintronics applications. Here, inspired by the successful synthesis of single‐layer CoCl, it predicts that Janus single‐layer CoClBr is a 2D intrinsic ferromagnetic semiconductor with a direct bandgap of 3.71 eV by first‐principles calculations. Single‐layer CoClBr exhibits an in‐plane magnetic anisotropic energy (MAE) of 542.25 eV per Co atom and a Curie temperature (T) of 89.49 K. Biaxial strain can effectively modulate its bandgap, MAE, and T, but will not change the ferromagnetic ground state. Compressive strain can increase the Curie temperature and switch the spin moment from in‐plane direction to out‐of‐plane direction. Tensile strain can enlarge the bandgap and introduce a direct‐to‐indirect bandgap transition in CoClBr. The MAE of CoClBr reaches 391.73 eV per Co atom and 1560.49 eV per Co atom at a compressive strain of ‐2% and a tensile strain of 5%, respectively. The tunable electronic and magnetic properties of Janus single‐layer CoClBr has potential application in low‐dimensional spintronics devices.

Tunable abundant valley Hall effect and chiral spin-valley locking in Janus monolayer VCGeN4.

It is both conceptually and practically fascinating to explore fundamental research studies and practical applications of two-dimensional systems with the tunable abundant valley Hall effect. In this work, based on first-principles calculations, the tunable abundant valley Hall effect is proved to appear in Janus monolayer VCGeN4. When the magnetization is along the out-of-plane direction, VCGeN4 is an intrinsic ferromagnetic semiconductor with a valley feature. The intriguing spontaneous valley polarization exists in VCGeN4 due to the common influence of broken inversion and time-reversal symmetries, which makes it easier to realize the anomalous valley Hall effect. Furthermore, we observe that the valley-non-equilibrium quantum anomalous Hall effect is driven by external strain, which is located between two half-valley-metal states. When reversing the magnetization, the spin flipping makes the position of the edge state to change from one valley to another valley, demonstrating an intriguing behavior known as chiral spin-valley locking. Although the easy magnetic axis orientation is along the in-plane direction, we can utilize an external magnetic field to transform the magnetic axis orientation. Moreover, it is found that the valley state, electronic and magnetic properties can be well regulated by the electric field. Our works explore the mechanism of the tunable abundant valley Hall effect by applying an external strain and electric field, which provides a perfect platform to investigate the spin, valley, and topology.

First-principles predictions of room-temperature ferromagnetism in orthorhombic MnX2 (X = O, S) monolayers.

Two-dimensional ferromagnets with high spin-polarization at ambient temperature are of considerable interest because they might be useful for making nanoscale spintronic devices. We report that even though bulk phases of MnO2 are generally antiferromagnetic with low ordering temperatures, the corresponding MnO2 and MnS2 monolayers are ferromagnetic, and MnS2 is a high temperature half metallic ferromagnet. Based on first-principles calculations, we find that the MnO2 monolayer is an intrinsic ferromagnetic semiconductor with a Curie temperature TC of ∼300 K, while the half-metallic MnS2 monolayer has a remarkably high TC of ∼1150 K. Both compounds have substantial magnetocrystalline anisotropy, out of plane in the case of MnO2 monolayers, and in plane along the b-axis of orthorhombic MnS2 monolayer. Interestingly, a metal-insulator phase transition occurs in the MnS2 monolayer when the applied biaxial strain is beyond -2%. Tuning near this metal-insulator transition offers additional possibilities for devices. The present work shows that MnX2 (X = O, S) monolayers have the properties required for ultrathin nano-spintronic devices.

Prediction of the two-dimensional ferromagnetic semiconductor Janus 2H-ZrTeI monolayer with large valley and piezoelectric polarizations.

Two-dimensional room-temperature Janus ferrovalley semiconductors with valley polarization and piezoelectric polarization offer new perspectives for designing multifunctional nanodevices. Herein, using first-principles calculations, we predict that the Janus 2H-ZrTeI monolayer is an intrinsic ferromagnetic semiconductor with in-plane magnetic anisotropy and a Curie temperature of 111 K. The Janus ZrTeI monolayer possesses a significant valley polarization of 141 meV due to time-reversal and inversion symmetry breaking. Based on the valley-contrasting Berry curvature, the anomalous valley Hall effect can be observed under an in-plane electric field. Meanwhile, the breaking of the inversion symmetry and mirror symmetry results in large longitudinal and transverse piezoelectric coefficients. By applying biaxial strain, the Janus 2H-ZrTeI monolayer can also be transformed into a Weyl nodal line semimetal. Furthermore, bilayers of ZrTeI with AB and BA stacking configurations allow the coexistence of valley polarization and ferroelectricity, enabling the manipulation of magnetism, ferroelectric polarization, and valley polarization through interlayer sliding. Our work provides a platform for studying valley polarization, piezoelectricity, and multiferroic coupling, which is significant for the application of multifunctional devices.

Intrinsic ferromagnetism with high Curie temperature in two dimensional XCrY3 (X = Al, Ga, In; Y = S, Se, Te) monolayers

Magnetic moments and specific heat capacity as a function of temperature for monolayer InCrTe3via Monte Carlo simulations. And side views of the XCrY3 (X = Al, Ga, In; Y = S, Se, Te) monolayers’ geometric structure.

Valley polarization in a two-dimensional high-temperature semiconducting TiInTe[formula omitted] honeycomb ferromagnet

Exploring valley-contrasting physics in scarce intrinsic high-temperature ferromagnetic semiconductors with feasible synthesis route is of great significance for ever-evolving moden information technology. By first-principles calculations, the TiInTe3 monolayer is predicted to demonstrate excellent structural stability and easy-plane semiconducting ferromagnetism arising from indirect superexchange mechanism. Monte Carlo simulations based on the Heisenberg model reveal a magnetic phase transition at 583 K well above room temperature. Due to the inversion and time-reversal asymmetries, tunable valley polarization in the conduction band up to 110 meV can be achieved by varying the magnetization orientation, which is further validated by a first-order perturbation theory. More excitingly, electron doping for realizing the anomalous valley Hall effect gives rise to an out-of-plane preference for magnetization in the TiInTe3 monolayer and thus generates spontaneous and larger valley polarizations. After doping, the ferromagnetism above room temperature remains robust in the experimentally attainable electron density regime. Our findings highlight that the TiInTe3 monolayer is a promising ferrovalley material for developing high-performance spintronic and valleytronic nanodevices.

Intrinsic Ferromagnetic Semiconductors with High Saturation Magnetization from Hybrid Perovskites.

Ferromagnetic semiconductors (FMS) enable simultaneous control of both charge and spin transport of charge carriers, which have emerged as a class of highly desirable but rare material for applications in spin field-effect transistors and quantum computing. Organic-inorganic hybrid perovskite with high compositional adjustability and structural versatility can offer unique benefits in the design of FMS but has not been fully explored. Here we demonstrate a series of molecular FMSs based on two-dimensional organic-inorganic hybrid perovskite structure, namely (2ampy)CuCl4 , (3ampy)CuCl4 and (4ampy)CuCl4 , which exhibits high saturation magnetization, dramatic temperature-dependent conductivity change and tunable ferromagnetic resonance. Magnetic measurements revealed high saturation magnetization up to 18.56 emu/g for (4ampy)CuCl4 , which is one of the highest value among reported hybrid FMSs to date. Conductivity studies of the three FMSs demonstrate that the smaller adjacent octahedron distance in the two-dimensional layer results in higher conductivity. Systematic ferromagnetic resonance investigation shows that the gyromagnetic ratio and Landau factor values are strongly dependent on the types of organic cations used. This work demonstrates that two-dimensional hybrid perovskite materials can simultaneously possess both tunable long-range ferromagnetic ordering and semiconductivity, providing a straightforward strategy for designing and synthesizing high-performance intrinsic FMSs. This article is protected by copyright. All rights reserved.

Prediction of Interesting Ferromagnetism in Janus Semiconducting Cr2AsP Monolayer

Abstract2D half‐metallic materials that have sparked intense interest in advanced spintronic applications are essential to the developing next‐generation nanospintronic devices. This study has adopted a first‐principles calculation method to predict the magnetic properties of intrinsic, Se‐doped, and biaxial strain tuning Cr2AsP monolayer. The Janus Cr2AsP monolayer is proven to be an intrinsic ferromagnetic (FM) semiconductor with an exchange splitting bandgap of 0.15 eV at the PBE+U level. Concentration‐dependent Se doping, such as Cr2AsSexP (x = 0.25, 0.50, 0.75), can regulate Cr2AsP from FM semiconductor to FM half‐metallicity. Specifically, the spin‐up channel crosses the Fermi level, while the spin‐down channel has a bandgap. More interestingly, the wide half‐metallic bandgaps and spin bandgaps make them have important implications for the preparation of spintronic devices. At last, it also explore the effect of biaxial strain from ‐14% to 10% on the magnetism of the Cr2AsP monolayer. There appears a transition from FM to antiferromagnetic (AFM) at a compressive strain of ‐10.7%, originating from the competition between the indirect FM superexchange interaction and the direct AFM interaction between the nearest neighboring Cr atoms. Additionally, when the compressive strain is ‐2% or the tensile strain is 6%, the semiconducting Cr2AsP becomes a half‐metallic material. These charming properties render the Janus Cr2AsP monolayer with great potential for applications in spintronic devices.

First-principles studies of charged defect states in intrinsic ferromagnetic semiconductors: the case of monolayer CrI3

Defects play significant roles in spin-current-related physical processes in intrinsic ferromagnetic semiconductors (FMSs), which are great promise for spintronics applications. However, current defect calculation methods cannot be used to investigate charged defects in FMSs due to the spin polarization of both the charged defect states and ionized carriers, which is not well treated in current defect calculation methods. In order to solve this problem, we propose a spin-distinguishable charge correction (SDCC) method that uses spin-polarized band edge charge density instead of spin-unpolarized uniform background charge density as the compensating charge for charged defects. We apply our method to study the defect properties of CrI3 monolayer and find it can be doped n-type under the Cr-rich growth condition but difficult to be doped p-type. The SDCC method proposed here is generally suitable for all FMSs, which will be useful for the studies of defect properties of magnetic semiconductors.

Magnetic and transport properties of two-dimensional ferromagnet VSe2 with Se vacancies

As a room-temperature van der Waals intrinsic ferromagnetic semiconductor, monolayer VSe2 was studied extensively for both fundamental research and potential low-dimensional semiconductor spintronic applications. Defects are inevitably encountered and can affect the electronic, magnetic, and transport properties during the experimental synthesis of VSe2 monolayer. However, the study of defects in VSe2 monolayer is still lacking at the atomic level. Here, the electronic, magnetic, and transport properties of VSe2 monolayer with Se vacancies are investigated by first-principles density-functional method. With the direction of easy magnetization axes unchanged, introducing Se vacancies results in the decrease of magnetic anisotropy energy and Curie temperature, which can be further increased by applying in-plane biaxial strain. After Se vacancies are introduced, the conductivity of the system decreases, while the larger doping range is obtained for pure spin-polarized current. The transition from semiconductor to half-metal with 100% spin polarization can be realized by introducing Se bivacancy in VSe2 monolayer, which is highly promising for next-generation spin filter, spin injection and spin transport nanodevices.PACS: 61.72.jd; 73.20.At; 75.50.Pp; 75.70.Ak

Enhancement of Perpendicular Magnetic Anisotropy and Curie Temperature in V-Doped Two-Dimensional CrSI Janus Semiconductor Monolayer

Two-dimensional (2D) intrinsic ferromagnetic semiconductors (FMSs) with high Curie temperatures (TC) and large perpendicular magnetic anisotropy (PMA) have immense potential in spintronic applications. Recently, the TC of the discovered 2D intrinsic FMSs is below room temperature, and the easy magnetization axis (EMA) is oriented in the in-plane direction. Here, using the first-principle calculations and the Monte Carlo simulations, we investigate the effect of V doping on structure, electronic structure, and magnetic properties of the CrVS2I2 monolayer. The calculated formation energy, phonon dispersion, ab initio molecular dynamics simulations, and elastic constants of the CrVS2I2 monolayer indicate that it is stable at room temperature. More importantly, V doping converts the EMA direction of the CrVS2I2 monolayer from in-plane to out-of-plane, accompanied by a significant enhancement of magnetic anisotropy energy from 0.0011 to 0.997 meV/atom, and it also enhances TC from 175 to 352 K. Moreover, the CrVS2I2 monolayer remains as a semiconductor with a direct band gap of 0.38 eV. Our findings provide a feasible route for the realization of high TC and large PMA in 2D intrinsic FMSs.

Enhanced Curie temperature and conductivity of van der Waals ferromagnet MgV2S4via electrostatic doping.

A van der Waals intrinsic ferromagnet with double magnetic atom layers is of great interest for both revealing fundamental physics and exploring promising applications in low-dimensional spintronics. Here, the magnetic and electronic properties of the van der Waals ferromagnet MgV2S4 monolayer are studied under electrostatic doping using first-principles calculations. A MgV2S4 monolayer presents the desired physical properties such as that of being a half-semiconductor with a direct bandgap of 1.21 eV and a ferromagnetic ground state, and having a high Curie temperature of 462 K. Unlike the robust ferromagnetic ground state, magnetic anisotropy and Curie temperature are sensitive to electrostatic doping. Meanwhile, the transition from a semiconductor to a half-metal and the significant improvement in conductivity under electrostatic doping make the MgV2S4 monolayer a promising candidate for low-dimensional spintronic field-effect transistors.

Two-dimensional ferromagnetic semiconductors of rare-earth Janus 2H-GdIBr monolayers with large valley polarization.

Based on a rare-earth Gd atom with 4f electrons, through first-principles calculations, we demonstrate that a Janus 2H-GdIBr monolayer exhibits an intrinsic ferromagnetic (FM) semiconductor character with an indirect band gap of 0.75 eV, a high Curie temperature Tc of 260 K, a significant magnetic moment of 8μB per f.u. (f.u. = formula unit), in-plane magnetic anisotropy (IMA) and a large spontaneous valley polarization of 118 meV. The MAE, inter-atomic distance or angle, and Tc can be efficiently modulated by in-plane strains and charge carrier doping. Under a strain range from -5% to 5% and charge carrier doping from -0.3 e to 0.3 e per f.u., the system still retains its FM ordering and the corresponding Tc can be modulated by strains from 233 K to 281 K and by charge carrier doping from 140 K to 245 K. Interestingly, under various strains, the matrix element differences (dz2, dyz), (dx2-y2, dxy) and (px, py) of Gd atoms dominate the MAE behaviors, which originates from the competition between the contributions of the Gd-d orbitals, Gd-p orbitals, and p orbitals of halogen atoms based on the second-order perturbation theory. Inequivalent Dirac valleys are not energetically degenerate due to the time-reversal symmetry breaking in the Janus 2H-GdIBr monolayer. A considerable valley gap between the Berry curvature at the K and K' points provides an opportunity to selectively control the valley freedom and states. External tensile (compressive) strain further increases (decreases) the valley gap up to a maximum (minimum) value of 158 (37) meV, indicating that the valley polarization in the Janus 2H-GdIBr monolayer is robust to external strains. This study provides a novel paradigm and platform to design spintronic devices for next-generation quantum information technology.

Janus V2AsP monolayer : a ferromagnetic semiconductor with a narrow band gap, a high Curie temperature and controllable magnetic anisotropy

The search and design of two-dimensional (2D) magnetic semiconductors for spintronics applications are particularly significant. In this work, we investigated the electronic and magnetic properties of Janus structure based on Dirac half-metallic vanadium phosphide (VP) monolayer (ML) by first-principles calculations. Due to the vertical symmetry breaking, Janus V2AsP ML becomes an intrinsic ferromagnetic semiconductor with a narrow band gap of 0.21 eV. We analyzed the electronic structure and origin of the in-plane easy axis in Janus V2AsP. The electron effective mass is anisotropic and only 0.129 m0 along the x-direction. The Curie temperature and magnetic anisotropy energy (MAE) of Janus V2AsP reach 490 K and 178 µeV per V atom, respectively. A uniaxial tensile stain ɛ x of 5% can increase its band gap and MAE to 0.39 eV and 210.6 µeV per V atom while maintaining its being above room temperature. Moreover, the direction of the easy axis can be changed between the in-plane x- and y-direction by a small uniaxial tensile strain ɛ x of 2%. Our study can motivate further research on the design the magnetic semiconductors in Janus structures based on 2D Dirac half-metals for spintronics applications.