Showing papers on "Half-metal published in 2019"

Itinerant ferromagnetic half metallic cobalt–iron couples: promising bifunctional electrocatalysts for ORR and OER

Abstract: Spin polarization has been recently recognized as a critical factor that affects the catalytic behavior of electrocatalysts. Half metals, with an exotic quantum state of 100% spin-polarized conduction electrons, and the spin associated atomic magnetization could show great potential in catalyst applications, however the corresponding research is insufficient. In this work, a prototype study of (Co and/or Fe)–Nx (x = 1–6) embedded configurations (FeCoNx–gra) is conducted within density functional theory (DFT) via spin-polarized calculation. The results indicate that an itinerant ferromagnetic half metal can be recognized as a promising candidate for a bifunctional electrocatalyst for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Specifically, the ferromagnetically coupled spin-polarized electrons in a half metal tend to attract oxygen molecules so as to stimulate ORR, while the formation of the O–O bond in OER requires spin conservation. The delocalized spin character (metallic spin of a half metal) guarantees the moderate binding strength of main reaction intermediates, i.e., *O2, *O, *OH, and *OOH, which can be measured by the atomic spin moment in the reaction center (0.4–1.5 μB for Fe in this study). This study identifies a new specific link between the electron structure and catalytic activities which has predictive power for catalyst design.

Weyl-loop half-metal in Li 3 ( FeO 3 ) 2

Abstract: Nodal-line metals and semimetals, as interesting topological states of matter, have been mostly studied in nonmagnetic materials. Here, based on first-principles calculations and symmetry analysis, we predict that fully spin polarized Weyl loops can be realized in the half-metal state of the three-dimensional material ${\mathrm{Li}}_{3}{({\mathrm{FeO}}_{3})}_{2}$. We show that this material has a ferromagnetic ground state, and it is a half-metal with only a single spin channel present near the Fermi level. The spin-up bands form two separate Weyl loops close to the Fermi level, which arise from band inversions and are protected by the glide mirror symmetry. One loop is type I, whereas the other loop is the hybrid type. Corresponding to these two loops in the bulk, on the (100) surface, there exist two fully spin polarized drumheads of surface states within the surface projections of the loops. The effects of the electron correlation and the spin-orbit coupling, as well as the possible hourglass Weyl chains in the nonmagnetic state, are discussed. The realization of fully spin polarized Weyl-loop fermions in the bulk and drumhead fermions on the surface for a half-metal may generate promising applications in spintronics.

Engineering the magnetic properties of PtSe2 monolayer through transition metal doping.

Abstract: Using first-principles calculations, we have studied the energetic feasibility and magnetic properties of transition metal (TM) doped PtSe2 monolayers. Our study shows that TM doped PtSe2 layers with 6.25% doping exhibit versatile spintronic behaviour depending on the nature of the dopant TM atoms. Groups IVB and VIII10 TM doped PtSe2 layers are non magnetic semiconductors, while groups IIIB, VB, VIII8, VIII9, IB TM doped PtSe2 layers are half-metals and finally, groups VIB, VIIB and IIB TM doped PtSe2 layers are spin polarized semiconductors. The presence of half-metallic and magnetic semiconducting characteristics suggest that TM doped PtSe2 layers can be considered as a new kind of dilute magnetic semiconductor and thus have the promise to be used in spintronics. By studying the magnetic interactions between two TM dopants in PtSe2 monolayers for dopant concentration of 12.5% and dopant distance of 12.85 [Formula: see text], we have found that in particular, Fe and Ru doped PtSe2 systems are ferromagnetic half-metal having above-room-temperature Curie point of 422 and 379.9 K, respectively. By varying the dopant distance and concentration we have shown that the magnetic interaction is strongly dependent on dopant distance and concentration. Interestingly, the Curie temperature of TM doped PtSe2 layers is affected by the correlation effects on the TM d states and also spin-orbit coupling. We have also studied the magnetic properties of defect complex composed of one TM dopant and one Pt vacancy (TMPt + VPt) which shows novel magnetism.

Prediction of intrinsic two dimensional ferromagnetism realized quantum anomalous Hall effect.

Abstract: Recently, the discovery of intriguing properties in intrinsic two dimensional ferromagnetism has spurred huge interest in investigating their applications in spintronics. Here, we predict that the monolayer of FeX3 (X = Cl, Br, I) possesses a quantum anomalous Hall insulating phase generated by the honeycomb lattice of iron atoms. We find that the ground state of FeX3 is a 100% spin polarized Dirac half metal with a ferromagnetic Curie temperature Tc = 116-175 K predicted from Monte Carlo simulations. Taking into account the spin orbit coupling, the Dirac cone opens a global band gap of 8.4-36 meV. The anomalous Hall conductivity calculation shows the Chern number C = -1, indicating the presence of a single chiral edge state in nanoribbons of finite width. Moreover, we also find the critical value Uc when we perform DFT+U calculations. The band gap firstly increases and then gradually decreases while keeping C = -1 for 0 Uc. Our work adds an experimentally feasible member to the quantum anomalous Hall insulator family, which is hoped to have great application potential in spintronics and nanoelectronics.

Rationally Designed High-Performance Spin Filter Based on Two-Dimensional Half-Metal Cr2NO2

Abstract: Summary Half-metals are promising candidates for designing efficient spin filters owing to their unique electronic structures, which show the electrical conductivity for spin-up states and a band gap for spin-down states. Herein, designing excellent ultrathin spin filters by using half-metal two-dimensional Cr2NO2, which has a Curie temperature of 566 K, is demonstrated based on first-principles calculations. Our results reveal that the required energy to overturn the spin arrangement of bilayer Cr2NO2, from parallel to anti-parallel states, is quite low. The bilayer Cr2NO2 maintains its half-metal behavior while sandwiched within the Au/Cr2NO2/Au heterojunction. Owing to the half-metal characteristic, the total current of Au/Cr2NO2/Au can reach a value of 15 nA per primitive cell under a low voltage of 10 mV in parallel states. The magnetoresistance ratio is 9,333% at low voltage. The robust half-metal behavior, high Curie temperature, and two-dimensional structure make Cr2NO2 an ideal ultrathin spin-filtering material with high switching ratio and low energy consumption.

Half-metal state of a Ti2C monolayer by asymmetric surface decoration.

Abstract: Searching for two-dimensional (2D) ferromagnetic materials is one of the key steps in 2D spintronics. 2D metal carbide/nitride materials (MXene) are widely regarded as promising candidates for this kind of material. However, when the surfaces are saturated with some functional groups during the preparation, the ground states of most of the MXenes transit from ferromagnetic (FM) to antiferromagnetic (AFM) or non-magnetic (NM) states. In this article, we propose a new method to avoid this problem by adopting asymmetric decoration of the MXene surface, which can make MXenes ferromagnetic ground states. Based on hybrid density functional theory calculations, our results show asymmetrical adsorption of negative ions or metal atoms makes the Ti atoms have different valence states, such as one sublayer Ti4+ and another Ti+, which prefer FM ground states. This research will deepen our understanding of the magnetic properties of 2D materials and contribute to the design of new 2D ferromagnetic materials.

Computationally predicting spin semiconductors and half metals from doped phosphorene monolayers

Abstract: First-principles computations are performed to investigate phosphorene monolayers doped with 30 metal and nonmetal atoms. The binding energies indicate the stability of all doped configurations. Interestingly, the magnetic atom Co doping induces the absence of the magnetism while the magnetism is realized in phosphorene with substitutional doping of nonmagnetic atoms (O, S, Se, Si, Br, and Cl). The magnetic moment of transition metal (TM)-doped systems is suppressed in the range of 1.0–3.97 µB. The electronic properties of the doped systems are modulated differently; O, S, Se, Ni, and Ti doped systems become spin semiconductors, while V doping makes the system a half metal. These results demonstrate potential applications of functionalized phosphorene with external atoms, in particular to spintronics and dilute magnetic semiconductors.

Surface resonance of thin films of the Heusler half-metal Co 2 MnSi probed by soft x-ray angular resolved photoemission spectroscopy

Abstract: Heusler compounds are promising materials for spintronics with adjustable electronic properties including 100% spin polarization at the Fermi energy. We investigate the electronic states of ${\mathrm{AlO}}_{x}$ capped epitaxial thin films of the ferromagnetic half-metal ${\mathrm{Co}}_{2}\mathrm{MnSi}$ ex situ by soft x-ray angular resolved photoemission spectroscopy (SX-ARPES). Good agreement between the experimental SX-ARPES results and photoemission calculations including surface effects was obtained. In particular, we observed in line with our calculations a large photoemission intensity at the center of the Brillouin zone, which does not originate from bulk states, but from a surface resonance. This provides strong evidence for the validity of the previously proposed model based on this resonance, which was applied to explain the huge spin polarization of ${\mathrm{Co}}_{2}\mathrm{MnSi}$ observed by angular-integrating UV-photoemission spectroscopy.

Half-metal calculations of CoZrGe half-Heusler compound by using generalized gradient approximation (GGA) and modified Becke-Johnson (mBJ) methods

Abstract: The structural and magnetic calculations of CoZrGe half-Heusler compound were investigated using generalized gradient approximation (GGA) and modified Becke-Johnson potential. First, the volume-energy curves were performed to determine which phase was more stable. According to that curve, the ferromagnetic (FM) phase was more energetically stable than non-magnetic (NM) phase. Then the total density of states and band structures were plotted using GGA and mBJ methods. Since the spin-up and spin-down electrons cut the Fermi energy level, they show metallic properties in the GGA method. Therefore, it is possible to say that CoZrGe half-Heusler compound is a metallic ferromagnetic material when GGA method is used. However, when the mBJ method was used, the band gap was seen around the Fermi energy level in spin up electrons. This showed that spin-up electrons had semiconductor properties and spin-down electrons had metallic nature. Thus, in the mBJ method, CoZrGe half-Heusler compound showed half-metal ferromagnetic nature. The Fermi energy level value in the GGA method was 0.586 Ry, while it was obtained as 0.601 Ry by using the mBJ method. This ensured that the Fermi energy level was higher when the mBJ method was used and the intersections in the GGA method was eliminated. Total magnetic moment is obtained as 1.00 µB/f.u. in both calculation methods. It has been clearly observed that these two different methods affected the Fermi energy level. Finally, CoZrGe half-Heusler compound can be used in spintronic calculations.

Robust half metallicity state with the hydrostatic and tetragonal distortion for a new quaternary Heusler ZrTiRhGa: FP-LAPW calculations

Abstract: The structural, mechanical, electronic and magnetic properties of quaternary Heusler ZrTiRhGa alloy have been investigated by using the first-principles calculations within the generalized gradient approximation GGA. The results of our calculations show that the type (I) of ZrTiRhGa, with the ferromagnetic (FM) state, is the most stable configuration among three possible configurations. The GGA + U calculations, where U is on-site Coulomb interaction correction, are performed to investigate the relationship between band gap and Hubbard potential. The calculated results showed that ZrTiRhGa compound is half-metallic material (HMM) with complete spin polarization around the Fermi level. The total magnetic moment is 2 μB per formula unit and follows the Slater-Pauling rule μt = Zt−18. The calculated elastic constants, cohesive and formation energy, for ZrTiRhGa compound, were analyzed in detail and reveal the mechanical stability. In addition, we investigated the sensitivity of the half-metallicity with two structural-type changes: hydrostatic and tetragonal deformation. Our calculations show that the half-metallicity can be preserved with lattice constants from 5.74 A to 7.12 A, under the influence of hydrostatic strain and in the range of the c/a ratio from −8% to 12% with the effect of the tetragonal strain. For ZrTiRhGa, since there is no available experimental data, our calculations are considered as first predictions.

Spin-1 Dirac half-metal, spin-gapless semiconductor, and spin-polarized massive Dirac dispersion in transition metal dihalide monolayers

Abstract: Spin-1 condensed matter systems characterized by the combination of a Dirac-like dispersion and flat bands are ideal for realizing high-temperature electronics and spintronic technologies in the absence of external magnetic field In this study, we propose a three-band tight binding model, with spin-polarized Haldane-like next-nearest-neighbour tunnelling, on dice lattice and show that spin-1 Dirac half-metal, spin-1 Dirac spin-gapless semiconductor, and spin-polarized spin-1 massive Dirac dispersion with nontrivial topology can exist in two-dimensional ferromagnetic condensed matter systems with electron spin polarization P = 1 The proposed spin-polarized spin-1 phases can be realized in ferromagnetic transition metal dihalides MX2 monolayers effectively By using first principle calculations, we show that a small compressive strain leads MX2 monolayers to be spin-one Dirac half-metal for M = Fe and X = Br, Cl while spin-one Dirac spin-gapless semiconductor for M = Co and X = Br, Cl Spin-one Dirac spin-gapless semiconductors CoBr2 and CoCl2 embeds flat band ferromagnetism where spin-orbit coupling opens a topologically non-trivial Dirac gap between dispersing valance and conduction band while leaving flat band unaffected The intrinsic flat-band ferromagnetism in spin-polarized spin-1 massive Dirac dispersion plays key role in materializing quantum anomalous Hall state with Chern number C = -2

Investigation of the Itinerant Electron Ferromagnetism of Ni2+xMnGa1-x and Co2VGa Heusler Alloys

Abstract: Experimental investigations into the field dependence of magnetization and temperature dependences of magnetic susceptibility in Ni2+xMnGa1-x (x = 0.00, 0.02, 0.04) and Co₂VGa Heusler alloy ferromagnets were performed following the spin fluctuation theory of itinerant ferromagnetism, called as "Takahashi theory". We investigated the magnetic field dependence of magnetization at the Curie temperature TC, which is the critical temperature of the ferromagnetic⁻paramagnetic transition, and also at T = 5 K, which concerns the ground state of the ferromagnetic state. The field dependence of the magnetization was analyzed by means of the H vs. M⁵ dependence, and the field dependence of the ground state at 5 K was investigated by means of an Arrott plot (H/M vs. M²) according to the Takahashi theory. As for Ni2+xMnGa1-x, the spin fluctuation parameter in k-space (momentum space, TA) and that in energy space (T₀) obtained at TC and 5 K were almost the same. On the contrary, as for Co₂VGa, the H vs. M⁵ dependence was not shown at TC. We obtained TA and T₀ by means of an Arrott plot at 5 K. We created a generalized Rhodes⁻Wohlfarth plot of peff/pS versus TC/T₀ for the other ferromagnets. The plot indicated that the relationship between peff/pS and T₀/TC followed Takahashi's theory. We also discussed the spontaneous magnetic moment at the ground state, pS, which was obtained by an Arrott plot at 5 K and the high temperature magnetic moment, pC, at the paramagnetic phase. As for the localized ferromagnet, the pC/pS was 1. As for weak ferromagnets, the pC/pS was larger than 1. In contrast, the pC/pS was smaller than 1 by many Heusler alloys. This is a unique property of Heusler ferromagnets. Half-metallic ferromagnets of Co₂VGa and Co₂MnGa were in accordance with the generalized Rhodes⁻Wohlfarth plot with a km around 1.4. The magnetic properties of the itinerant electron of these two alloys appeared in the majority bands and was confirmed by Takahashi's theory.

Observation of a Dirac state in borophene hetero-bilayers by Cr intercalation

Abstract: By intercalating Cr atoms into borophene hetero-bilayers (BHBs), three kinds of Cr@BHBs, namely, Cr@BHB(α1,β), Cr@BHB(α1,β1) and Cr@BHB(β,β1) are constructed and intriguing electronic and magnetic properties are predicted by density functional theory calculations. Interestingly, a Dirac cone is found in Cr@BHB(α1,β), which is well conserved even under 8% tensile strain along uniaxial x and biaxial xy directions, and up to 4% tensile strain along the y direction. On the contrary, the Dirac cone disappears when exerting biaxial or uniaxial compressive strains, and Cr@BHB(α1,β) is transformed into a ferromagnetic half metal or metal eventually. In addition, the Cr@BHB(α1,β1) and Cr@BHB(β,β1) isomers are found to be an antiferromagnetic semiconductor and ferromagnetic half metal, respectively, and their electronic and magnetic properties are sensitive to the external strains. Our study proposes a pathway for designing borophene based materials with intriguing properties, which may have potential applications in electronics and spintronics.

Site-Preference, Electronic, Magnetic, and Half-Metal Properties of Full-Heusler Sc2VGe and a Discussion on the Uniform Strain and Tetragonal Deformation Effects

Abstract: A hypothetical full-Heusler alloy, Sc2VGe, was analyzed, and the comparison between the XA and L21 structures of this alloy was studied based on first-principles calculations. We found that the L21-type structure was more stable than the XA one. Further, the electronic structures of both types of structure were also investigated based on the calculated band structures. Results show that the physical nature of L21-type Sc2VGe is metallic; however, XA-type Sc2VGe is a half-metal (HM) with 100% spin polarization. When XA-type Sc2VGe is at its equilibrium lattice parameter, its total magnetic moment is 3 μ B , and its total magnetism is mainly attributed to the V atom. The effects of uniform strain and tetragonal lattice distortion on the electronic structures and half-metallic states of XA-type Sc2VGe were also studied. All the aforementioned results indicate that XA-type Sc2VGe would be an ideal candidate for spintronics studies, such as spin generation and injection.

First-principles study of the metal-insulator transition in the Ti-substituted rutile CrO2

Abstract: The effect of Ti-substitution for Cr in CrO2 is extensively studied by employing first-principles electronic structure calculations. The host material is a ferromagnetic half-metal. This material encounters first step transition from half-metal to a metallic phase at 50% Ti-substitution for Cr. The application of Coulomb interaction U significantly changes the electronic and magnetic properties of Cr0.5Ti0.5O2. This material remains in its metallic phase up to U = 2 eV and encounters a second step transition from the metallic to a half-metallic phase at U = 3 eV. Eventually, this system exhibits a metal-insulator transition (MIT) at U = 4 eV with a band gap of 0.15 eV. Nevertheless, Cr0.5Ti0.5O2 preserves its ferromagnetism (FM) in all the metallic, half-metallic and insulating phases. The metal to half-metal transition in Cr0.5Ti0.5O2 is observed due to full spin polarizations accompanied by strong dynamical correlations of Cr-dyz/xz electrons. Besides this, weak static but the strong dynamical correlation of electrons in the bonding and anti-bonding components of Cr-dyz/xz orbitals is accounted for the key element of MIT in Cr0.5Ti0.5O2. The FM in the half-metallic Cr0.5Ti0.5O2 arises from the double exchange interaction of electrons in the partially occupied bonding and anti-bonding components of Cr-dyz/xz orbitals. In addition, the double exchange interaction of electrons in the bonding components of Cr-dyz/xz orbitals triggers FM in the insulating phase of Cr0.5Ti0.5O2. The ferromagnetic Curie temperature increases due to Ti-substitution in CrO2. Finally, a trivial structural distortion is observed due to Ti-substitutions in CrO2.

Ab initio study of rhombohedral ErMnO3 as a high Tc half-metal with multiple Dirac cones and promising applications in spintronics

Abstract: Advanced materials with high spin polarization and Dirac features have attracted widespread attention due to their potential applications in the field of spintronics. But this type of material is still very rare, especially for three-dimensional bulk materials. In this manuscript, via first-principles, we predicted a new rhombohedral type material ErMnO3. Remarkably, this material possesses the following excellent characteristics: (1) robust half-metallic properties, (2) multiple Dirac-cones, (3) strong resistance to the spin-orbit coupling, (4) high Curie temperature and (5) very resistant to the hole and electron doping. Therefore, ErMnO3 could potentially have broad application prospects in spintronics. Also, to better understand the specific behaviors of ErMnO3 under extreme conditions as high temperature or pressure, we further investigated its thermodynamic properties through the quasi-harmonic Debye model.

Crossover between standard and inverse spin-valve effect in atomically thin superconductor/half-metal structures

Abstract: Spin-singlet Cooper pairs consisting of two electrons with opposite spins cannot directly penetrate from a superconductor to a half-metal (fully spin polarized ferromagnets) which blocks the superconducting proximity effect between these materials. In this paper we demonstrate that, nevertheless, two half-metallic layers electrically coupled to the superconducting film substantially affect its critical temperature and produce the spin valve effect. Within the tight-binding model for the atomically thin multilayered spin valves we show that depending on the details of the electron energy spectra in half-metals the critical temperature as a function of the angle between the spin quantization axes in half-metals can be either monotonically increasing or decreasing. This finding highlights the crucial role of the band structure details in the proximity effect with half-metals which cannot be adequately treated in the quasiclassical theories.