Structure, phase stability, half-metallicity, and fully spin-polarized Weyl states in compound NaV2O4: a new example for topological spintronic material
Tingli He, Xiaoming Zhang, Tie Yang, Ying Liu, Xuefang Dai, Guodong Liu
Here, we systematically investigate the structure, phase stability, half-metallicity, and topological electronic structure for a new topological spintronic material NaV2O4. The material has a tetragonal structure with excellent dynamical and thermal stabilities. It shows a half-metallic ground state, where only the spin-up bands present near the Fermi level. These bands form a Weyl nodal line close to the Fermi level, locating in the kz = 0 plane. The nodal line is robust against SOC, under the protection of the mirror symmetry. The nodal line band structure is very clean, thus the drumhead surface states can be clearly identified. Remarkably, the nodal line and drumhead surface states have the 100% spin polarization, which are highly desirable for spintronics applications. In addition, by shifting the magnetic field in-plane, we find that the Weyl nodal line can transform into single pair of Weyl nodes. The Weyl-line and Weyl-node fermions in the bulk, as well as the drumhead fermions on the surface are all fully spin-polarized, which may generate new physical properties and promising applications.
Dirac and Weyl materials refer to a class of solid materials which host low-energy quasiparticle excitations that can be described by the Dirac and Weyl equations in relativistic quantum mechanics. Starting with the advent of graphene as the first prominent example, these materials have been attracting tremendous interest owing to their novel fundamental properties as well as the great potential for applications. Here we introduce the basic concepts and notions related to Dirac and Weyl materials and briefly review some recent works in this field, particularly on the conceptual development and…
This work presents results for the magnetic properties of the compound GeTe doped with 3d transition metals V, Cr and Mn from the viewpoint of potential application in spintronics. We report a systematic density-functional study of the electronic structure, magnetic and cohesive properties of these ternary compounds in both rock salt and zinc blende structures. In both cases, it is the Ge sublattice that is doped with the three transition metals. Some of these compounds are found to be half-metallic at their optimized cell volumes. For these particular cases, we calculate both exchange interac…
Heusler compounds offer potential as spintronic devices due to their spin-polarization and half-metallicity properties, where electron spin-majority (minority) manifold exhibits states (band gap) at the electronic chemical potential, yielding full spin-polarization in a single manifold. Yet, Heuslers often exhibit intrinsic disorder that degrades its half-metallicity and spin-polarization. Using density-functional theory, we analyze the electronic and magnetic properties of equiatomic Heusler ($L$2$_{1}$) CoMnCrAl and CoFeCrGe alloys for effects of hydrostatic pressure and intrinsic disorder (…
First Principal Investigations to Explore the Half-metallicity, Structural, Mechanical, and Optoelectronic Properties of Sodium-Based Fluoroperovskites NaYF3 (Y = Sc and Ti) for Applications in Spintronics and Optoelectronics
To obtain high-performance spintronic devices with high integration density, two-dimensional (2D) half-metallic materials are eagerly pursued all along. Here, we propose a stable 2D material with a honeycomb-kagome lattice, i.e., the Mg3C2 monolayer, based on first-principles calculations. This monolayer is an anti-ferromagnetic (AFM) semiconductor at its ground state. We further demonstrate that a transition from AFM semiconductor to ferromagnetic half-metal in this 2D material can be induced by carrier (electron or hole) doping. This magnetic transition can be understood by the Stoner criter…
Antiferromagnetic spin motion at terahertz (THz) frequencies attracts growing interests for fast spintronics, however their smaller responses to external field inhibit device application. Recently the noncollinear antiferromagnet Mn$_3$Sn, a Weyl semimetal candidate, was reported to show large anomalous Hall effect (AHE) at room temperature comparable to ferromagnets. Dynamical aspect of such large responses is an important issue to be clarified for future THz data processing. Here the THz anomalous Hall conductivity in Mn$_3$Sn thin films is investigated by polarization-resolved spectroscopy.…
Based on systematic first-principles density-functional theory (DFT) simulations, we predict that the zigzag GaN nanoribbons (ZGaNNR) can be used both as highly efficient CO detectors as well as spin filters. Our calculations performed both on infinitely long nanoribbons, and also on finite strands, suggest that: (a) CO binds strongly at the edges of ZGaNNRs, and (b) that several of the resultant configurations exhibit half-metallic behavior. We considered various edge-passivation sites and found that all the resultant structures are thermodynamically stable. The metallic, half-metallic, and s…
In this letter, we report the discovery of a new class of spintronic materials, namely spin semi-metals (SSM), employing both theoretical and experimental tools. The band structure of this class of materials is such that one of the spin bands resembles that of a semi-metal, while the other is similar to that of an insulator/semiconductor. This report is the experimental verification of the first SSM, FeRhCrGe, a quaternary Heusler alloy with a magnetic moment 3 $μ_B$ and a Curie temperature of 550 K. The measurement below 300 K shows nearly temperature independent conductivity and a relatively…
Spintronics is the fast growing field that will play a key role in optimizing power consumption, memory, and processing capabilities of nanoelectronic devices. Heusler alloys are potential candidates for application in spintronics due to their room temperature (RT) half-metallicity, high Curie temperature, low lattice mismatch with most substrates, and strong control on electronic density of states at Fermi level. In this work, we investigate the effect of {substitutional doping and disorder} on the half-metallicity, phase stability, and magnetism of Heusler alloys using density functional the…
The pyrite compound CoS2 has been intensively studied in the past due to its itinerant ferromagnetism and potential for half-metallicity, which make it a promising material for spintronic applications. However, its electronic structure remains only poorly understood. Here we use complementary bulk- and surface-sensitive angle-resolved photoelectron spectroscopy and ab-initio calculations to provide a complete picture of its band structure. We discover Weyl-cones at the Fermi-level, which presents CoS2 in a new light as a rare member of the recently discovered class of magnetic topological meta…
Heusler alloys containing Co and Mn are amongst the most heavily studied half-metallic ferromagnets for future applications in spintronics. Using state-of-the-art electronic structure calculations, we investigate the effect of doping and disorder on their electronic and magnetic properties. Small degrees of doping by substituting Fe or Cr for Mn scarcely affect the half-metallicity. A similar effect is also achieved by mixing the sublattices occupied by the Mn and sp atoms. Thus the half-metallicity is a robust property of these alloys.
Two-dimensional Cr2N MXene as well as the surface-passivated Cr2NF2, Cr2N(OH)2 and Cr2NO2 are investigated by using density functional theory. The Cr2N is an anti-ferromagnetic metal. The F atom or OH group-passivation does not change the anti-ferromagnetic characteristics. However, Cr2NO2 has a ferromagnetic ground state, which is a half-metal. The half-metallicity of Cr2NO2 is still robust when bias is applied to the nanometer sized device. The half-metallicity is intrinsic and does not require atomically clean surfaces. Therefore the stable surface-oxygen-passivated MXene is a good candidat…
We report a first-principles electronic-structure calculation on C and BN hybrid zigzag nanoribbons. We find that half-metallicity can arise in the hybrid nanoribbons even though stand-alone C or BN nanoribbon possesses a finite band gap. This unexpected half-metallicity in the hybrid nanos-tructures stems from a competition between the charge and spin polarizations, as well as from the pi orbital hybridization between C and BN. Our results point out a possibility of making spintronic devices solely based on nanoribbons and a new way of designing metal-free half metals.
Density functional theory calculations demonstrate that rocksalt MgN is a magnetic material at the verge of half-metallicity, with an electronic structure robust against strong correlations and spin-orbit interaction. Furthermore the calculated heat of formation describes the compound as metastable and suggests that it can be fabricated by tuning the relative Mg and N abundance during growth. Intriguingly the equilibrium lattice constant is close to that of MgO, so that MgN is likely to form as an inclusion during the fabrication of N-doped MgO. We then speculate that the MgO/MgN system may re…
Studies of Cr-doped III-V nitrides, dilute magnetic alloys, in the zinc-blende crystal structure are presented. The objective of the work is to investigate half-metallicity in Al(0.75)Cr(0.25)N, Ga(0.75)Cr(0.25)N and In(0.75)Cr(0.25)N for their possible application in the spin based electronic devices. The calculated spin polarized band structures, electronic properties and magnetic properties of these compounds reveal that Al0.75Cr0.25N and Ga0.75Cr0.25N are half-metallic dilute magnetic semiconductors while In0.75Cr0.25N is metallic in nature. The present theoretical predictions provide evid…
Weyl semimetals with time reversal symmetry breaking are expected to show various fascinating physical behaviors, such as intrinsic giant anomalous Hall effect, chiral anomaly effect in the bulks, and Fermi arcs on the surfaces. Here we report a scanning tunneling microscopy study on the magnetic Weyl semimetal candidate Co$_3$Sn$_2$S$_2$. According to the morphology and local density of states of the surface, we provide assignments to different surface terminations. The measured local density of states reveals a semimetal gap of ~300 mV, which is further verified as the gap in spin-minority b…
One-dimensional Cr2NO2 nanoribbons cutting from the oxygen-passivated Cr2NO2 MXene are investigated by using density functional theory. The wide nanoribbons have ferromagnetic ground states and are half-metals, independent of their chirality. The half-metallic band gaps of the wide nanoribbons are larger than 1 eV, which are large enough for avoiding thermally activated spin flip. The magnetism does not rely on the edge states but originates from all the Cr atoms. Furthermore, the half-metallicity is still robust in an electronic device even if the bias is up to 1 V. Therefore, one-dimensional…
Nodal line semimetals in two-dimensional (2-D) materials have attracted intense attention currently. From fundamental physics and spintronic applications points of view, high Curie temperature ferromagnetic (FM) ones with nodal lines robust against spin-orbit coupling (SOC) are significantly in desirable. Here, we propose that FM K2N monolayer is such Weyl nodal line semimetal. We show that K2N monolayer is dynamically stable, and has a FM ground magnetic state with the out-of-plane [001] magnetization. It shows two nodal lines in the low-energy band structures. Both nodal lines are robust aga…
Since their discovery, topological insulators have been expected to be ideal spintronic materials owing to the spin currents carried by surface states with spin--momentum locking. However, the bulk doping problem remains an obstacle that hinders such application. In this work, we predict that a newly discovered family of topological materials, the Weyl semimetals, exhibits large intrinsic spin Hall effects that can be utilized to generate and detect spin currents. Our $ab~initio$ calculations reveal a large spin Hall conductivity that is comparable to that of $4d$ and $5d$ transition metals. T…
We propose a class of graphene nanoribbons showing strong intrinsic ferromagnetic behavior due to their asymmetry. Such ribbons are based on a zig-zag edged backbone surmounted by a periodic, triangular notched region of variable size. The electronic properties as a function of the topology are investigated. Interestingly, substitutional doping by boron or nitrogen induces half-metallicity. The most effective doping sites can be inferred from the band structure. Given the present rapid development of bottom-up strategies for the synthesis of atomically precise carbon nanostructures the propose…
Recently, magnetic topological semimetals have received a lot of attention due to their potential applications in the field of spintronics. By using first-principles calculations, we propose that two ferromagnetic spinel materials of X2MnO4 (X=Li, Na) have Weyl-type nodal chains around the Fermi level. Their stabilities are validated by cohesive energies, phonon dispersions, and elastic constants. The nodal chains are composed of two types of nodal loops, which are protected by the glide operation Mz, the mirror operation M101 and their equivalent. The drumhead surface states are observed on t…
Recent experimental realization of long sought Weyl fermions in non-magnetic crystals has greatly motivated condensed matter physicists to search for materials supporting Weyl fermions. Weyl fermions appear to be very promising for future electronics, often referred as Weyltronics. Here, by means of first-principle calculations, we report a stoichiometric crystal structure of BiSb with broken space-inversion symmetry. This structure is insulating in bulk and has non-trivial band topology. We observe a pressure driven Weyl semi-metallic phase transition in this crystal structure. The obtained W…
Topological Weyl semimetal WTe2 with large-scale film form has a promising prospect for new-generation spintronic devices. However, it remains a hard task to suppress the defect states in large-scale WTe2 films due to the chemical nature. Here, we significantly improve the crystalline quality and remove the Te vacancies in WTe2 films by post annealing. We observe the distinct Shubnikov-de Haas quantum oscillations in WTe2 films. The nontrivial Berry phase can be revealed by Landau fan diagram analysis. The Hall mobility of WTe2 films can reach 1245 cm2V-1s-1 and 1423 cm2V-1s-1 for holes and el…
We perform density functional calculations on one-dimensional zigzag edge graphene nano-ribbons (ZGNRs) of different widths, with and without edge doping including semi-local exchange-correlations. Our study reveals that, although the ground state of edge passivated (with hydrogen) ZGNRs prefers to be anti-ferromagnetic, the doping of both the edges with Boron atoms stabilizes the system in a ferromagnetic ground state. Both the local and semi-local exchange-correlations result in half-metallicity in edge passivated ZGNRs at a finite cross-ribbon electric field. However, the ZGNR with Boron ed…
Two-dimensional (2D) half-metallic materials are of great interest for their promising applications in spintronics. Although numerous of 2D half-metals have been proposed theoretically, rarely of them can be synthesized experimentally. Here, exemplified by monolayer FeCl2, we show three mechanisms in such quantum magnets that would cause the metal-insulator transition by using first-principles calculations. In particular, half-metallicity, especially that protected by symmetry-induced degeneracies, predicted by the previous theoretical simulations could be destroyed by electron correlation, sp…
By using first-principles calculations, we predict that an in-plane homogenous electrical field can induce half-metallicity in hydrogen-terminated zigzag silicene and germanene nanoribbons (ZSiNRs and ZGeNRs). A dual-gated finite ZSiNR device reveals a nearly perfect spin-filter efficiency of up to 99% while a quadruple-gated finite ZSiNR device serves as an effective spin field effect transistor (FET) with an on/off current ratio of over 100 from ab initio quantum transport simulation. This discovery opens up novel prospect of silicene and germanene in spintronics.
Topological semimetals have recently attracted great attention due to prospective applications governed by their peculiar Fermi surfaces. Weyl semimetals host chiral fermions that manifest as pairs of non-degenerate massless Weyl points in their electronic structure, giving rise to novel macroscopic quantum phenomena such as the chiral anomaly, an unusual magnetoresistance, and various kinds of Hall effects These properties enable the engineering of non-local electric transport devices, magnetic sensors and memories, and spintronics devices. Nevertheless, little is known about the underlying s…
This study reports the magnetization switching induced by spin-orbit torque (SOT) from the spin current generated in Co2MnGa magnetic Weyl semimetal (WSM) thin films. We deposited epitaxial Co2MnGa thin films with highly B2-ordered structure on MgO(001) substrates. The SOT was characterized by harmonic Hall measurements in a Co2MnGa/Ti/CoFeB heterostructure and a relatively large spin Hall efficiency of -7.8% was obtained.The SOT-induced magnetization switching of the perpendicularly magnetized CoFeB layer was further demonstrated using the structure. The symmetry of second harmonic signals, t…
Reliable and energy efficient magnetization switching by electrically-induced spin-orbit torques is of crucial technological relevance for spintronic devices implementing memory and logic functionality. Here we predict that the strength of spin-orbit torques and the related Dzyaloshinskii-Moriya interaction in topologically non-trivial magnetic insulators can exceed by far that of conventional metallic magnets. In analogy to the quantum anomalous Hall effect, we explain this extraordinary response in absence of longitudinal currents as a hallmark of magnetic monopoles in the electronic structu…
The Cr2Ge2Te6 (CGT) compound is an intrinsic ferromagnetic material with a van der Waals layered structure that shows great promise in spintronics applications. In this work, we investigated the edge effect in the formation of CGT nanoribbons with different terminations as well as the change in electronic and magnetic properties considering spin-orbit effects on the calculations. We studied the thermodynamic stability of the nanoribbon employing the surface formation energy formalism. According to the calculations, in regions of Ge-rich, Te-rich, and Ge-poor, Te-poor of chemical potential the …
An outstanding feature of topological quantum materials is their novel spin topology in the electronic band structures with an expected large charge-to-spin conversion efficiency. Here, we report a charge current-induced spin polarization in the type-II Weyl semimetal candidate WTe2 and efficient spin injection and detection in a graphene channel up to room temperature. Contrary to the conventional spin Hall and Rashba-Edelstein effects, our measurements indicate an unconventional charge-to-spin conversion in WTe2, which is primarily forbidden by the crystal symmetry of the system. Such a larg…
Two-dimensional (2D) surface of the topological materials is an attractive channel for the electrical conduction reflecting the linearly-dispersive electronic bands. By applying a reliable systematic thickness t dependent measurement of sheet conductance, here we elucidate the dimensionality of the electrical conduction paths of a Weyl semimetal Co3Sn2S2. Under the ferromagnetic phase, the 2D conduction path clearly emerges in Co3Sn2S2 thin films, indicating a formation of the Fermi arcs projected from Weyl nodes. Comparison between 3D conductivity and 2D conductance provides the effective thi…
We report first-principles calculations of zinc-blende half-metals, identifying systems for epitaxial growth with semiconductors, and present calculations for CrAs/GaAs multilayers. We find that half-metallicity is conserved troughout the heterostructure, making this a good candidate for spintronics applications.
Manipulating the spin degrees of freedom of electrons affords an excellent platform for exploring novel quantum states in condensed-matter physics and material science. Based on first-principles calculations and analysis of crystal symmetries, we propose a fully spin-polarized composite semimetal state, which is combined with the one-dimensional nodal lines and two-dimensional nodal surfaces, in the half-metal material CaFeO$_3$. In the nodal line-surface states, the Baguenaudier-like nodal lines feature six rings linked together, which are protected by the three independent symmetry operation…
The realization of Weyl systems with the minimum nonzero number of Weyl points (WPs) and full spin polarization remains challenging in topology physics and spintronic. In this study, for the first time, we used first-principle calculations and symmetry analysis to demonstrate that BaNiIO$_6$, a dynamically and thermodynamically stable half-metallic material, hosts fully spin-polarized single-pair WPs (SP-WPs) with a charge number ($\cal{C}$) of $\pm$2 and a type-\uppercase\expandafter{\romannumeral3} band dispersion around the Fermi level. Moreover, the fully spin-polarized SP-WPs induce doubl…
Magnetic topological quantum materials have attracted great attention due to their exotic topological quantum physics induced by the interplay among crystalology, magnetism, and topology, which is of profound importance to fundamental research and technology applications. However, limited materials are experimentally available, most of whom are realized by magnetic impurity doping or heterostructural constructions. In this work, based on the first-principles calculations, we predict that double perovskite Ba2CdReO6 is an intrinsic ferromagnetic topological semi-half-metal, while the ferrimagne…
Dirac points and Weyl phase in a honeycomb altermagnet
Guangdong Provincial Key Laboratory of Magnetoelectric Physics and Devices, State Key Laboratory of Optoelectronic Materials and Technologies, Center for Neutron Science and Technology, School of Physics, Sun Yat-Sen University（中山大学物理学院）
Recent studies show superconductivity provides new perspectives on spintronics. We study a heterostructure composed of an s-wave superconductor and a cubic chiral-magnet that can stabilize a topological spin texture, a skyrmion. We propose a supercurrent-induced spin torque that originates from the spin-orbit coupling, and we show that the spin torque can drive a skyrmion in an efficient way that reduces Joule heating. We also study the band structure of Bogoliubov quasiparticles and show the existence of Weyl points, whose positions can be controlled by the direction of the magnetization. Thi…
Weyl semimetals (WSM) have been extensively studied due to their exotic properties such as topological surface states and anomalous transport phenomena. Their band structure topology is usually predetermined by material parameters and can hardly be manipulated once the material is formed. Their unique transport properties appear usually at very low temperature, which sets challenges for practical device applications. In this work, we demonstrate a way to modify the band topology via a weak magnetic field in a ferromagnetic topological semimetal, Co2MnAl, at room temperature. We observe a tunab…
Diamond-structure materials have been extensively studied for decades, which form the foundation for most semiconductors and their modern day electronic devices. Here, we discover a e$_g$-orbital ($d_{z^2}$,$d_{x^2-y^2}$ ) model within the diamond lattice (e$_g$-diamond model) that hosts novel topological states. Specifically, the e$_g$-diamond model yields a 3D nodal cage (3D-NC), which is characterized by a $d$-$d$ band inversion protected by two types of degenerate states (i.e., e$_g$-orbital and diamond-sublattice degeneracies). We demonstrate materials realization of this model in the wel…
Weyl semimetal Td-phase WTe2 possesses the spin-resolved band structure with strong spin-orbit coupling, holding promises as a useful spin source material. The noncentrosymmetric crystalline structure of Td-WTe2 endows the generation of the out-of-plane polarized spin, which is of great interest in magnetic memory applications. Previously, WTe2 was explored in spin devices based on mechanically exfoliated single crystal flakes with a size of micrometers. For practical spintronics applications, it is highly desirable to implement wafer-scale thin films. In this work, we utilize centimeter-scale…
The Technical Committee of the IEEE Magnetics Society has selected 7 research topics to develop their roadmaps, where major developments should be listed alongside expected timelines; (i) hard disk drives, (ii) magnetic random access memories, (iii) domain-wall devices, (iv) permanent magnets, (v) sensors and actuators, (vi) magnetic materials and (vii) organic devices. Among them, magnetic materials for spintronic devices have been surveyed as the first exercise. In this roadmap exercise, we have targeted magnetic tunnel and spin-valve junctions as spintronic devices. These can be used for ex…
