Half-metallic Phase Research Articles

Quasinodal lines in rhombohedral magnetic materials

A well-established result in condensed matter physics states that materials crystallizing in symmetry groups containing glide reflection symmetries possess nodal lines on the energy bands. These nodal lines are topologically protected and appear on the fixed planes of the reflection in reciprocal space. In the presence of inversion symmetry, the energy bands are degenerate and the nodal lines on the fixed plane may hybridize or may cross. In the former case, the crossing is avoided, thus producing lines on reciprocal space where the energy gap is small, and in the latter, the nodal lines will endure, thus producing Dirac or double nodal lines. In addition, if the material crystallizes in a ferromagnetic phase where the glide reflection symmetry is broken, the nodal lines hybridize, thus defining lines in reciprocal space where the energy gap is small. In this work we concentrate our efforts on the study of nodal lines that hybridize due to magnetization; we have coined the term of quasi-nodal lines for those lines in reciprocal space where the energy gap is small (less than what can be detected experimentally). We study magnetic trifluorides and trioxides which crystallize in magnetic space groups 167.107 and 161.71 and we show the existence of quasi-nodal lines on these materials. We furthermore show that whenever the quasi-nodal lines are located around the Fermi level then interesting charge and spin transport effects are induced and can be used to detect experimentally these lines. Of particular interest are the half-metallic ferromagnetic phases of PdF3 and LiCuF3 where the large signal of the anomalous Hall conductance is due to the presence of the quasi-nodal lines on the Fermi level.

First-principles investigation of the metal-insulator transition in rutile RuO2

The electronic and magnetic properties of rutile ruthenium dioxide (RuO2) are extensively investigated by employing density functional theory. The material RuO2 is observed as a nonmagnetic metal. The sharing of a single Ru 4d electron by the dyz/xz orbitals is responsible for the metallicity of RuO2. The ground-state properties of this material change significantly upon the application of on-site Coulomb interaction U. RuO2 preserves its nonmagnetic metallic character up to U=3 eV. Nevertheless, RuO2 encounters a first step transition from nonmagnetic metal to a ferromagnetic metal (FM) at U=4 eV. The present system remains in its FM phase till U=5 eV. It exhibits a second step transition from the FM to a half-metallic ferromagnetic phase at U=6 eV. Eventually, RuO2 encounters metal-insulator transition (MIT) at U=7 eV with a band gap of Eg ∼0.3 eV upholding ferromagnetism. It is revealed that the spin polarization arising from the strong dynamical electron correlation due to the application of U is responsible for the metal to half-metal and MIT in RuO2.

Field-induced half-metallic phase in epitaxial TcO[formula omitted] bilayer on rutile TiO[formula omitted] surface

For spintronic applications, it is highly desirable to realize 100%-spin-polarized two-dimensional (2D) electron systems in field-controllable epitaxial ultrathin films or 2D materials on semiconductor substrates. Through systematic first-principles investigation, we find that one epitaxial TcO2 bilayer on rutile TiO2 (001) substrate is an antiferromagnet-like narrow-gap semiconductor, and its electronic and magnetic properties can be manipulated through electric field. When electric field reaches 0.03 V/Å, it transits to a half-metallic ferrimagnet with 100% spin polarization. Our analysis indicates that in both phases the magnetization density and the electronic states near the Fermi level originate mainly from the TcO2 bilayer, and across the interface the bond lengths and angles quickly converge to the corresponding values of the bulk TiO2. Therefore, the heterostructure actually hosts a 2D electron system determined by the TcO2 bilayer and the TiO2 substrate. Because the half-metallic phase can be achieved from a nonmetallic phase, such epitaxial 2D electron systems should be usable in spintronic devices.

Electrical control of crossed Andreev reflection and spin-valley switch in antiferromagnet/superconductor junctions

We study the subgap transport through the antiferromagnet/superconductor (AF/S) and antiferromagnet/superconductor/antiferromagnet (AF/S/AF) junctions controlled by electric field in a generic buckled honeycomb system, such as silicene, germanene, and stanene. In the presence of electric field and antiferromagnetic exchange field, the spin-valley polarized half-metallic phase can be achieved in the honeycomb system due to the spin-orbit coupling, which affords an opportunity to generate the pure crossed Andreev reflection (CAR). It is found that the pure CAR can be generated without local Andreev reflection (AR) and elastic cotunneling (EC) over a wide range of electric field. A spin-valley switch effect can be realized between the pure CAR and the pure EC by adjusting the electric field. The properties of AR process and CAR process strongly depend on the spin-valley polarized states. Our results suggest that the device can implement an electrical measurement of the CAR process and spin-valley switch.

Effect of Co doping in tailoring the crystallite size, surface morphology and optical band gap of CuO thin films prepared via thermal spray pyrolysis

In this paper, we reported the nonmagnetic semiconductor to half metal transition behavior of Co doped CuO (CCO) thin films with p-to n-type characteristics for the first time. The thin films are prepared using a thermal spray pyrolysis technique with variations of Co doping concentration from 0 to 8 at.% in the steps of 2 at.%. Subsequently, the surface morphological, structural, optical, electrical properties and electronic band structures by density functional theory (DFT) have been studied intensively. The surface morphology of the CCO thin films is significantly influenced by Co doping concentrations and agglomerated spherical grains are formed, as indicated by field emission scanning electron microscopy (FESEM). The quantitative analysis is carried out by the energy dispersive X-ray (EDX) analysis that confirms the presence of Cu, O and Co atoms. X-ray diffraction (XRD) reveals all the deposited CCO thin films are belong to monoclinic structure with mixed phases, and average crystallite size shrinkages from 28.67 to 18.96 nm with Co doping concentrations. The optical transmittance and the band gap decrease noticeably from 85% to 58%, and 3.00 to 1.98 eV, respectively. Several significant optical parameters such as Urbach energy, steepness parameter, electron–phonon interaction, refractive index, dielectric constant and static dielectric constant is examined. Enhancement of Cu-O bond length is determined and found similarities between the Rietveld refinement and DFT studies. With the spin polarized density of state (DOS), polarized value lies between 0 and above 1 which confirms that semiconductor to half metal phase transition. Using the Hall Effect measurement, p- to n-type conductivity is determined, and finally a lower resistivity and mobility is found to be 07.83×10−3 Ω cm and 0.003 cm2V−1s−1, respectively at 8 at.% CCO thin film. Thus, the above results depict that 8 at.% of CCO thin films may have potential application in the field of spintronic and optoelectronic devices.

Correlation driven metallic and half-metallic phases in a band insulator

We demonstrate a mechanism for realizing ferrimagnetic metal, antiferromagnetic half-metal, and paramagnetic metal phases by tuning the onsite Coulomb repulsion $U$ in a band insulator. In a simple model of a correlated band insulator, namely the ionic Hubbard model at half-filling, and in the presence of a second neighbor hopping, we show that as $U$ is increased, first he insulating band gap is suppressed to zero at a critical ${U}_{1}$ and remains zero within the paramagnetic metallic phase. Interestingly, at a larger ${U}_{c}$, ferrimagnetic order turns on, leading to a ferrimagnetic metallic phase for ${U}_{c}<U<{U}_{2}$. For $U>{U}_{2}$, the system exhibits an antiferromagnetic half-metallic phase, followed finally by a transition into an antiferromagnetic Mott insulator. Our results, based on dynamical mean-field theory, suggest alternate routes for achieving magnetically ordered metallic and half-metallic phases which have potential applications in spintronics.

First-principles study on the mechanical and electromagnetic properties of porous two-dimensional C7N2 crystals

By uniformly introducing pyrazine defects on monolayer C 3 N, the porous 2D C 7 N 2 (p2D-C 7 N 2 ) crystals were obtained. Based on first-principles calculations, we systematically investigate the mechanical, electronic, and magnetic properties of p2D-C 7 N 2 crystals. The p2D-C 7 N 2 crystals have excellent performance in terms of structural and thermodynamic stability. Furthermore, we systemically study the electronic and magnetic properties of various nanoribbons by cutting from a monolayer p2D-C 7 N 2 crystals along the typical crystallographic orientations. All the nanoribbons are magnetic metals. The C 7 N 2 nanoribbons exhibit sensitive responses to edge modifications using H and O atoms. We find that the edge modifications can induce the spin semiconductor phase, non-magnetic metal phase, half-semiconductor phase, and half-metal phase. These results can open up the two-dimensional material frontier to some extent and can also provide some theoretical support for the potential application and development of nanoelectronic devices based on C 7 N 2 . • The holey 2D C 7 N 2 sheet has excellent performance in terms of structural and thermodynamic stability. • The holey 2D C 7 N 2 has the intrinsic ferromagnetic state magnetic order. • All the original C 7 N 2 nanoribbons are magnetic metals. • The edge modifications by H and O atoms in the C 7 N 2 nanoribbons can induce the different phase.

Multiple Weyl fermions and tunable quantum anomalous Hall effect in 2D half-metal with huge spin-related energy gap

Topological states of matter in two-dimensional (2D) system, especially those combined with magnetism have received special attention currently, because of their new physics and the potential applications in spintronics and nanoscale. At present, realizing multiple topological phases in a single 2D Magnetic material and studying their potential entanglement still face great challenges. Here, we predict a novel 2D topological material namely monolayer EuO2, which can realize multiple types of topological states, namely, type-I Weyl point (WP), type-II WP, critical-type WP, and Quantum anomalous Hall (QAH) effect. Specifically, Without considering Spin-orbit coupling (SOC), these Weyl points perfectly possess the 100% spin-polarization because monolayer EuO2 is a half metal phase with huge spin-related energy gap as large as 6.2 eV. Remarkably, With considering SOC, the topological states in monolayer EuO2 can be tuned by direction of magnetization. Under out-plane magnetization, monolayer EuO2 can realize the QAH effect with single chiral edge state occurring inside the energy gap. Under in-plane magnetization, the system can remain 2D Weyl states at special directions. In other in-plane magnetization, the system shows the in-plane QAH effect with the Chern number of −1 or 1. Therefore, Our work promotes the realization of tunable topological phase in 2D half metals.

A DFT Study of the Electronic, Magnetic and Structural Properties of Rutile VO2

The electronic, magnetic and structural properties of rutile VO2 are investigated by employing density functional theory (DFT). In the high-temperature tetragonal structure (I42/mnm), VO2 is a nonmagnetic metal. All the V-t2g states are partially occupied by the single V 3d electron, which is responsible for the metallic behavior of VO2. The electronic and magnetic properties of rutile VO2 change significantly upon the application of on-site Coulomb interaction U. The system undergoes a first step transition from nonmagnetic metal to a ferromagnetic metallic phase at U = 1 eV. Eventually, VO2 encounters a metal to half-metal transition for U = 2 eV, preserving ferromagnetism in the half-metallic phase. From this study, the polarization of V-3d electrons arising from the electron correlation due to the application of U is accounted for metal to half-metal transition in VO2. The combined effect of p–d hybridizations and the anti-ferromagnetic coupling of V and O atoms is responsible for the ferromagnetism of half-metallic VO2. Nevertheless, an insignificant structural distortion is observed across the metal to half-metal transition.

Two-dimensional FeTe2 and predicted Janus FeXS (X: Te and Se) monolayers with intrinsic half-metallic character: tunable electronic and magnetic properties via strain and electric field.

Driven by the fabrication of bulk and monolayer FeTe2 (ACS Nano, 2020, 14, 11473-11481), we explore the lattice, dynamic stability, electronic and magnetic properties of FeTeS and FeSeS Janus monolayers using density functional theory calculations. The obtained results validate the dynamic and thermal stability of the FeTeS and FeSeS Janus monolayers examined. The electronic structure shows that the FeTe2 bulk yields a total magnetization higher than the FeTe2 monolayer. FeTeS and FeSeS are categorized as ferromagnetic metals due to their bands crossing the Fermi level. So, they can be a good candidate material for spin filter applications. The biaxial compressive strain on the FeTe2 monolayer tunes the bandgap of the spin-down channel in the half-metal phase. By contrast, for FeTeS, the biaxial strain transforms the ferromagnetic metal into a half-metal. The electric field applied to the FeSeS monolayer in a parallel direction transforms the half-metal to a ferromagnetic metal by closing the gap in the spin-down channel.

Structural, electronic and magnetic properties of Mn doped CeO2: An ab-initio study

Onwards the ab-initio approach, we investigate the electronic structure of the Mn doped CeO2 Ceria Fluorite-like. We have studied the stability of the Diluted Magnetic Oxides (DMO)-compounds as function of the impurity. A stable half-metallic ferromagnetic phase is revealed after introducing Mn impurities in the non-magnetic semi-conductor intrinsic CeO2. The total moments of the doped system vary from 0.234 to 0.744 μB for 8% and 25% of Mn. The interaction responsible for magnetism is also determined. Finally, the variation of the Curie temperature, which has reached about 840 (K) for 25% of Mn impurity in instead of 340 (K) for 8% is also determined.

Structural stability, magneto-electronic properties, and tuning effects for transition metal-doped net-Y nanoribbons

Low-dimensional carbon allotropes have attracted much attention due to their great potential for applications in future electronic and magnetic devices. Recently, new graphene-like nanoribbons consisting of four-, six-, and eight-membered carbon rings have been synthesized successfully, and a related two-dimensional structure net-Y was subsequently proposed. Here, we report theoretical investigations on the structural stability and magneto-electronic properties for the net-Y armchair nanoribbons doped with low-concentration transition metal (TM) atoms (Mn, Ni, Fe, Co, and V), especially focusing on their electric-magnetic and mechano-magnetic coupling effects. The calculated binding energy and molecular dynamics simulation confirm that all these ribbons hold a high stability. Particularly, by TM-doping, the intrinsic non-magnetic semiconducting ribbon can be converted to a bipolar magnetic semiconductor (BMS), a half metal (HM), or a spin-degenerate magnetic semiconductor (MSC), presenting rich magnetic features. Meanwhile, this magnetism is flexibly tunable by the transverse electric field or stretched strain. For example, a low electric field can transform the ribbon into the HM phase, and the mechanic strain can realize a continuous magnetic phase transition among BMS, magnetic metal (MS), MSC, HM, and half semiconductor. These findings indicate that the TM-doped net-Y ribbon possesses favorable magnetism, which might have promising applications in magnetic nanodevices.

Understanding the atomistic origin of the magnetic phases in Cobalt-TM (V, Nb, Ta, Zr, Hf, W) pair co-doped boron nitride monolayer and the hydrogenation effect

Abstract 2D nanomaterials have been reported to demonstrate interesting magnetic phases when doped with transition metal double impurities. To understand the atomistic origin of such magnetism in pristine, vacancy defected and cobalt (Co)-transition metal (V, Nb, Ta, Zr, Hf, W) pairs co-doped hexagonal boron nitride nanosheet (h-BNNS), we report a systematic investigation of the structural, electronics and magnetic properties of such transition metal (TM) co-doped 2D monolayer through first-principles calculation based on density functional theory (DFT) using the Generalized Gradient Approximation. Due to the wide band gap of pristine h-BNNS, the electronic structure of TM atoms co-doped systems are determined from the electronic state around the Fermi level of the co-doped system. Co–Zr, Co–Hf, Co–V, Co–Nb and Co–Ta pair co-doped h-BNNS induced half-metallic ferromagnetism as evident from the spin polarized DOS spectra. Our calculation also revealed hydrogenation induced half metallic phase in the Co–W pair co-doped h-BNNSs. The Co-TM and hydrogenated Co-TM co-doped h-BNNSs under study suggest that the ground-state spin polarized ferromagnetic half-metallic (FHM) as well as antiferromagnetic half-metallic (AFHM) phases could be promising materials for applications in the field of spintronics.

Weyl fermions in ferromagnetic high-temperature phase of K2Cr8O16

By combining first-principles calculations and symmetry arguments, we propose that the half-metallic phase of K2Cr8O16 presents ferromagnetic Weyl fermions. In particular, K2Cr8O16 possesses two pairs of Weyl nodes, which originate from two groups of nodal lines connected by the mirror reflection symmetry. We show that the non-trivial topological properties of K2Cr8O16 come from the partially occupied t2g states of Cr, and we also demonstrate that such exotic topological feature is robust. The topological surface states and corresponding Fermi arcs are revealed. As K2Cr8O16 is a realistic and widely studied material, our results suggest that K2Cr8O16 is an ideal candidate for studying ferromagnetic Weyl fermions. In addition, K2Cr8O16 possesses several interesting phenomena, such as the co-existence of charge density wave and Weyl fermions, even pairs of FM Weyl points, and tunable distribution of Weyl points, which will attract intensive attentions in this field.

Photon-Induced Weyl Half-Metal Phase and Spin Filter Effect from Topological Dirac Semimetals.

Recently discovered Dirac semimetals (DSMs) with two Dirac nodes, such as Na_{3}Bi and Cd_{2}As_{3}, are regarded as carrying the Z_{2} topological charge in addition to the chiral charge. We study the Floquet phase transition of Z_{2} topological DSMs subjected to a beam of circularly polarized light. Owing to the resulting interplay of the chiral and Z_{2} charges, the Weyl nodes are not only chirality dependent but also spin dependent, which constrains the behavior in creation and annihilation of the pair of Weyl nodes. Interestingly, we find a novel phase: One spin band is in the Weyl semimetal phase while the other is in the insulator phase, and we dub it the Weyl half-metal (WHM) phase. We further study the spin-dependent transport in a Dirac-Weyl semimetal junction and find a spin filter effect as a fingerprint of the existence of the WHM phase. The proposed spin filter effect, based on the WHM bulk band, is highly tunable in a broad parameter regime and robust against magnetic disorder, which is expected to overcome the shortcomings of the previously proposed spin filter based on the topological edge or surface states. Our results offer a unique opportunity to explore the potential applications of topological DSMs in spintronics.

Strain engineering of magnetic and orbital order in perovskite LuMnO3 epitaxial films

Epitaxial strain has been extensively used to tailor the functionality of perovskite oxides, in which strain control of magnetism is highly desirable, especially for perovskite manganites. Here on the basis of the first-principles calculations we demonstrated the control of magnetic phase and orbital order in ${\mathrm{LuMnO}}_{3}$ film by epitaxial strain imposed by a square substrate and revealed the surface and interface effects by combining the strain-bulk and heterostructure models. The spiral and $E$ -type multiferroic phases present in perovskite manganites bulks can exist in the tensile strained films, while a ferromagnetic half-metal phase with high Curie temperature and an antiferromagnetic polar-metal phase arise in the range of compressive strain. Increasing compressive strain changes the sign of the ${Q}_{3}$ mode of Jahn-Teller distortion, resulting in the transition of the in-plane stagger orbital order to a uniform orbital order. The reconstruction of Jahn-Teller distortion and orbital order occur in the first two layers near the surface. The symmetry breaking of the crystal field at the surface leads to a uniform ${d}_{3{z}^{2}\ensuremath{-}{r}^{2}}$ type orbital order and surface metallization. The electron accumulation at the interface with ${\mathrm{SrTiO}}_{3}$ substrate has been demonstrated and decreases dramatically with the increase in the number of ${\mathrm{LuMnO}}_{3}$ layers.

Emergence of a Multiferroic Half-Metallic Phase in Bi_{2}FeCrO_{6} through Interplay of Hole Doping and Epitaxial Strain.

Epitaxial strain has been shown to drive structural phase transitions along with novel functionalities in perovskite-based thin films. Aliovalent doping at the A site can drive an insulator-to-metal and magnetic transitions in perovskites along with a variety of interesting structural and electronic phenomena. Using first-principles calculations, we predict the formation of a multiferroic half-metallic phase with a large magnetic moment in the double perovskite, Bi_{2}FeCrO_{6}, by coupling epitaxial strain with A-site hole doping. We also demonstrate that epitaxial strain can be used to manipulate the hole states created by doping to induce half-metal to insulator, antipolar to polar, antiferromagnetic to ferromagnetic, orbital ordering and charge ordering transitions. Our work also suggests that hole doping under strain could lead to mitigation of issues related to antisite defects and lowered magnetization in thin films of the material.

Tunable spin transport and quantum phase transitions in silicene materials and superlattices

In this paper, we study spin transport properties of silicene structures such as ribbons and superlattices with the Kane–Mele model. We investigate the effects of ferromagnetic and antiferromagnetic exchange fields, vertical and transverse electric fields and defects on the band structure, density of states, as well as conductance of the system. Our calculated results indicate that by applying a vertical electric field and/or an antiferromagnetic exchange field, metal–semimetal and also metal–semiconductor quantum phase transitions occur. We show that a zigzag silicene ribbon, when exposed to a transverse electric field in combination with an antiferromagnetic exchange field, behaves as a half-metal that allows one spin state electrons to move. Furthermore, under a vertical electric field and in the simultaneous presence of ferromagnetic and antiferromagnetic exchange fields applied to two half-widths of the ribbon, we observe the half-metallic phase. Our results help to control spin currents and to have new applications in silicene spintronics.

Investigation of half-metallic and magnetic phase transition in Co2TiZ (Z = Al, Ga, In) Heusler alloys

ABSTRACTThe half-metallic and ferromagnetic properties of Co2TiZ (Z = Al, Ga, In) Heusler alloys are investigated. The structural stability is analyzed among the two possible structures, namely, L21 (Cu2MnAl phase) and XA (Hg2CuTi phase) structures. It is found that these alloys are stable in L21 (Cu2MnAl phase) structure. The electronic structure of Co2TiZ reveals that these alloys exhibit half-metallic ferromagnetism with small spin-flip gap at the minority spin state. As the pressure is increased, ferromagnetic to non-magnetic phase transition is observed at the pressures of 380.9, 363.0 and 317.8 GPa for Co2TiAl, Co2TiGa and Co2TiIn, respectively. Half-metallic to metallic phase transition occurs in Co2TiAl, Co2TiGa and Co2TiIn at the pressures of 76.5 GPa, 73.1 GPa and 63.9 GPa respectively.

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

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.