Half-metallic State Research Articles

Nanoscale and edge effect on electronic properties of graphene

A brief review is given on nanoscale and edge effect on electronic properties of graphene from theoretical point of view. The presence of edges in graphene has strong implications for low-energy spectrum of the π electrons. The topics include the electronic states of graphene nanoribbons on the basis of tight-binding model, ferrimagnetic edge spin polarization due to the edge states and half-metallic states induced by the electric field effect or edge chemical modification.

Vacancy-driven ferromagnetism in ferroelectric PbTiO3

The possible origin of ferromagnetism in PbTiO3 containing vacancies is investigated by performing first-principles calculations. We demonstrate that O and Ti vacancies both induce ferromagnetism but by different mechanisms: the ferromagnetism driven by the O vacancy originates from the spin-polarized eg state of the nearest Ti atom, whereas that driven by the Ti vacancy is due to the half-metallic px state of the nearest O atom. The results presented here provide fundamental insights into the design of multiferroics in conventional ferroelectrics.

Insulator–metal transition driven by pressure and B‐site disorder in double perovskite La2CoMnO6

The ground state of double perovskite oxide La2CoMnO6 (LCMO) and how it is influenced by external pressure and antisite disorder are investigated systematically by first-principles calculations. We find, on the consideration of both the electron correlation and spin-orbital coupling effect, that the LCMO takes on insulating nature, yet is transformed to half metallicity once the external pressure is introduced. Such tuning is accompanied by a spin-state transition of Co(2+) from the high-spin state (t(5)2ge(2)g) to low-spin state (t(6)2ge(1)g) because of the enhancement of crystal-field splitting under pressure. Using mean-field approximation theory, Curie temperature of LCMO with Co(2+) being in low-spin state is predicted to be higher than that in high-spin state, which is attributed to the enhanced ferromagnetic double exchange interaction arising from the shrinkage of Co-O and Mn-O bonds as well as to the increase in bond angle of Co-OMn under pressure. We also find that antisite disorder in LCMO enables such transition from insulating to half-metallic state as well, which is associated with the spin-state transition of antisite Co from high to low state. It is proposed that the substitution of La(3+) for the rare-earth (RE) ions with smaller ionic radii could open up an avenue to induce a spin-state transition of Co, rendering thereby the RE2CoMnO6 a promising half-metallic material.

First-principles study on the origin of ferromagnetism in n-type Cu-doped ZnO

Based on the density functional calculations with the GGA+U correction, we elucidate the origin of the experimentally reported ferromagnetism in n-type Cu-doped ZnO. Pure Cu-doped ZnO shows the unoccupied 3d states in the gap introduced by Cu, resulting in the insulating ground state and weak magnetic exchange interactions, in contrast to the half-metallic ground state and high ferromagnetic stability predicted by the calculations without U correction. However, the electron traps induced by Cu in n-type Cu-doped ZnO may lead to the partial occupancy of the Cu gap states, which stabilize the ferromagnetic ordering between two Cu atoms.

First-principles linear muffin-tin orbital investigations in the electronic and magnetic structures of double perovskites Ba2TMoO6 (T=V, Cr, Mn, Fe and Co)

The electronic and magnetic structures of ordered double perovskites Ba2TMoO6 (T=V, Cr, Mn, Fe and Co) are systematically investigated by means of the first-principle linear muffin-tin orbitals with the atomic-sphere approximation (LMTO-ASA) method. The calculations are performed by using the both local spin density approximation (LSDA) and the LSDA+U Coulomb interaction schemes. The results show a half-metallic ferrimagnetic ground states for T=Cr, Fe and Co in LSDA+U treatment, whereas half-metallic ferromagnetic character is observed for T=V. For T=Mn, insulating ground state is obtained, stabilized in the antiferromagnetic state. The LSDA+U calculations yield better agreement with the theoretical and the experimental results than do the LSDA.

Electronic structure and magnetic properties of metallocene multiple-decker sandwich nanowires

We present a study of the electronic and magnetic properties of the multiple-decker sandwich nanowires ($CP-M$) composed of cyclopentadienyl (CP) rings and 3d transition metal atoms (M=Ti to Ni) using first-principles techniques. We demonstrate using Density Functional Theory that structural relaxation play an important role in determining the magnetic ground-state of the system. Notably, the computed magnetic moment is zero in $CP-Mn$, while in $CP-V$ a significant turn-up in magnetic moment is evidenced. Two compounds show a half-metallic ferromagnetic ground state $CP-Fe/Cr$ with a gap within minority/majority spin channel. In order to study the effect of electronic correlations upon the half-metallic ground states in $CP-Cr$, we introduce a simplified three-bands Hubbard model which is solved within the Variational Cluster Approach. We discuss the results as a function of size of the reference cluster and the strength of average Coulomb $U$ and exchange $J$ parameters. Our results demonstrate that for the range of studied parameters $U=2-4eV$ and $J=0.6-1.2eV$ the half-metallic character is not maintained in the presence of local Coulomb interactions.

Quaternary Heusler compounds Co2−xRhxMnZ (Z = Ga, Sn, Sb): crystal structure, electronic structure, and magnetic properties

Within the huge family of Heusler compounds only a few quaternary derivatives are known that crystallize in the space group. In this work, the yet unreported compounds CoRhMnZ (Z = Ga, Sn, Sb) and the alloy Co0.5Rh1.5MnSb were investigated in detail by experimental techniques and theoretical methods. The ab initio calculations predict the CoRhMnZ compounds to be half-metallic ferromagnets or to be close to the half-metallic ferromagnetic state. Calculations of the elastic constants show that the cubic structure is stable in compounds containing Mn. Both calculations and experiment reveal that Mn cannot be exchanged by Fe (CoRhFeGa). The low temperature magnetization of the compounds is in the range of 3.4–5.5 μB depending on the composition. The best agreement between experiment and calculation has been achieved for CoRhMnSn (5 μB). The other compounds are also cubic but tend to anti-site disorder. Compared to Co2MnSn it is interesting to note that the magnetic properties and half-metallicity are preserved when replacing one of the ‘magnetic’ Co atoms by a ‘non-magnetic’ Rh atom. This allows us to increase the spin–orbit interaction at one of the lattice sites while keeping the properties as a precondition for applications and physical effects relying on a large spin–orbit interaction. The Curie temperatures were determined from measurements in induction fields of up to 1 T by applying molecular field fits respecting the applied field. The highest Curie temperature was found for CoRhMnSn (620 K) that makes it, together with the other well defined properties, attractive for above room temperature spintronic applications.

Electronic and magnetic properties of SrCoO3:the first principles study

Using density functional calculations within the generalized gradient approximation and Hubbard U framework, the electronic and the magnetic properties of SrCoO3 are investigated. The result shows that the spin-up t2g and eg states of Co shift toward the lower energy with the increase of the U value, whereas the spin-down t2g and eg states of Co shift toward the higher energy. The O 2p state almost does not shift with the increase of U value. The electronic structure of SrCoO3 changes from metal state into half-metal state around U of 7-8 eV. The magnetic moment of Co ions increases linearly with U increasing for U 7.0 eV, and almost does not change for U 8.0 eV. Compared with the experimental results, U = 8.0 eV is thought to be suited for the study of SrCoO3. The result shows that with U = 8.0 eV, the magnetic moment on Co site is 3.19 B and SrCoO3 has the half-metallic nature.

Orbital ordering and its origin of KCrF3

The electronic, the magnetic and the orbital structures of KCrF3 are studied by first principles in its recently identified crystallographic phases (tetragonal and cubic) [Margadonna and Karotsis 2006 J. Am. Chem. Soc. 128 16436]. In tetragonal phase, both generalized gradient approximation (GGA) and GGA+U calculations show that the ground state is the A-type antiferromagnetic (A-AFM) configuration with G-type orbital ordering pattern. Our calculations show that the orbital structures and the magnetic configurations can be measured by the optical conductivity. In the cubic state, the GGA calculations show that the ground state is ferromagnetic half metal state, while the GGA+U(Ueff = 3.0 eV) calculations show that the A-AFM insulator phase is the ground state. Our calculations indicate that the electron-electron interactions rather than the electron-phonon interactions are the driving forces behind the orbital order.

Fermi surfaces of iron pnictide high-Tcsuperconductors from the limit of local magnetic moments

A two-orbital $t$-$J$ model over the square lattice that describes low-energy electronic excitations in iron-pnictide high-${T}_{c}$ superconductors is analyzed with Schwinger-boson-slave-fermion mean-field theory and by exact numerical diagonalization on a finite system. When interorbital hole hopping is suppressed, a quantum critical point (QCP) is identified that separates a commensurate spin-density wave (cSDW) state at strong Hund's rule coupling from a hidden half metal state at weak Hund's rule coupling. Low-energy spin waves that disperse anisotropically from cSDW momenta are predicted at the QCP. Nested Fermi surfaces similar to those observed experimentally in iron-pnictide materials are also predicted in such case.

Theory of Half-Metallic Ferrimagnetism in Double Perovskites

Double perovskites such as Sr(2)FeMoO(6) are rare examples of materials with half-metallic ground states and a ferrimagnetic T(c) above room temperature. We present a comprehensive theory of the temperature and disorder dependence of their magnetic properties by deriving and validating a new effective spin Hamiltonian for these materials, amenable to large-scale three-dimensional simulations. We show how disorder, ubiquitous in these materials, affects T(c), the magnetization, and the conduction electron polarization. We conclude with a novel proposal to enhance T(c) without sacrificing polarization.

Electron transport through the p-n junction of zigzag graphene nanoribbon with external transverse electric fields

We study the electron transport properties of the p-n junction based on zigzag graphene nanoribbon, including its magnetic moment, charge-density distribution, and also the spin-resolved transmission probability. A nanoscale system of width ≈2 nm and length ≈15 nm is considered. It is shown that the magnetization of the edges is quenched in the p and n regions, but can be preserved in the center part of the p-n junction. We find that the electrons can tunnel through the junction by the localized modes, which exist at the interface of the p and n regions. In the presence of external transverse electric fields, we show that the electron current through the p-n junction will be spin-polarized when the zigzag graphene nanoribbon is in a half-metal state, and the current is switched off when a charge-polarized state is obtained.

ELECTRON DOPING EFFECTS ON ELECTRONIC AND MAGNETIC PROPERTIES IN Sr2FeReO6

We have investigated the electronic and magnetic properties of electron-doped Sr 2-x La x FeReO 6 (x = 0.0, 0.25, 0.5) using first-principles density functional theory within the generalized gradient approximation (GGA) and GGA + U schemes. Our results reveal that the symmetry of the La -doped compounds is decreased from tetragonal I4/m for perfect sample to monoclinic P21/n. With increasing La content the absolute magnetic moment of the Re site increases distinctly and the doped electrons are considered to occupy mainly the down-spin Re 5d band from the band calculation. Electronic doping is found to enhance the Curie temperature (T c ) and stabilize the ferromagnetic half-metallic ground states of Sr 2 FeReO 6. And it is found that the increase of T c is mainly caused by the increase of ferromagnetic interaction between the Fe – O – Fe .

Magnetism induced by nonmagnetic dopant in Li2O, Na2O, K2O and Rb2O: first-principles calculations

First-principles calculations based on the tight-binding linear muffin-tin orbital (TB-LMTO) method are performed to investigate the occurrence of spin polarization in the alkali-metal oxides (M2O) [M: Li, Na, K, Rb] in antifluorite (anti-CaF2-type) structure with non-magnetic sp (F, Cl, Br and I) dopants. The calculations reveal that non-magnetic substitutional doping at cation site can induce stable half-metallic ferromagnetic ground state in I2-VI compounds. Total energy calculations show that the antifluorite ferromagnetic state is energetically more stable than the antifluorite non-magnetic state at equilibrium volume. Ground state properties such as equilibrium lattice constant and bulk modulus were calculated. The calculated magnetic moment is found to be 2.00 μB per dopant atom.

Magnetic and electronic properties of ordered double-perovskite La2VMnO6thin films

We report the stabilization of the $B$-site-ordered phase of epitaxial La${}_{2}$VMnO${}_{6}$ double-perovskite films grown by pulsed-laser deposition. Previous theoretical calculation predicted the possibility of realizing a spin-compensated half-metallic state in this system with an Mn${}^{3+}$ ion at a low spin state. In contrast, our magnetic measurements suggest that the system in ordered phase is ferrimagnetic with Mn${}^{3+}$ at a high spin state. Synchrotron radiation photoemission measurement indicates that the electronic ground state of this material is insulating: occupied Mn 3$d$ ${t}_{2g}$, Mn 3$d$ ${e}_{g}$, and V 3$d$ ${t}_{2g}$ states are located far from the Fermi level. The optical band gap is estimated to be about 0.9 eV, which is similar to the charge transfer gap of LaMnO${}_{3}$ based on the optical absorption spectra.

Electronic and crystallographic structure, hard x-ray photoemission, and mechanical and transport properties of the half-metallic Heusler compound Co2MnGe

This work reports on the electronic and crystalline structure and the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co${}_{2}$MnGe. The crystalline structure was examined in detail by extended x-ray absorption fine-structure spectroscopy and anomalous x-ray diffraction. The compound exhibits a well-ordered $L{2}_{1}$ structure as is typical for Heusler compounds with 2:1:1 stoichiometry. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab initiocalculations. Transport measurements and hard x-ray photoelectron spectroscopy were performed to explain the electronic structure of the compound. The obtained valence band spectra exhibit small energy shifts that are the result of the photoexcitation process, whereas electron-electron correlation in the ground state is negligible. The vibration and mechanical properties of the compound were calculated. The observed hardness values are consistent to a covalent-like bonding of Co${}_{2}$MnGe.

Zigzag graphene nanoribbons without inversion symmetry

Graphene on a substrate will suffer an inversion-symmetry-breaking (ISB) lattice potential. Taking electron-electron interaction into account, we study in this paper the possibility of half-metallicity and noncollinear (NC) magnetic phase for graphene zigzag nanoribbons without inversion symmetry. At half-filling it is found that half-metallic(HM) state can be achieved at an intermediate value of the ISB potential due to its competition with the electron-electron interaction. Away from half-filling, the phase diagrams of doping versus ISB potential for different ribbon width are given, where the regimes for the HM states and NC magnetic state are clearly indicated and discussed. For ribbons with perfect edges, we predict a topological transition between two HM states with different magnetic structures, which is accompanied by an abrupt transition of electrical conductance along the ribbon from $2e^2/h$ to $e^2/h$.

Incommensurable state of a spin density wave and superconductivity in quasi-two-dimensional systems with an anisotropic energy spectrum

We investigate phase transitions in quasi-two-dimensional systems with an anisotropic energy spectrum and a deviation from the half-filling of the energy band (µ ≠ 0). We demonstrate the possibility of the transition of an insulator into a half-metallic state when the nesting condition is violated because the parameter µ ≠ 0 and of taking the umklapp processes into account. We obtain the basic equations for the parameters of the superconducting (Δ) and magnetic (M) orders and determine the conditions for the emergence of superconductivity on the background of a spin-density-wave state and also for the coexistence of superconductivity and magnetism. We show that the transition of a magnetic system into a superconducting state as the parameter µ increases can be a first-order phase transition at low temperatures. We also obtain an expression for the heat capacity jump C S -C N at T = T c , which depends on M and µ and differs essentially from the case of the Bardeen-Cooper-Schrieffer theory. We also consider the transformations related to the density of electron states of the relevant anisotropic system, which can undergo essential changes under pressure or doping.

Half-metallic surface states and topological superconductivity in NaCoO2from first principles

Based on first-principles calculations, we predict a half-metallic surface state in layered bulk insulator NaCoO${}_{2}$, with tunable surface hole concentration. The half-metallic surface has a single Fermi surface with a helical spin texture, similar to the surface state of topological insulators, but with the key difference of time-reversal symmetry breaking in the present case. We propose the realization of topological superconductivity and Majorana fermions when the half-metallic surface states are in proximity contact with a conventional superconductor.

Edge States and Half-Metallicity in TiO2 Nanoribbons

We apply first-principles calculations to investigate the existence and properties of edge states in TiO2 nanoribbons. We show that edge states may be found either in the gap region or defining the bottom of the conduction band. In both cases, the addition of electrons to the ribbons, which is facilitated by the large work function of TiO2 materials, may drive the systems to a half-metallic state, with conduction taking place along the edges and with only one spin component. We also show that a U-negativity phenomenology may show up in some configurations, resulting in the stabilization of the charged edges.