Enhanced half-metallicity in edge-oxidized zigzag graphene nanoribbons

We present a comprehensive theoretical study of the electronic properties and relative stabilities of edge-oxidized zigzag graphene nanoribbons. The oxidation schemes considered include hydroxyl, lactone, ketone, and ether groups. Using screened exchange density functional theory, we show that these oxidized ribbons are more stable than hydrogen-terminated nanoribbons except for the case of the etheric groups. The stable oxidized configurations maintain a spin-polarized ground state with antiferromagnetic ordering localized at the edges, similar to the fully hydrogenated counterparts. More important, edge oxidation is found to lower the onset electric field required to induce half-metallic behavior and extend the overall field range at which the systems remain half-metallic. Once the half-metallic state is reached, further increase of the external electric field intensity produces a rapid decrease in the spin magnetization up to a point where the magnetization is quenched completely. Finally, we find that oxygen-containing edge groups have a minor effect on the energy difference between the antiferromagnetic ground state and the above-lying ferromagnetic state.     

Ferromagnetism and the Optical Properties of Mn-Doped CdSe with the Wurtzite Structure

The geometrical structure of CdSe was optimized by using the ultrasoft pseudopotential method of a total energy plane wave based on density functional theory. The band structure, density of states, and optical properties were calculated and discussed in detail. The Mn-doped CdSe is found to be a half-metallic ferromagnet with 100% carrier spin polarization at the Fermi level. At a Mn concentration of 12.5%, the calculated total energy of the spin-polarized state is 614 meV lower than that of the nonspin-polarized state. The net magnetic moment of 5 μ _B is found per supercell for 12.5% Mn-doped CdSe. The estimated Curie temperature of 748.6 K for Mn-doped CdSe is above room temperature. The ferromagnetic ground state in Mn-doped CdSe can be explained in terms of the p ? d hybridization mechanism. These results suggest that Mn-doped CdSe may present a promising dilute magnetic semiconductor, and may have potential applications in the field of spintronics.     

Functionalization of low-dimensional honeycomb germanium with 3d transition-metal atoms

The energetics, electronic and magnetic properties of 3d transition-metal (TM) atoms adsorbed low-dimensional Ge honeycomb structures have been systematically investigated from the spin-polarized density-functional theory calculations. For the two-dimensional Ge honeycomb structure, all TM atoms considered prefer to adsorb on the hollow site of the buckled hexagon in both single-sided and double-sided adsorption cases, with binding energies ranging between 3.27 and 5.92 eV. Upon adsorption, the semimetallic 2D honeycomb Ge can change to either ferromagnetic or antiferromagnetic metals depending on both TM species and coverage density. For the one-dimensional structure, we found binding of TM atoms to hollow site of the edge hexagon yields the minimum energy state for all TM species considered and in all three AGeNRs examined which belong to different families. Depending on ribbon width, adsorbed TM species and adsorption concentration, most of the TM decorated AGeNRs can either be metals or semiconductors with ferromagnetic or antiferromagnetic spin alignment, except for Co-adsorbed ones which remain to be nonmagnetic. Interestingly, Cr or Mn adsorption can make certain AGeNRs to be half-metallic with a 100% spin-polarization at Fermi level which can be good candidates for future application in spintronic fields. Furthermore, the effect of the on-site Coulomb interaction on the stability of these half-metallic ribbons is also considered by performing L(S)DA + U calculations, and the results show that the half-metallic ground state of the Cr-adsorbed ribbons is more robust than that of the Mn-adsorbed one.     

Magnetic boron nitride nanoribbons with tunable electronic properties

We present theoretical evidence, based on total-energy first-principles calculations, of the existence of spin-polarized states well localized at and extended along the edges of bare zigzag boron nitride nanoribbons. Our calculations predict that all the magnetic configurations studied in this work are thermally accessible at room temperature and present an energy gap. In particular, we show that the high spin state, with a magnetic moment of 1 muB at each edge atom, presents a rich spectrum of electronic behaviors as it can be controlled by applying an external electric field in order to obtain metallic <--> semiconducting <--> half-metallic transitions.     

Theoretical Investigation of Cubic BaVO<Subscript>3</Subscript> and LaVO<Subscript>3</Subscript> Perovskites via Tran–Blaha-Modified Becke–Johnson Exchange Potential Approach

The full potential linearized augmented plane wave method of density functional theory has been used to investigate the structural, electronic, magnetic and thermoelectric properties of cubic perovskites BaVO₃ and LaVO₃. The ferromagnetic ground state has been found to be stable by comparing the total energies of non-spin-polarized and spin-polarized calculations performed for optimized unit cells. For both compounds, the bond length and tolerance factor are also measured. From the band structures and density of states plots, it is found that both compounds are half-metallic. We found that the presence of V at the octahedral site of these perovskites develops exchange splitting through p-d hybridization, which results in a stable ferromagnetic state. The observed exchange splitting is further clarified from the magnetic moment, charge and spin of the anion and cations. Finally, we also presented the calculated thermoelectric properties of these materials, which show that half-metallic BaVO₃ and LaVO₃ materials are potential contenders for thermoelectric applications.     

Magnetism induced by nonmagnetic dopant in Li<Subscript>2</Subscript>O,Na<Subscript>2</Subscript>O,K<Subscript>2</Subscript>O and Rb<Subscript>2</Subscript>O: 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.     

Quantum Phase Transition in the BCS-to-BEC Evolution of p-wave Fermi Gases

We discuss the possibility of a quantum phase transition in ultra-cold spin-polarized Fermi gases which exhibit a p-wave Feshbach resonance. We show that when fermionic atoms form a condensate that can be externally tuned between the BCS and BEC limits, the zero temperature compressibility and the spin susceptibility of the fermionic gas are non-analytic functions of the two-body bound state energy. This non-analyticity is due to a massive rearrangement of the momentum distribution in the ground state of the system. Furthermore, we show that the low temperature superfluid density is also non-analytic and exhibits a dramatic change in behavior when the critical value of the bound state energy is crossed.     

Graphene/Half-Metallic Heusler Alloy: A Novel Heterostructure toward High-Performance Graphene Spintronic Devices

Graphene-based vertical spin valves (SVs) are expected to offer a large magnetoresistance effect without impairing the electrical conductivity, which can pave the way for the next generation of high-speed and low-power-consumption storage and memory technologies. However, the graphene-based vertical SV has failed to prove its competence due to the lack of a graphene/ferromagnet heterostructure, which can provide highly efficient spin transport. Herein, the synthesis and spin-dependent electronic properties of a novel heterostructure consisting of single-layer graphene (SLG) and a half-metallic Co₂Fe(Ge_0.5Ga_0.5) (CFGG) Heusler alloy ferromagnet are reported. The growth of high-quality SLG with complete coverage by ultrahigh-vacuum chemical vapor deposition on a magnetron-sputtered single-crystalline CFGG thin film is demonstrated. The quasi-free-standing nature of SLG and robust magnetism of CFGG at the SLG/CFGG interface are revealed through depth-resolved X-ray magnetic circular dichroism spectroscopy. Density functional theory (DFT) calculation results indicate that the inherent electronic properties of SLG and CFGG such as the linear Dirac band and half-metallic band structure are preserved in the vicinity of the interface. These exciting findings suggest that the SLG/CFGG heterostructure possesses distinctive advantages over other reported graphene/ferromagnet heterostructures, for realizing effective transport of highly spin-polarized electrons in graphene-based vertical SV and other advanced spintronic devices.     

Electronic and Magnetic Properties of SnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Spinel Ferrites

In this work, the electronic and magnetic properties of SnFe₂O₄ spinel ferrite with various cation distributions (mixed, normal and inverse spinel phases) were studied. The calculations were performed by Korringa–Kohn–Rostoker (KKR)-coherent potential approximation (CPA) method with generalized gradient (GGA) approximation for the exchange and correlation functional. Our spin-polarized calculations give a half-metallic character for SnFe₂O₄ inverse spinel phase and near half-metallic character for SnFe₂O₄ mixed spinel phase and metal character for SnFe₂O₄ normal spinel phase. These results, which contribute to understanding the effect of the cation distribution in SnFe₂O₄ ferrite with spinel structure in the existence of gap states occupied by majority and minority spin electrons, the spin magnetic moments, the magnetization and the half-metallic behaviour.     

First-Principles Investigation of Half-metallic Ferromagnetism in V-doped BeS, BeSe, and BeTe

We have investigated the electronic structure and half-metallic ferromagnetism in zinc blende phase of Be_1?x V _x M (M=S, Se, Te) at concentration x=0.125 by employing a first-principles calculations within the framework of density functional theory (DFT) based on the linearized augmented plane wave method (FP-LAPW), as implanted in the WIEN2k code with generalized gradient approximation functional proposed by Wu and Cohen (WC-GGA). The electronic properties exhibit half-metallic behavior. So the density of states shows the hybridization between the p (S, Se, Te) and 3d (V) states that creates the antibonding states in the gap, which stabilizes the ferromagnetic ground state associated with the double-exchange mechanism, whereas the spin polarized band structures depict half-metallic gap that increases from Be_0.875V_0.125S to Be_0.875V_0.125Se to Be_0.875V_0.125Te. These compounds are robust half-metallic ferromagnets with spin polarization of 100 % and predicted to be potential candidates for spin injection applications in spintronic devices. Therefore, our predictions require an experimental confirmation in the future.     

Spin Dynamics in the Ferromagnetic Phase of Colossal Magnetoresistive Oxides

A review is given that focuses on the spin dynamics in the ferromagnetic regime of the magnetoresistive oxides. At small wave vectors the quadratic dispersion relation is remarkably isotropic throughout the composition range, with a gap that is too small to measure with conventional neutron scattering techniques. At larger wave vectors the spin waves are strongly damped in the ground state, in contrast to expectations based on a simple half-metallic band structure, while the temperature dependence of the damping and renormalization of the spin waves is anomalous. An unusual spin-diffusion component to the fluctuation spectrum develops as the Curie temperature is approached, which appears to be related to the formation of polarons in the system and the consequent charge localization. Near and above T_C the diffuse scattering from polarons is directly observed, as well as the correlations between the polarons.     

Theoretical Investigation of Structural,Electronic, and Magnetic Properties of V-Doped MgSe and MgTe Semiconductors

In this study, we have explored the structural, electronic, and magnetic properties of V-doped zincblende MgSe and MgTe compounds using density functional calculations. The Wu-Cohen generalized gradient approximation is used for optimizing the structural properties, while the modified Becke and Johnson local (spin) density approximation functional has been employed to compute the electronic and magnetic properties. The spin dependent band structures, electronic density of state, and magnetic moments calculated for V-doped MgSe and MgTe semiconductors exhibit occurrence of 100 % spin polarization at the Fermi level which confirms stable half-metallic ferromagnetism in these materials. The spin-down gaps and the half-metallic gaps are analyzed in terms of V-3d and Se-4p (Te-5 p) hybridization, where it is observed that the V-3dstates play a key role in generating spin polarization and the magnetic moment in these compounds. The exchange constants N ₀ αand N ₀ β have been calculated to demonstrate the effects resulting from exchange splitting process. Furthermore, spin-polarized charge density calculation is presented for elucidating the bonding nature, while pressure dependence of total magnetic moment for three concentrations of V-doped MgSe and MgTe are also discussed.     

YN<Subscript>2</Subscript> monolayer: Novel p-state Dirac half metal for high-speed spintronics

In spintronics,it is highly desirable to find new materials that can simultaneously possess complete spin-polarization,high-speed conduction electrons,large Curie temperature,and robust ferromagnetic ground states.Using first-principles calculations,we demonstrate that the stable YN2 monolayer with octahedral coordination is a novel p-state Dirac half metal (DHM),which not only has a fully spin-polarized Dirac state,but also the highest Fermi velocity (3.74 x 105 m/s) of the DHMs reported to date.In addition,its half-metallic gap of 1.53 eV is large enough to prevent the spin-flip transition.Because of the strong nonlocal p orbitals of N atoms (N-p) direct exchange interaction,the Curie temperature reaches over 332 K.Moreover,its ferromagnetic ground state can be well preserved under carrier doping or external strain.Therefore,the YN2 monolayer is a promising DHM for high-speed spintronic devices and would lead to new opportunities in designing other p-state DHMs.     

Spin-Injection

During the past decade there has been a growing interest in the properties of electronic systems driven out of equilibrium by the injection of spin-polarized carriers. Since ferromagnetic metals are characterized by majority and minority spin sub-bands, such a state can be realized and studied by current biasing structures consisting of ferromagnetic/non-ferromagnetic layers. This review describes several aspects of this area of research including the concept and measurement of spin polarization, the generic spin injection structure, spin-injection into various materials (noble metals, semiconductors, and superconductors), and magnetic tunneling phenomena.     

Electronic,Elastic, and Magnetic Properties of the Full-Heusler with the 4d Transition Metal Element,Co<Subscript>2</Subscript>YSi,Co<Subscript>2</Subscript>ZrSi,and Co<Subscript>2</Subscript>Y<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>Si: a First-Principle Study

The full-potential all-electron linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method, as implemented in the suite of software WIEN2K, has been used to systematically investigate the structural, electronic, elastic, and magnetic properties of the half-metallic ferromagnetic Heusler compounds with 4d transition elements Co₂YSi, Co₂ZrSi, and alloy Co2Y_0.5Zr_0.5Si presented. The theoretical formalism used is the generalized gradient approximation to density functional theory (GGA) with the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional. The calculated lattice parameters and magnetic moments agree well with the available theoretical results. The spin-polarized density of states (DOS) of Co₂YSi and Co₂ZrSi indicate a half-metallic behavior with vanishing electronic density of states for minority spin at the Fermi level, which yields a perfect spin polarization while for Co₂Y_0.5Zr_0.5Si shows a nearly half-metallic behavior with small spin-down electronic density of states at the Fermi level.     

Spin-Injection

During the past decade there has been a growing interest in the properties of electronic systems driven out of equilibrium by the injection of spin-polarized carriers. Since ferromagnetic metals are characterized by majority and minority spin sub-bands, such a state can be realized and studied by current biasing structures consisting of ferromagnetic/non-ferromagnetic layers. This review describes several aspects of this area of research including the concept and measurement of spin polarization, the generic spin injection structure, spin-injection into various materials (noble metals, semiconductors, and superconductors), and magnetic tunneling phenomena.     

GGA and GGA + <Emphasis Type="Italic">U</Emphasis> Study of Rare Earth-Based Perovskites in Cubic Phase

The structural, elastic, electronic, and magnetic properties of cubic perovskite RAlO₃ (R = Sm, Eu, Gd, Dy, Tb, Ho, Tm, Er, Yb) compounds have been calculated using a full-potential linearized augmented plane wave (FP-LAPW) method within the density functional theory. The exchange-correlation potential was treated with the generalized gradient approximation of Wu and Cohen (WC-GGA) to calculate the total energy. Moreover, the GGA + U?based potential was also applied for the electronic and magnetic properties. The calculated structural properties such as lattice parameter are consistent with the accessible data. The spin-polarized electronic band structure and the calculation of density of state show that DyAlO₃, EuAlO₃, SmAlo₃, TmAlO₃, HoAlO₃, YbAlO₃, and TbAlO₃ compounds have a half-metallic nature, and only GdAlO₃ and ErAlO₃ have a semiconductor nature. The spin-polarized magnetic moment of these compounds reveals that they show a ferromagnetic nature.     

A First-Principles Calculation on Structural,Electronic, Magnetic,Mechanical, and Thermodynamic Properties of SrAmO<Subscript>3</Subscript>

For the first time, the americium-based perovskite SrAmO₃ has been studied with respect to its structural, electronic, magnetic, mechanical, and thermodynamic properties. The study has been carried within the well-known density functional theory (DFT) using different approximations such as local spin density approximation (LSDA), generalized gradient approximation (GGA), LSDA + U, GGA + U. In order to check for the stable ground state, optimization was performed for non-magnetic, ferromagnetic, and anti-ferromagnetic phases, and the compound was found to be stable in the ferromagnetic phase. The spin magnetic moment was obtained with different exchange correlations and was found to be an integer which is one of the consequences of half-metallic nature. The half-metallic nature of SrAmO₃ was also confirmed from spin-polarised band structure calculations using GGA, GGA + U, and mBJ, showing metallic nature in spin-up states and semi-conducting in spin-down states. The elastic constants, Young modulus, shear modulus, Poisson ratio, and anisotropic factor were also calculated. SrAmO₃ was found to establish ductile and anisotropic nature. Debye temperature was predicted to be 353 K from elastic constants. The thermodynamic properties, like variation of specific heat capacity, thermal expansion, and entropy, were studied in the temperature range of 0 to 600 K.     

Magnetic properties of vacancies on the triangular plane 3He crystal

The ground state of the plane quantum Fermi crystal with a triangular lattice is considered. Using symmetry properties, the ground state wave function is constructed and a variational estimate of the ground state energy is obtained. The ground state corresponds to an unsaturated ferromagnet with spin equal to one-third of the maximum spin. Some other nonalternate lattices are also considered. The spectrum of elementary excitations for a finite concentration of the vacancies is studied. The existence of low-lying spin excitations with a small gap is predicted. Ordinary gapless spin waves also exist.     

Prediction of Half-Metallic Properties in Non-transition Metal-based Binary Compounds <Emphasis Type="Italic">X</Emphasis> Bi (<Emphasis Type="Italic">X</Emphasis> = Ba,Sr and Ca) with Zinc-Blende and Wurtzite Structures

In this study, structural, electronic and magnetic properties of non-transition metal-based binary compounds X Bi (X = Ba, Sr and Ca) in five different phases: rock salt, NiAs, wurtzite, zinc blende and CsCl, are investigated in order to find new sp magnetic materials used for real spintronic and other related applications. The calculations are performed by a developed full-potential augmented plane wave plus local orbitals (FP-L/APW + lo) method within the spin density functional theory. As exchange-correlation potential, we used the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) and the modified Becke-Johnson potential mBJ-GGA-PBE form. It is found that the magnetic moment in these compounds is mainly contributed by the spin-polarized p orbitals of Bi atoms. The WZ XBi are true half-metallic ferromagnet with opposite spin direction at Fermi energy having a magnetic moment of 2.00 μ B per formula unit.