Ferromagnetism,half-metallicity and spin-polarised electronic structures characterisation insights in Ca1−xTixO

ABSTRACT

In this study, we have computed the structural, electronic and half-metallic ferromagnetic properties of Ca_1?xTi_xO compounds at concentrations x?=?0.125, 0.25, 0.5 and 0.75 by employing the first-principle approaches of density functional theory. The generalised gradient approximation of Wu and Cohen (GGA-WC) is used to calculate the structural parameters, whereas the electronic structures and magnetic properties are characterised by the accurate Tran–Blaha-modi?ed Becke–Johnson potential (TB-mBJ). The lattice constant, bulk modulus and indirect gap of CaO are in good agreement with other theoretical and experimental results. The Ca_0.25Ti_0.75O at x?=?0.75 has metallic ferromagnetic nature. The Ca_0.875Ti_0.125O, Ca_0.75Ti_0.25O and Ca_0.50Ti_0.50O compounds have total magnetic moments of 2?μ_B per Ti atom with a half-metallic character, a spin polarisation of 100% and a large half-metallic gap of 1.345?eV for x?=?0.125. Therefore, the Ca_1?xTi_xO material with a low concentration of Ti is a true half-metallic ferromagnet and seems to be a promising candidate for semiconductor spintronics.     

Spintronic properties of the Ti2CoB Heusler compound by density functional theory

The electronic structure and magnetic properties of the Ti₂CoB Heusler compound with a high-ordered CuHg₂Ti structure were investigated using the self-consistent full potential linearized augmented plane wave (FPLAPW) method within the density functional theory (DFT). Spin-polarized calculations show that the Ti₂CoB compound is half-metallic ferromagnetic with a magnetic moment of 2 μ_B at the equilibrium lattice constant, a=5.74 Å. The Ti₂CoB Heusler compound is ferromagnetic below the equilibrium lattice constant and ferrimagnetic above the equilibrium lattice constant. A large peak in majority-spin DOS and an energy gap in minority-spin DOS are observed at the Fermi level, yielding a spin polarization of 100%. A spin polarization higher than 90% is achieved for a wide range of lattice constants between 5.6 and 6.0 Å.     

Surface effects on the magnetic properties of Co2FeAl(0 0 1): An ab initio study

Electronic structures of the Co₂FeAl(0 0 1) surface are studied theoretically via first-principles calculations based on density functional theory. It is found that the minority spin band gap at the Fermi level in bulk Co₂FeAl disappears at the surface due to space localization of the states. However, beneath the surface, the density of states of individual atoms shows a trend of minority spin gap opening at the Fermi level, which indicates that the electronic structures become close to that of bulk Co₂FeAl. The termination of FeAl is more favorable for spin polarization of Co₂FeAl films than that of Co. Accordingly, we present a composite tri-layer model to illustrate the fading of the half-metallic property in Co₂FeAl films against the ideal character in bulk materials.     

First principles study of the electron structures of CaCu3Mn4O12 and CaCu3Ti4O12

The electronic structures of CaCu₃Mn₄O₁₂ and CaCu₃Ti₄O₁₂ are investigated from HF SCF LCAO calculation. In CaCu₃Mn₄O_12, the band and the density of states show a spin asymmetric ferrimagnetic character with a small energy gap. The Mn spin is anti-aligned with the Cu spin, and the total spin moment is 9 μ_B. Our calculation correctly reproduces the observed antiferromagnetic insulating character of CaCu₃Ti₄O₁₂. The gap in the band structure, which is 2.15 eV, reasonably agrees with the experimental value 1.5 eV. The electron density populations at different planes show clearly that the electron density has symmetric character. A tilted Mn(Ti) orbital implies a typical tilted three-dimensional network of MnO₆ (TiO₆) octahedra due to doping of the Jahn–Teller ion Cu. There is no covalency between Ca, Cu and Mn(Ti) atoms. In contrast, there are stronger bonds and somewhat likely covalency between Cu and O atoms, and also between Mn(Ti) and O atoms.     

The electronic and magnetic properties with intrinsic defects in ZnO nanosheets: First-principles prediction

Using full-potential linearized augmented plane wave (FLAPW) method, we investigated the effects of intrinsic vacancies on electronic and magnetic properties in graphene-like ZnO nanosheets. The results show that the oxygen vacancy (V_O) has no influence on magnetism in ZnO nanosheet, whereas the Zn vacancy (V_Zn) lead to spin polarization of the nanostructures with a total magnetic moment of 2.0μ_B due to O-2p and Zn-3d hybridization. When the distance of two V_Zn defects increases to 6.499 Å, the system shows an intriguing half-metallic character with 100% spin-polarized carriers due to O(2p)–Zn(3d)–O(2p) coupling chain between two V_Zn defects.     

Structural,electronic and elastic properties of potassium hexatitanate crystal from first-principles calculations

The structural, electronic and elastic properties of potassium hexatitanate (K₂Ti₆O₁₃) whisker were investigated using first-principles calculations. The calculated cell parameters of K₂Ti₆O₁₃ including lattice constants and atomic positions are in good agreement with the experimental data. The obtained formation enthalpy (−61.1535 eV/atom) and cohesive energy (−137.4502 eV/atom) are both negative, showing its high structural stability. Further analysis of the electronic structures shows that the potassium hexatitanate is a wide-band semiconductor. Within K₂Ti₆O₁₃ crystal, the TiO bonding interactions are stronger than that of KO, while no apparent KTi bonding interactions can be observed. The structural stability of K₂Ti₆O₁₃ was closely associated with the covalent bond interactions between Ti (d) and O (p) orbits. Further calculations on elastic properties show that K₂Ti₆O₁₃ is a high stiffness and brittle material with small anisotropy in shear and compression.     

The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.     

Electronic structure and magnetic properties of X2YZ (X = Co,Y = Mn,Z = Ge,Sn) type Heusler compounds: a first principle study

We performed the structure optimization followed by the calculation of electronic structure and magnetic properties on Co₂MnGe and Co₂MnSn. The structure optimization was based on generalized gradient approximation exchange correlation and full potential linearized augmented plane wave (FP-LAPW) method. The calculation of electronic structure was based on FP-LAPW method using local spin density approximation. We have studied the electronic structure and magnetic properties. The calculated density of states and band structures shows the half-metallic ferromagnets character of Co₂MnGe and Co₂MnSn.     

Density functional study of the half-metallic ferromagnetism in Co-based Heusler alloys Co2MSn (M = Ti, Zr, Hf) using LSDA and GGA

The half-metallic state in the Heusler alloys Co₂MSn (M = Ti, Zr, Hf) was studied by means of first principles calculation, using both, the Local Spin Density Approximation (LSDA) and the Generalized Gradient Approximation (GGA) to the exchange-correlation energy. While the GGA calculation shows that the three alloys are half-metallic ferromagnets, the LSDA results show that they are ferromagnetic but not half-metallic systems. The difference between the exchange-correlation functionals is analyzed through the electronic structure of the alloys. The origin of the gap in the minority spin channel for GGA calculations is discussed.     

Peculiarities of the electronic transport in Co2CrAl and Co2CrGa half-metallic ferromagnets

We present results on the transport properties of the half-metallic ferromagnetic Heusler alloys Co₂CrAl and Co₂CrGa in the temperature range from 4 to 900 K. The peculiarities of the resistivity and the absolute differential thermoelectric power are considered within a two-current model of conductivity, taking into account the energy gap at the Fermi level in the electronic spectrum of alloys for electrons with spin opposite to the direction of the magnetization vector.     

Half-metallic behavior of Co2YZ (Y = V,Cr; Z = Al,Ga) under pressure: a DFT study

The structural, electronic and magnetic properties of Co-based Heusler compounds Co₂YZ (Y = V, Cr; Z = Al, Ga) under pressure are studied using first principles density functional theory. The calculations are performed within generalized gradient approximation. The total magnetic moment decreases slightly on compression. Under application of external pressure, the valence band and conduction band are shifted downward which leads to the modification of electronic structure. There exists an indirect band gap along Г–X for all the alloys studied. Co₂CrAl shows half-metallic nature up to 85 GPa. After this pressure transition from true half-metallic behavior to nearly half-metallic behavior is observed and at 90 GPa it shows metallic behavior. Co₂CrGa shows nearly half-metallic behavior at ambient pressure, but true half-metallic behavior is observed as pressure is increased to 100 GPa. For Co₂VGa, true half-metallic to nearly half-metallic transition is observed at 40 GPa and around 100 GPa, Co₂VGa shows metallic behavior. For Co₂VAl, true half-metallic behavior is not observed at ambient as well as higher pressures. The half metal-to-metal transition in Co₂VAl and Co₂CrAl is accompanied by quenching of magnetic moment.     

The role of oxygen vacancies in magnetism of CoxTi1−xO2−δ films on Si(001) prepared by sol-gel method

Co_xTi_1−xO_2−δ films have been prepared on Si(001) substrates by sol-gel method. When heat treated in air, Co_xTi_1−xO_2−δ films are non-ferromagnetic at room temperature. However, after further vacuum annealing or hydrogenation, Co_xTi_1−xO_2−δ films show room-temperature ferromagnetism (RTFM). When the vacuum annealed Co_xTi_1−xO_2−δ films are reheated in air, the magnetic moments of the films strongly reduce. After these films are vacuum annealed once again, the magnetic moments are greatly enhanced, confirming the role of vacuum annealing in ferromagnetism of Co_xTi_1−xO_2−δ films. The x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and measurements of magnetization (M) vs temperature (T) fail to detect Co clusters in the vacuum annealed and the hydrogenated Co_xTi_1−xO_2−δ films. Oxygen vacancies are formed in Co_xTi_1−xO_2−δ films after vacuum annealing and hydrogenation, determined by XRD and XPS measurements. These results indicate that oxygen vacancies created by vacuum annealing and hydrogenation play an important role in the generation of RTFM in Co_xTi_1−xO_2−δ films.     

Half-metallic properties of Ti2FeSi full-Heusler compound

In this study, the electronic structure and magnetic properties of novel half-metallic Ti₂FeSi full-Heusler compound with CuHg₂Ti-type structure were examined by density functional theory (DFT) calculations. The electronic band structures and density of states of the Ti₂FeSi compound show the spin-up electrons are metallic, but the spin-down bands are semiconductor with a gap of 0.45 eV, and the spin-flip gap is of 0.43 eV. Fe atom shows only a small magnetic moment and its magnetic moment is antiparallel to that of Ti atoms, which is indicative of ferrimagnetism in Ti₂FeSi compound. The Ti₂FeSi Heusler compound has a magnetic moment of 2 μ_B at the equilibrium lattice constant a=5.997 Å.     

Defect properties of Ti-doped Cr2O3

The defects in Cr_2−xTi_xO₃ (x = 0, 0.2 and 0.3) were studied by a combination of X-ray diffraction, density and electrical conductivity measurements supported by atomistic simulation. The results are consistent with the Ti being dissolved as Ti⁴⁺ compensated by Cr vacancies which associate to form complex defects of lower energy. Ti doping gives n-type semiconductivity due to a small concentration of Ti³⁺ in equilibrium with the complexes.     

First-principles study of half-metallic properties of the Heusler alloy Ti2CoGe

First-principles calculations have been performed on the electronic structures and magnetic properties of a new Ti₂Co-based full-Heusler alloy Ti₂CoGe. The calculations predict the Ti₂CoGe is a half-metallic ferromagnet at the equilibrium lattice constant with the minority-spin energy gap of 0.60 eV. It is found that the total magnetic moment (M_t) and the number of valence electrons (Z_t) in Ti₂CoGe obey a new Slater–Pauling (SP) rule of M_t=Z_t−18 and the rule also can be applied to other Ti₂Co-based half-metallic full-Heusler alloys. The Ti₂CoGe alloy keeps a 100% polarization at Fermi level and maintains the half-metallic character for lattice constants ranging between 6.05 and 6.67 Å.     

First-principles study of atomic hydrogen adsorption on Fe3O4(100)

The adsorption of atomic hydrogen on an Fe₃O₄(100) surface is investigated using first-principles calculations. Our calculations reveal that hydrogen atoms prefer bonding with surface oxygen atoms not with tetrahedral iron atoms. The hydrogen-adsorbed Fe₃O₄(100) surface can be represented by a (1 × 1) unit cell, which is consistent with our recent experimental result. The spin-up surface-state bands are found to be shifted toward the deep level due to hydrogen adsorption. As a result, a band gap appears in the spin-up electronic states and half-metal behavior occurs at the H/Fe₃O₄(100) surface. The transition from a metallic to half-metallic surface due to hydrogen adsorption is discussed through analysis of the calculated spin-resolved band structure and differential charge density distribution. The reason for the enhancement of the spin polarization is attributed to a donation-redistribution process by O―H bond formation but not to detailed atomic structures of Fe and O atoms such like Jahn–Teller distortion.     

Structure and physical properties of Li4Ti5O12 synthesized at deoxidization atmosphere

Powders of spinel Li₄Ti₅O₁₂ (LTO) were successfully synthesized at reducing conditions by solid-state method. The structure and physical properties of Li₄Ti₅O₁₂ were examined by X-ray diffraction (XRD), Raman spectroscopy, scanning electronic microscopy (SEM), and differential capacitance, respectively. XRD shows that both samples are single-phase spinel compounds. LTO synthesized in Ar/H₂(8% mol) has a larger lattice parameter than that in Ar. SEM indicates that all of the prepared powders have the uniform, nearly cubic structure morphology with narrow size distribution in the range of 200–300 nm. Raman spectra indicate that the Raman bands corresponding to the Ti–O vibration has a blue shift from 674 to 680 cm⁻¹ due to the few H₂ in the synthesized condition, indicating that there is very few oxygen vacancies in the Li₄Ti₅O₁₂ synthesized under Ar/H₂ (8% mol). The dQ/dV vs. voltage plots reveals the redox potentials for the synthesized Li₄Ti₅O₁₂-negative electrode materials.     

Electronic, magnetic and transport properties of Co2TiZ (Z=Si, Ge and Sn): A first-principle study

We have studied the electronic structure, magnetic and transport properties of some Co based full Heusler alloys, namely Co₂TiZ (Z=Si, Ge and Sn), in the frame work of first-principle calculations. The calculations show that Co₂TiZ (X=Si, Ge and Sn) are to be half-metallic compounds with a magnetic moment of 2 μ_B, well consistent with the Slater-Pauling rule. The electronic structure results reveal that Co₂TiZ has the high density of states at the Fermi energy in the majority-spin state and show 100% spin polarization. Our results also suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The origin of energy gap in the minority-spin states is discussed in terms of the electron splitting of Z (Z=Si, Ge and Sn) and 3d Co atoms and also the d-d hybridization between the Co and Ti atoms. The transport properties of these materials are discussed on the basis of Seebeck coefficients, electrical conductivity coefficients and thermal conductivity coefficients.     

The effects of O deficiency on the electronic structure of rutile TiO2

Ab initio density functional calculations (plane wave GGA, CASTEP) were performed to determine the effect of O deficiency on the electronic structure of rutile, TiO₂. O deficiency was introduced through either the removal of O or the insertion of interstitial Ti atoms. At physically realistic concentrations of O vacancies in the rutile lattice (i.e. 25% and less) O deficiency results in the population of the bottom of the conduction band, the location of the Ti 3d orbitals in the pure structure, increasingly with increasing vacancy concentration. We propose that this could be confused with the formation and population of gap states especially where O vacancies occur in isolated positions in the lattice. In contrast, Ti interstitials introduce a defect state into the energy gap, without an overall reduction in the size of the energy gap. O vacancies result in a spin polarized solution, whereas Ti interstitials do not.     

Mössbauer emission spectroscopy of doped 57Co1−xO—II. donor impurities: 57Co1−xO:Fe,Ti, In

Mossbauer emission studies of Fe, In, and Ti doped ⁵⁷Co_1?xO have been earned out The appearance of the anomalous ferrice line is due to the existence of cationic vacancies and impurity acceptor levels An electron recombination model is proposed which takes into account the electronic character of the impurity defects.