Self-consistent Spin-polarized Calculations Research Articles

Negative Differential Resistance and Spin-Filtering Effects in Zigzag Graphene Nanoribbons with Nitrogen-Vacancy Defects

We explore the electronic and transport properties of zigzag graphene nanoribbons (GNRs) with nitrogen-vacancy defects by performing fully self-consistent spin-polarized density functional theory calculations combined with non-equilibrium Green's function technique. We observe robust negative differential resistance (NDR) effect in all examined molecular junctions. Through analyzing the calculated electronic structures and the bias-dependent transmission coefficients, we find that the narrow density of states of electrodes and the bias-dependent effective coupling between the central molecular orbitals and the electrode subbands are responsible for the observed NDR phenomenon. In addition, the obvious difference of the transmission spectra of two spin channels is observed in some bias ranges, which leads to the near perfect spin-filtering effect. These theoretical findings imply that GNRs with nitrogen-vacancy defects hold great potential for building molecular devices.

Magnetic and hyperfine properties of Fe 8 molecule

Density functional self-consistent spin-polarized calculations with the discrete variational method were performed to investigate the electronic structure, magnetic and hyperfine properties of the single molecule magnet [Fe 8O 2(OH) 12(tacn) 6] +8 (shortly named Fe 8). Magnetic moments, total spin, density of states, charge and spin density contour maps are obtained. The average moment and net charge per Fe site are about 4.2 μ B and +2.5e, respectively. The total magnetic moment of 20 μ B is obtained which is in accordance with experimental results. The anisotropy of Fe 8 is investigated from the charge and spin density distribution. The hyperfine magnetic field and isomer shift for different Fe ions are also studied by comparing with the Mössbauer spectrum.

Theoretical analysis of spin-dependent Mn K-edge x-ray absorption of MnP

Spin-dependent x-ray absorption near edge structure (XANES) spectra measured by selectively monitoring of the Mn K-beta emission while scanning the excitation energy through the Mn K absorption edge were analyzed. The method of calculation is based on a combination of self-consistent spin-polarized calculation of muffin-tin potential and a full multiple scattering theory for interpretation of x-ray absorption. This approach allows to separate influence of dipole transition matrix elements and density of empty electronic states on spin-dependent XANES. It is found that matrix elements affect the splitting between spin-up and spin-down spectra only near the absorption threshold, while differences in densities of states slightly shift spectra in the region 35–55 eV above the main edge. Effects of multielectron term dependent broadening of spin-dependent XANES spectra and different mean free paths of photoelectrons with spin-up and spin-down in ferromagnetic phase were taken into account. It is shown that these factors also contribute to the splitting of spectra with different spin directions near the absorption threshold.

Structural and electronic properties ofCo/Al2O3/Comagnetic tunnel junction from first principles

A detailed first-principles study of the atomic and electronic structure of the $\mathrm{Co}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}/\mathrm{Co}$ magnetic tunnel junction has been performed in order to elucidate the key features determining the spin-dependent tunneling. The atomic structure of the multilayer with the O- and Al-terminated interfaces between fcc Co(111) and crystalline $\ensuremath{\alpha}$-${\mathrm{Al}}_{2}{\mathrm{O}}_{3}(0001)$ has been optimized using self-consistent spin-polarized calculations within density-functional theory and the generalized gradient approximation. We found that the relaxed atomic structure of the O-terminated interface is characterized by a rippling of the Co interfacial plane, the average Co-O bond length being 2.04 \AA{} which is within 5% of that in bulk CoO. The corresponding electronic structure is influenced by the covalent bonding between the O $2p$ and Co $3d$ orbitals resulting in exchange-split bonding and antibonding states and an induced magnetic moment of 0.07${\ensuremath{\mu}}_{B}$ on the interfacial oxygen atoms. The Al-terminated interface contains Co-Al bonds with an average bond length of 2.49 \AA{} compared to 2.48 \AA{} in bulk CoAl. Due to charge transfer and screening effects the Co interfacial layer acquires a negative charge which results in a reduced magnetic moment of $1.15{\ensuremath{\mu}}_{B}$ per Co atom. We found that the electronic structure of the O-terminated $\mathrm{Co}/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}/\mathrm{Co}$ tunnel junction exhibits negative spin polarization at the Fermi energy within the first few monolayers of alumina but it eventually becomes positive for distances beyond 10 \AA{}.

Density functional study of electronic, magnetic and hyperfine properties of [M(CN) 5NO] 2− (M=Fe, Ru) and reduction products

The discrete variational method (DVM) in density functional theory (DFT) was employed to investigate the electronic structure of the complexes: [Fe(CN) 5NO] 2− (nitroprusside), [Fe(CN) 5NO] 3−, [Fe(CN) 4NO] 2−, [Ru(CN) 5NO] 2− and [Ru(CN) 5NO] 3−. Total energy calculations revealed that in pentacyanonitrosylferrate(I) and pentacyanonitrosylruthenate(I), which are paramagnetic ions containing one unpaired electron, the M–N–O bond angle is bent. From self-consistent spin-polarized calculations, the distribution of the unpaired electron in the paramagnetic complexes [Fe(CN) 5NO] 3−, [Fe(CN) 4NO] 2− and [Ru(CN) 5NO] 3− was obtained, as well as spin-density maps. A long-standing controversy regarding the configuration of [Fe(CN) 5NO] 3− was elucidated, and it was found that the unpaired electron in this complex is in an orbital primarily localized on π * (NO). Mössbauer quadrupole splittings on Fe and Ru were derived from calculations of the electric-field gradients. Magnetic hyperfine coupling constants on N of the NO ligand were also obtained for the paramagnetic complexes.

Bonding and Cohesive Properties of Cobalt/Alumina Magnetic Tunnel Junctions

AbstractMagnetic tunnel junctions (MTJs) are promising candidates for applications in spintronic devices such as magnetic random access memories, read heads and sensors. Previous studies have revealed the critical influence of the chemistry and structure of the ferromagnet metal/alumina interface on both sign and magnitude of the tunneling current in MTJs. We have performed a detailed first-principles study of the structure and bonding at the cobalt/alumina interface in order to understand the relationship between magnetic properties, local bonding environment and cohesion at the interface. Both Al- and 0- terminated interfaces were considered and full geometry optimization of the structure was done by selfconsistent spin-polarized calculations within density functional theory and the generalized gradient approximation. We found that the relaxed atomic structure of the 0-terminated interface is characterized by a rippling of the Co interfacial plane, the average Co-O bond length being 2.04 Å which is within 5% of that in bulk CoO and it is characterized by the covalent bonding between the 0 2p and Co 3d orbitals. The Al-terminated interface contains Co-Al bonds with an average bond length of 2.49 Å compared to 2.48 Å in bulk CoAl and it is characterized by metallic behaviour of the first aluminium layer. As a consequence of different character of bonding, the cohesion at the O-terminated interface is stronger as that at the Al-terminated interface, the work of separation being equal to 6.35 J/m2 and 3.64 J/m2 respectively.

Chemical bonding and magnetic properties of the high-spin molecule [Mn 12O 12(HCOO) 16(H 2O) 4

Density functional self-consistent spin-polarized calculations with the Discrete Variational Method were performed to obtain the electronic structural and magnetic properties of the cluster [Mn 12O 12(HCOO) 16(H 2O) 4] approaching the true high-spin molecule [Mn 12O 12(CH 3COO) 16(H 2O) 4]. Magnetic moments, total spin, densities of states, and charge- and spin-density maps are obtained. The bond order and magnetic hyperfine field on the Mn nucli in this molecule are obtained from our calculations and compared to reported experimental values of similar compounds when available.

A study on the effect of hydrogen in the compounds with -type structure

The structural and magnetic properties of hydrides with -type structure have been studied by means of magnetic measurements, the neutron powder diffraction technique and self-consistent spin-polarized band calculations (LMTO-ASA). We found that the hydrides retain the -type structure, but with an increase of unit-cell volume. The neutron diffraction results indicate that hydrogen atoms occupy the interstitial 2b sites. Magnetic measurements carried out on , and their hydrides show that both the Curie temperature and the saturation magnetization can be enhanced by introducing interstitial hydrogen atoms. Band-structure calculations and spin-fluctuation theory give a fair description of the enhancement of the magnetization and Curie temperature.

Electronic structural and magnetic properties of cadmium chalcogenide beryllosilicate sodalites

Density functional self-consistent spin-polarized calculations with the Discrete Variational Method were performed to obtain the electronic structures of cadmium chalcogenide beryllosilicate sodalites Cd8[BeSiO4]6Z2 and their bulk materials CdZ (Z = S, Se, Te). Similar as their bulk materials, these sodalites are also semiconductors. From their density of states and charge distributions, it was found that there is greater localization of the electron density in the sodalites compared with that of the bulk. The band gaps in their bulk compounds are different from those in the sodalite structures due to the electrons of Si, Be and O filled in. Different from their bulk materials, the d electrons of Cd play an important role to form an unseparated valence band with all orbitals of other elements. The calculated magnetic moments of these three materials are very small since all d atomic orbitals are fully occupied.

The electronic structure and magnetic properties of (100) surface of TiFe alloy

Ab initio self-consistent spin-polarized FLAPW calculations of the electronic structure of the FeTi (100) surface have been performed. The seven-layer symmetrical film with the surface layer formed by atoms of Fe was considered. The data on distribution of electronic charge and spin density in the unit cell, total local and partial densities of states are presented. A high density of states in the first layer for spin-down electrons due to highly localized surface states at Fermi level has been obtained. The calculated value of work function is 4.46 eV. The magnetic moment of the surface layer is 2.41 μ B and rapidly falls off to the center of the film. The magnetic moments on Ti atoms are negative.

Spin-polarized photoemission of

The first spin-resolved photoemission experiment on an iron - boron amorphous alloy using a synchrotron source is presented. The experimental spin polarization of the d band of has been compared with three theoretical predictions and found to be in best agreement with self-consistent spin-polarized calculations based on a supercell LMTO approach. The observed average spin polarization of the valence band is approximately twice that of 10 eV secondary electrons. Hysteresis loops for determined from the 1 eV and 20 eV secondary-electron asymmetry are similar to those determined using the magnetooptic Kerr effect, but show a lower coercivity. The differences are attributed to a combination of different sampling depths of the two techniques and to the surface inhomogeneity of the sample.

Effect of pressure on the Curie temperature of Fe3Pt.

We have performed self-consistent spin-polarized calculations for ${\mathrm{Fe}}_{3}$Pt (which crystallizes in the ${\mathrm{Cu}}_{3}$Au structure) at various Wigner-Seitz radii using the linear muffin-tin orbital atomic sphere approximation method. The average magnetic moment obtained at equilibrium lattice constant (1.99${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) is close to the experimental value (2.15${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$). We have calculated the Curie temperature of this compound at various Wigner-Seitz radii, using the model proposed by Mohn and Wohlfarth. The quantities which are obtainable from the band-structure calculations are the only requisite for this model. The variation of the Curie temperature with Wigner-Seitz radii, i.e., with pressure, ${\mathit{T}}_{\mathit{C}}$(P) is then compared with the experimental data.

Surface magnetic moments and electronic structure of Cr(100), Fe(100), and Ni(100) near the Fermi level

The results of the ab initio self-consistent spin-polarized FLAPW calculations of the electronic structure of seven-layer Cr(100), Fe(100), and Ni(100) films are presented. Theoretical work function and layer-by-layer magnetic moments are obtained. The possibility of comparing data obtained in experiment and first-principles calculations is discussed. In this connection the values of the layer-by-layer magnetic moments m( r , E F), corresponding to some energy interval (about 0.2 eV) just below the Fermi level are defined. It is shown for the cases of Cr(100), Fe(100), and Ni(100) films that the values of ordinary layer-by-layer magnetic moments m( r ) may differ greatly from those calculated near the Fermi level. The magnetization of surface layers may even have the opposite signs. This difference can account for the possible strong deviations between the computational results on the surface magnetic properties and those obtained by some experimental techniques engaging electrons located near the Fermi level in the energy spectrum (e.g., scanning tunneling microscopy).

Band structure of spin-Peierls cuprate CuGeO3

The spin-restricted and spin-polarized fully self-consistent linear muffin tin orbital band calculations for cuprate CuGeO3 are presented with Cu atoms in normal and displaced positions within the orthorhombic cells. This, computationally much cheaper, approach gives practically the same one-electron bands as recent linear augmented plane wave calculations, and the same qualitative explanation for the spin-Peierls transition observed at low temperatures in this compound. New self-consistent spin-polarized calculations of CuGeO3, for some supposed antiferromagnetic order in copper chains along the orthorhombic c axis, have given very small values for magnetic moments on Cu atoms. Again the dimerization of CuO2 chains can open a band gap at EF, if the Cu displacements in neighbouring chains are out of phase.

Surface magnetic moment of Fe(100) and Ni(100) and electronic structure near the Fermi level

The results of the self-consistent spin-polarized FLAPW calculations of the electronic structures of seven-layer Fe(100) and Ni(100) films are presented. The possibility of comparing data obtained from experiment and from first-principles calculations is discussed. In this connection the layer-projected magnetic moments corresponding to a narrow energy interval (0.2 eV) just below the Fermi level are introduced. 2D Fermi surfaces are presented.

Calculation of the magneto-optical properties of ferromagnetic metals using the spin-polarized relativistic LMTO method

An efficient method of self-consistent spin-polarized relativistic calculations and an original technique of calculation of the conductivity tensor have been developed. The method has been applied to ferromagnetic Fe, Co, Ni and Pu. Spin and orbital magnetic moments and the Kerr rotation angle have been evaluated.

Magnetic phase stability of 3d-metals and plutonium

We have performed self-consistent spin-polarized calculations for the magnetic structure and exchange parameters of pure 3d-metals and Pu. Local field effects and the strength of non-Heisenberg interactions are estimated. For α-Mn and α-Pu antiferromagnetic structures are dominant, and a complexe magnetic ordering was found for the low temperature structures. An analytical expression for the magnetic torque and a non-collinear low energy magnetic state of γ-Fe are presented.

Electronic structures and Curie temperatures of iron-based rare-earth permanent-magnet compounds.

The modification of the electronic structures of Sm[sub 2]Fe[sub 17[minus][ital x]]Al[sub [ital x]]N[sub [ital y]], NdFe[sub 11]TiN[sub [ital y]], and YFe[sub 12[minus][ital x]]Mo[sub [ital x]] upon alloying and nitriding are examined with self-consistent spin-polarized calculations and soft-x-ray photoemission measurements between 18 and 135 eV. The changes in the Curie temperature [ital T][sub [ital c]] with substitutional modifications and nitrogen addition are modeled with self-consistent spin-polarized electronic structure calculations and the spin-fluctuation theory of Mohn and Wohlfarth which relates the electronic structure to [ital T][sub [ital c]]. The calculations show that the spin-summed density of states at the Fermi energy is related to [ital T][sub [ital c]]. The photoemission spectra are dominated by the Fe 3[ital d] electrons within 3 eV of the Fermi energy in agreement with calculations. Changes in the density of states at the Fermi energy for interstitial and and substitutional modification compare well with calculations. Using photoemission results with experimental magnetic moments for the substitutional modification of the compound Sm[sub 2]Fe[sub 17[minus][ital x]]Al[sub [ital x]], the spin-fluctuation theory predicts a change in [ital T][sub [ital c]] in agreement with the measured change in [ital T][sub [ital c]]. Spin-resolved photoemission spectra for [ital c]-axis oriented Sm[sub 2]Fe[sub 17]N[submore » 2.6] with magnetization perpendicular to the surface are presented and compared to theoretical calculations.« less

Structural, electronic, and magnetic properties of Fe-Y alloys.

Results of ab initio calculations of the structural, electronic, and magnetic properties of crystalline and amorphous Fe-Y alloys are presented. The structure of the amorphous phases is generated via a simulated molecular-dynamics quench, based on effective interatomic forces derived from tight-binding-bond theory. The results show that a pronounced chemical short-range order (preferred heterocoordination) exists at all compositions and that at short distances the local topology is similar in the crystalline and glassy phases. In the Y-rich alloys, medium-range concentration fluctuations are superposed to the local order. The investigation of the electronic structure using self-consistent spin-polarized linear muffin-tin orbital calculations confirms that the bonding properties are similar in the crystalline and amorphous phases. The crystalline compounds are ferrimagnetic, the negative Y moment is induced by a strong covalent interaction of the Y d band with the minority-spin Fe d band. Ferrimagnetism is also found in the amorphous alloys. In the Fe-rich range (more than 80 at. % Fe) competing ferro- and antiferromagnetic Fe-Fe exchange interactions lead to coexisting positive and negative Fe moments. The decrease of the magnetic moments of the Fe atoms with increasing Y content is strongly influenced by the strong coupling of the Fe spins to the Y moments---as a consequence a spontaneous Fe moment exists even at very small Fe-Fe coordination numbers. In addition the structural inhomogeneity of the Y-rich amorphous alloys leads to cluster magnetism.

Spin-Polarized Electronic Structure of MnBiAl

The self-consistent spin-polarized band-structure calculation of ferromagnetic compound MnBiAl in its low-temperature phase has been performed. In this paper the calculation results are given. Comparison with the results of MnBi is performed in order to find the effect on electronic structure by doping with Al.