Manifestation of the shape and edge effects in spin-resolved transport through graphene quantum dots

We report on theoretical studies of transport through graphene quantum dots weakly coupled to external ferromagnetic leads. The calculations are performed by exact diagonalization of a tight-binding Hamiltonian with finite Coulomb correlations for graphene sheet and by using the real-time diagrammatic technique in the sequential and cotunneling regimes. The emphasis is put on the role of graphene flake shape and spontaneous edge magnetization in transport characteristics, such as the differential conductance, tunneling magnetoresistance (TMR) and the shot noise. It is shown that for certain shapes of the graphene dots a negative differential conductance and nontrivial behavior of the TMR effect can occur

The Overlapping Muffin-Tin Approximation

We present the formalism and demonstrate the use of the overlapping muffin-tin approximation (OMTA). This fits a full potential to a superposition of spherically symmetric short-ranged potential wells plus a constant. For one-electron potentials of this form, the standard multiple-scattering methods can solve Schr\"{o}dingers' equation correctly to 1st order in the potential overlap. Choosing an augmented-plane-wave method as the source of the full potential, we illustrate the procedure for diamond-structured Si. First, we compare the potential in the Si-centered OMTA with the full potential, and then compare the corresponding OMTA $N$-th order muffin-tin orbital and full-potential LAPW band structures. We find that the two latter agree qualitatively for a wide range of overlaps and that the valence bands have an rms deviation of 20 meV/electron for 30% radial overlap. Smaller overlaps give worse potentials and larger overlaps give larger 2nd-order errors of the multiple-scattering method. To further remove the mean error of the bands for small overlaps is simple.Comment: 9 pages, 3 figures Proceedings of The European Conference "Physics of Magnetism 2008", Poznan, Polan

Multiplet ligand-field theory using Wannier orbitals

We demonstrate how ab initio cluster calculations including the full Coulomb vertex can be done in the basis of the localized, generalized Wannier orbitals which describe the low-energy density functional (LDA) band structure of the infinite crystal, e.g. the transition metal 3d and oxygen 2p orbitals. The spatial extend of our 3d Wannier orbitals (orthonormalized Nth order muffin-tin orbitals) is close to that found for atomic Hartree-Fock orbitals. We define Ligand orbitals as those linear combinations of the O 2p Wannier orbitals which couple to the 3d orbitals for the chosen cluster. The use of ligand orbitals allows for a minimal Hilbert space in multiplet ligand-field theory calculations, thus reducing the computational costs substantially. The result is a fast and simple ab initio theory, which can provide useful information about local properties of correlated insulators. We compare results for NiO, MnO and SrTiO3 with x-ray absorption, inelastic x-ray scattering, and photoemission experiments. The multiplet ligand field theory parameters found by our ab initio method agree within ~10% to known experimental values

First-principles study of magnetization relaxation enhancement and spin-transfer in thin magnetic films

The interface-induced magnetization damping of thin ferromagnetic films in contact with normal-metal layers is calculated from first principles for clean and disordered Fe/Au and Co/Cu interfaces. Interference effects arising from coherent scattering turn out to be very small, consistent with a very small magnetic coherence length. Because the mixing conductances which govern the spin transfer are to a good approximation real valued, the spin pumping can be described by an increased Gilbert damping factor but an unmodified gyromagnetic ratio. The results also confirm that the spin-current induced magnetization torque is an interface effect.Comment: 10 pages, 8 figures, RevTeX; modified according to Referees' request

Spin-injection through an Fe/InAs Interface

The spin-dependence of the interface resistance between ferromagnetic Fe and InAs is calculated from first-principles for specular and disordered (001) interfaces. Because of the symmetry mismatch in the minority-spin channel, the specular interface acts as an efficient spin filter with a transmitted current polarisation between 98 an 89%. The resistance of a specular interface in the diffusive regime is comparable to the resistance of a few microns of bulk InAs. Symmetry-breaking arising from interface disorder reduces the spin asymmetry substantially and we conclude that efficient spin injection from Fe into InAs can only be realized using high quality epitaxial interfaces.Comment: 4 pages, 4 figure

Spin accumulation and decay in magnetic Schottky barriers

The theory of charge and spin transport in forward-biased Schottky barriers reveals characteristic and experimentally relevant features. The conductivity mismatch is found to enhance the current-induced spin imbalance in the semiconductor. The GaAs¿MnAs interface resistance is obtained from an analysis of the magnetic-field-dependent Kerr rotation experiments by Stephens et al. and compared with first-principles calculations for intrinsic interfaces. With increasing current bias, the interface transparency grows toward the theoretical values, reflecting increasingly efficient Schottky barrier screening

Perpendicular transport and magnetization processes in magnetic multilayers with strongly and weakly coupled magnetic layers

Within the framework of a two-band tight-binding model, we have performed calculations of giant magnetoresistance, exchange coupling and thermoelectric power (TEP) for a system consisting of three magnetic layers separated by two non-magnetic spacers with the first two magnetic layers strongly antiferromagnetically exchange-coupled. We have shown how does the GMR relate with the corresponding regions of magnetic structure phase diagrams and computed some relevant hysteresis loops, too. The GMR may take negative values for specific layers thicknesses, and the TEP reveals quite pronounced oscillations around a negative bias.Comment: 16 pages, 10 figures, submited to Journal of Magnetism and Magnetic Material