Ab initio study of the interface properties of Fe/GaAs(110)

We have investigated the initial growth of Fe on GaAs(110) by means of density functional theory. In contrast to the conventionally used (001)-surface the (110)-surface does not reconstruct. Therefore, a flat interface and small diffusion can be expected, which makes Fe/GaAs(110) a possible candidate for spintronic applications. Since experimentally, the actual quality of the interface seems to depend on the growth conditions, e.g., on the flux rate, we simulate the effect of different flux rates by different Fe coverages of the semiconductor surface. Systems with low coverages are highly diffusive. With increasing amount of Fe, i.e., higher flux rates, a flat interface becomes more stable. The magnetic structure strongly depends on the Fe coverage but no quenching of the magnetic moments is observed in our calculations.Comment: 9 pages, 8 figure

Modelling Electron Spin Accumulation in a Metallic Nanoparticle

A model describing spin-polarized current via discrete energy levels of a metallic nanoparticle, which has strongly asymmetric tunnel contacts to two ferromagnetic leads, is presented. In absence of spin-relaxation, the model leads to a spin-accumulation in the nanoparticle, a difference ($\Delta\mu$) between the chemical potentials of spin-up and spin-down electrons, proportional to the current and the Julliere's tunnel magnetoresistance. Taking into account an energy dependent spin-relaxation rate $\Omega (\omega)$, $\Delta\mu$ as a function of bias voltage ($V$) exhibits a crossover from linear to a much weaker dependence, when $|e|\Omega (\Delta\mu)$ equals the spin-polarized current through the nanoparticle. Assuming that the spin-relaxation takes place via electron-phonon emission and Elliot-Yafet mechanism, the model leads to a crossover from linear to $V^{1/5}$ dependence. The crossover explains recent measurements of the saturation of the spin-polarized current with $V$ in Aluminum nanoparticles, and leads to the spin-relaxation rate of $\approx 1.6 MHz$ in an Aluminum nanoparticle of diameter $6nm$, for a transition with an energy difference of one level spacing.Comment: 37 pages, 7 figure

Canted Magnetization Texture in Ferromagnetic Tunnel Junctions

We study the formation of inhomogeneous magnetization texture in the vicinity of a tunnel junction between two ferromagnetic wires nominally in the antiparallel configuration and its influence on the magnetoresistance of such a device. The texture, dependent on magnetization rigidity and crystalline anisotropy energy in the ferromagnet, appears upon an increase of ferromagnetic inter-wire coupling above a critical value and it varies with an external magnetic field.Comment: 5 pages, 4 figure

Half-metallic ferromagnets for magnetic tunnel junctions

Using theoretical arguments, we show that, in order to exploit half-metallic ferromagnets in tunneling magnetoresistance (TMR) junctions, it is crucial to eliminate interface states at the Fermi level within the half-metallic gap; contrary to this, no such problem arises in giant magnetoresistance elements. Moreover, based on an a priori understanding of the electronic structure, we propose an antiferromagnetically coupled TMR element, in which interface states are eliminated, as a paradigm of materials design from first principles. Our conclusions are supported by ab-initio calculations

Electronic Phase Separation in Manganite/Insulator Interfaces

By using a realist microscopic model, we study the electric and magnetic properties of the interface between a half metallic manganite and an insulator. We find that the lack of carriers at the interface debilitates the double exchange mechanism, weakening the ferromagnetic coupling between the Mn ions. In this situation the ferromagnetic order of the Mn spins near the interface is unstable against antiferromagnetic CE correlations, and a separation between ferromagnetic/metallic and antiferromagnetic/insulator phases at the interfaces can occur. We obtain that the insertion of extra layers of undoped manganite at the interface introduces extra carriers which reinforce the double exchange mechanism and suppress antiferromagnetic instabilities.Comment: 8 pages, 7 figures include

Tunnelling between non-centrosymmetric superconductors with significant spin-orbit splitting studied theoretically within a two-band treatment

Tunnelling between non-centrosymmetric superconductors with significant spin-orbit splitting is studied theoretically in a two-band treatment of the problem. We find that the critical Josephson current may be modulated by changing the relative angle between the vectors describing absence of inversion symmetry on each side of the junction. The presence of two gaps also results in multiple steps in the quasiparticle current-voltage characteristics. We argue that both these effects may help to determine the pairing states in materials like CePt$_3$Si, UIr and Cd$_2$Re$_2$O$_7$. We propose experimental tests of these ideas, including scanning tunnelling microscopy.Comment: 5 pages, 1 figure. Minor changes. Some new references added. Journal-ref. adde

Magnetic and orbital blocking in Ni nanocontacts

We address the fundamental question of whether magneto-resistance (MR) of atomic-sized contacts of Nickel is very large because of the formation of a domain wall (DW) at the neck. Using {\em ab initio} transport calculations we find that, as in the case of non-magnetic electrodes, transport in Ni nanocontacts depends very much on the orbital nature of the electrons. Our results are in agreement with several experiments in the average value of the conductance. On the other hand, contrary to existing claims, DW scattering does {\em not} account for large MR in Ni nanocontacts.Comment: 5 pages, 3 Figure

Influence of Roughness and Disorder on Tunneling Magnetoresistance

A systematic, quantitative study of the effect of interface roughness and disorder on the magnetoresistance of FeCo$|$vacuum$|$FeCo magnetic tunnel junctions is presented based upon parameter-free electronic structure calculations. Surface roughness is found to have a very strong effect on the spin-polarized transport while that of disorder in the leads (leads consisting of a substitutional alloy) is weaker but still sufficient to suppress the huge tunneling magneto-resistance (TMR) predicted for ideal systems

Effects of different geometries on the conductance, shot noise and tunnel magnetoresistance of double quantum dots

The spin-polarized transport through a coherent strongly coupled double quantum dot (DQD) system is analyzed theoretically in the sequential and cotunneling regimes. Using the real-time diagrammatic technique, we analyze the current, differential conductance, shot noise and tunnel magnetoresistance (TMR) as a function of both the bias and gate voltages for double quantum dots coupled in series, in parallel as well as for T-shaped systems. For DQDs coupled in series, we find a strong dependence of the TMR on the number of electrons occupying the double dot, and super-Poissonian shot noise in the Coulomb blockade regime. In addition, for asymmetric DQDs, we analyze transport in the Pauli spin blockade regime and explain the existence of the leakage current in terms of cotunneling and spin-flip cotunneling-assisted sequential tunneling. For DQDs coupled in parallel, we show that the transport characteristics in the weak coupling regime are qualitatively similar to those of DQDs coupled in series. On the other hand, in the case of T-shaped quantum dots we predict a large super-Poissonian shot noise and TMR enhanced above the Julliere value due to increased occupation of the decoupled quantum dot. We also discuss the possibility of determining the geometry of the double dot from transport characteristics. Furthermore, where possible, we compare our results with existing experimental data on nonmagnetic systems and find qualitative agreement.Comment: 15 pages, 12 figures, accepted in Phys. Rev.

Probing the Role of the Barrier Layer in Magnetic Tunnel Junction Transport

Magnetic tunnel junctions with a ferrimagnetic barrier layer have been studied to understand the role of the barrier layer in the tunneling process - a factor that has been largely overlooked until recently. Epitaxial oxide junctions of highly spin polarized La0.7Sr0.3MnO3 and Fe3O4 electrodes with magnetic NiMn2O4 (NMO) insulating barrier layers provide a magnetic tunnel junction system in which we can probe the effect of the barrier by comparing junction behavior above and below the Curie temperature of the barrier layer. When the barrier is paramagnetic, the spin polarized transport is dominated by interface scattering and surface spin waves; however, when the barrier is ferrimagnetic, spin flip scattering due to spin waves within the NMO barrier dominates the transport.Comment: 10 pages, 3 figure