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We present theoretical and experimental results on tunneling current in single Fe/MgO/GaAs and double Fe/MgO/GaAs/MgO/Fe tunnel junctions. The charge and spin currents are calculated as a function of external voltage for different sets of parameters characterizing the semiconducting GaAs layer. Transport characteristics of a single Fe/MgO/GaAs junction reveal typical diode as well as spin diode features. The results of numerical calculations are compared with current-voltage characteristics measured experimentally for double tunnel junction structures, and a satisfactory agreement of the theoretical and experimental results has been achieved. [ABSTRACT FROM AUTHOR]

The macroscopic description of spin-dependent electronic transport in magnetic-layered structures is extended by including the effects due to spin-flip scattering at the interfaces between magnetic and nonmagnetic layers. Such processes lead to spin-memory losses at the interfaces and therefore play a significant role in the giant magnetoresistance and current-induced magnetic switching phenomena. Interface spin-flip scattering significantly modifies the distribution of spin currents and spin accumulation in the vicinity of the interface. Two semi-infinite systems in direct contact, as well as magnetic/nonmagnetic (F/N) sandwiches and superlattices, are considered for both parallel and antiparallel magnetic configurations. [ABSTRACT FROM AUTHOR]

The effect of transverse magnetic anisotropy on spin-dependent transport through a large-spin molecule strongly tunnel-coupled to ferromagnetic electrodes is analyzed theoretically. In particular, we investigate whether it is possible to observe in transport signatures of oscillations of the ground-state doublet splitting due to the application of an external magnetic field along the molecule's hard axis. We show that magnetic field leads to revivals of the Kondo effect, with the Kondo temperature depending on the magnetic configuration of the device. [ABSTRACT FROM AUTHOR]

Abstract: Spin torque and current-induced magnetic switching (dynamics) in spin valve nanopillars is considered theoretically in the diffusive transport regime. Basic characteristics of the precessional states are discussed. Current-induced switching in other spin valves is analyzed, particularly in ferromagnetic single-electron transistors based on magnetic molecules. [Copyright &y& Elsevier]

The generation of oscillations in the microwave frequency range is one of the most important applications expected from spintronics devices exploiting the spin-transfer phenomenon, which is the reorientation of the magnetization of a ferromagnetic domain by spin-polarized current. Here we report transport and microwave power measurements on specially designed nanopillars, for which a non-standard angular dependence of the spin-transfer torque is predicted by theoretical models. We observe a new kind of current-induced dynamics that is characterized by large angle precessions in the absence of any applied field. This is also predicted by simulations including a ‘wavy’ angular dependence of the torque. This type of nanopillar, which is able to generate microwave oscillations in zero applied magnetic field, could represent an interesting method for the implementation of spin-transfer oscillators. We also emphasize the theoretical implications of our results on the angular dependence of the torque. [ABSTRACT FROM AUTHOR]

Theoretical analysis of the spin-transfer torque acting on the magnetic moment of the central electrode (island) in a single-electron ferromagnetic transistor has been performed for the spin relaxation time in the island ranging from fast to slow spin relaxation limits. The magnetic configuration of the system can be generally arbitrary. Spin accumulation on the island, due to the spin asymmetry of tunnelling processes, is taken into account. Electric current flowing through the device is calculated in the regime of sequential transport, and the master equation is used to calculate probabilities of different charge and spin states in the island. The torque acting on the central electrode is then calculated from the spin current absorbed by magnetic moment of the island. [ABSTRACT FROM AUTHOR]

Transport properties of ferromagnetic quantum wires at room temperature are not yet fully understood, and the role of electronic structure of magnetic atoms in the conductance quantization is still under discussion. We present experimental results on the conductance quantization in point contacts between ferromagnetic (Co) or nonmagnetic (Au) wires and semiconductor (Ge) samples. The main features of the conductance histograms for the nonmagnetic wires are consistent with the conductance quantization in the units of quantum conductance G0 = 2e²/h. For the ferromagnetic Co nanowires, the conductance shows plateaus at nG0, generally with non-integer n. Such behaviour is a consequence of the complex electronic structure of magnetic 3d transition-metal atoms. A description of the quantization phenomena is presented in terms of the Landauer formalism for the current flowing through a small nanoconstriction. [ABSTRACT FROM AUTHOR]

Resonant tunnelling through a non-interacting single-level quantum dot attached to ferromagnetic leads is analysed theoretically. The magnetic moments of the leads are assumed to be non-collinear. Apart from this, an external magnetic field is applied to the system, which is non-collinear with the magnetizations. The magnetic moments of the leads and the external magnetic field are, however, in a common plane. Basic transport characteristics, including current-voltage curves, differential conductance, and tunnel magnetoresistance associated with magnetization rotation, are calculated using the non-equilibrium Green function technique. The dependence of transport characteristics on the bias voltage has been calculated numerically. [ABSTRACT FROM AUTHOR]

Electric current in a double-barrier junction with an atomic-size spacer (quantum dot) and ferromagnetic external electrodes is analyzed theoretically in the sequential tunneling regime. Master equation method is applied to calculate the average occupation numbers, electric current and tunnel magnetoresistance (TMR). It is shown that the Coulomb correlations on the dot can suppress electric current and lead to diode-like behavior. It is also shown that the Coulomb correlations can enhance TMR in a certain bias range. Variation of electric current and the TMR effect with increasing temperature is also studied. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

We calculate shot noise and the corresponding Fano factors in magnetic double-barrier tunnel junctions. Two situations are analyzed: (i) the central metallic layer is nonmagnetic while the external ones are ferromagnetic, and (ii) all of the metallic layers are ferromagnetic. In the latter case, the number of various magnetic configurations of the junctions is larger, which improves the functionality of such devices. The corresponding shot noise and Fano factor are shown to depend on the magnetic configuration of the junctions. The effect of spin relaxation in the central layer is also taken into account. The theoretical results are compared with experimental data on the shot noise in Fe/MgO/Fe/MgO/Fe structures. [ABSTRACT FROM AUTHOR]

The influence of an asymmetric in-plane magnetic anisotropy K x ≠ K y on the thermally activated spin current is studied theoretically for two different systems: (i) the F | N system consisting of a ferromagnetic insulator ( F ) in a direct contact with a nonmagnetic metal ( N ) and (ii) the sandwich structure N | F | N consisting of a ferromagnetic insulating part sandwiched between two nonmagnetic metals. It is shown that when the difference between the temperatures of the two nonmagnetic metals in a N | F | N structure is not large, the spin pumping currents from the magnetic part to the nonmagnetic ones are equal in amplitude and have opposite directions, so only the spin torque current contributes to the total spin current. The spin current flows then from the nonmagnetic metal with the higher temperature to the nonmagnetic metal having a lower temperature. Its amplitude varies linearly with the difference in temperatures. In addition, we have found that if the magnetic anisotropy is in the layer plane, then the spin current increases with the magnon temperature, while in the case of an out-of-plane magnetic anisotropy the spin current decreases when the magnon temperature enhances. Enlarging the difference between the temperatures of the nonmagnetic metals, the linear response becomes important, as confirmed by analytical expressions inferred from the Fokker–Planck approach and by the results obtained upon a full numerical integration of the stochastic Landau–Lifshitz–Gilbert equation. [ABSTRACT FROM AUTHOR]