Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. While metal spintronics has already found its niche in the computer industry-giant magnetoresistance systems are used as hard disk read heads-semiconductor spintronics is yet to demonstrate its full potential. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent interaction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In view of the importance of ferromagnetic semiconductor materials, a brief discussion of diluted magnetic semiconductors is included. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field. 72.25.Rb, 75.50.Pp, PACS

We report the observation of tunneling anisotropic magnetoresistance effect (TAMR) in the epitaxial metal-semiconductor system Fe/GaAs/Au. The observed two-fold anisotropy of the resistance can be switched by reversing the bias voltage, suggesting that the effect originates from the interference of the spin-orbit coupling at the interfaces. Corresponding model calculations reproduce the experimental findings very well. PACS numbers: 73.43.Jn, 72.25.Dc, 73.43.Qt Tunneling magnetoresistance (TMR) devices consist of a tunneling barrier, typically an oxide, sandwiched between two ferromagnetic layers of different coercive fields. Such systems find widespread use in sensor and memory application as they exhibit a large resistance difference for parallel and antiparallel alignment of the ferromagnets' magnetization [1]. The TMR effect relies, within the simplest model [2], on the different spin polarizations at the Fermi energy E F in the ferromagnets; it is absent if one ferromagnetic layer is replaced by a normal metal. Hence it came as a surprise that a spin-valve-like tunnel magnetoresistance was found in (Ga,Mn)As/alumina/Au sandwiches [3]. The origin of the effect, labeled tunneling anisotropic magnetoresistance (TAMR), was associated with the anisotropic density of states in the ferromagnet (Ga,Mn)As. An enhanced anisotropic magnetoresistance (AMR) effect measured across a constriction in a (Ga,Mn)As film was ascribed to the TAMR effect, too [4]. In both experiments the fourfold symmetry, expected if the (Ga,Mn)As hole density of states is involved, was broken and ascribed to strain in (Ga,Mn)As.Here we show that a TAMR effect can also be observed in sandwiches involving a conventional ferromagnet like iron. A stack of Fe, GaAs and Au, with iron grown epitaxially on the GaAs tunneling barrier, shows pronounced spin-valve-like signatures. We observe a uniaxial anisotropy of the tunneling magnetoresistance. Depending on the bias voltage the high resistance state is either observed for the magnetization M oriented in [110] or in [110] direction. We propose a theoretical model in which the C 2v symmetry, resulting from the interference of Bychkov-Rashba and Dresselhaus spin-orbit interactions, is transferred to the tunneling probability, giving rise to the observed two-fold symmetry.A sketch of the system is shown in Fig. 1(a). The 13 nm thick epitaxial iron layer was grown on an 8 nm thin GaAs (001) barrier by transferring the freshly grown GaAs heterojunction from the molecular beam epitaxy chamber to a magnetron sputtering system without breaking the ultrahigh vacuum (UHV). The quality of the interface of a sample from the same wafer was checked by high-resolution transmission electron microscopy [5]. The Fe layer was covered by 50 nm cobalt and 100 nm gold which serves as back contact. The wafer then was glued upside down to another substrate and the original substrate was etched away. Finally, the circular, 150 nm thick top gold contact was made by employing optical lithography, selective etching of AlGa...

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By considering a linear in momentum but otherwise arbitrary spin-orbit coupling ͑SOC͒, we derive a simple analytical expression for the current-driven spin torque in a single ferromagnetic layer. Explicit forms of the spin torque are given for structures with different SOC fields, in dependence of strain effects, growth direction, and/or symmetry under spatial inversion. The Landau-Lifshitz-Gilbert equation including the effects of the SOC mediated spin torque on the magnetization dynamics is briefly discussed.

The effects of the spin-orbit coupling ͑SOC͒ on the tunneling magnetoresistance of ferromagnet/ semiconductor/normal-metal tunnel junctions are investigated. Analytical expressions for the tunneling anisotropic magnetoresistance ͑TAMR͒ are derived within an approximation in which the dependence of the magnetoresistance on the magnetization orientation in the ferromagnet originates from the interference between Bychkov-Rashba and Dresselhaus SOCs that appear at junction interfaces and in the tunneling region. We also investigate the TAMR effect in ferromagnet/semiconductor/ferromagnet tunnel junctions. The conventional tunneling magnetoresistance ͑TMR͒ measures the difference between the magnetoresistance in parallel and antiparallel configurations. We show that in ferromagnet/semiconductor/ferromagnet heterostructures, because of the SOC effects, the conventional TMR becomes anisotropic-we refer to it as the anisotropic tunneling magnetoresistance ͑ATMR͒. The ATMR describes the changes in the TMR when the axis along which the parallel and antiparallel configurations are defined is rotated with respect to a crystallographic reference axis. Within the proposed model, depending on the magnetization directions in the ferromagnets, the interplay of Bychkov-Rashba and Dresselhaus SOCs produces differences between the rates of transmitted and reflected spins at the ferromagnet/semiconductor interfaces, which results in an anisotropic local density of states at the Fermi surface and in the TAMR and ATMR effects. Model calculations for Fe/GaAs/Fe tunnel junctions are presented. Finally, based on rather general symmetry considerations, we deduce the form of the magnetoresistance dependence on the absolute orientations of the magnetizations in the ferromagnets.

The temporal and spatial controllability of charge distribution in submicron structures opens new avenues for potential applications and for the understanding of nonequilibrium processes. Here we suggest a novel way to trigger and control within picoseconds charge currents and magnetic moments in nanoscopic and mesoscopic ring structures by applying two shaped, time-delayed light pulses. Our quantum dynamic calculations show that the magnitude and direction of the induced currents are tunable by varying the time delay and strengths of the pulses. Furthermore, in an array of rings desirable magnetic orders are generated depending on the ring sizes and particle number.

Using analytical formulas as well as a finite-difference scheme, we investigate the magnetic field dependence of the energy spectra and magnetic edge states of HgTe/CdTe-based quantum wells in the presence of perpendicular magnetic fields and hard walls for the band-structure parameters corresponding to the normal and inverted regimes. Whereas one can not find counterpropagating, spin-polarized states in the normal regime, below the crossover point between the uppermost (electronlike) valence and lowest (holelike) conduction Landau levels, one can still observe such states at finite magnetic fields in the inverted regime, although these states are no longer protected by time-reversal symmetry. Furthermore, the bulk magnetization and susceptibility in HgTe quantum wells are studied, in particular their dependence on the magnetic field, chemical potential, and carrier densities. We find that for fixed chemical potentials as well as for fixed carrier densities, the magnetization and magnetic susceptibility in both the normal and the inverted regimes exhibit de Haas-van Alphen oscillations, the amplitude of which decreases with increasing temperature. Moreover, if the band structure is inverted, the ground-state magnetization (and consequently also the ground-state susceptibility) is discontinuous at the crossover point between the uppermost valence and lowest conduction Landau levels. At finite temperatures and/or doping, this discontinuity is canceled by the contribution from the electrons and holes and the total magnetization and susceptibility are continuous. In the normal regime, this discontinuity of the ground-state magnetization does not arise and the magnetization is continuous for zero as well as finite temperatures.

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