FONE-Spin Transport und Korrelationen in Nanostrukturen_SPINTRA

The project SPINTRA is designed to develop fundamental research in creating, storing, manipulating and transporting the electron spin using hybrid ferromagnet/semiconductor structures and multilayers. 10 teams from 7 different countries participate in the project. The teams from Linz, Leuven, Nottingham and Prague are experts in fabrication of various magnetic nanostructures by MBE technique. They have also facilities for characterization of samples and fabrication of nanodevices. The groups from Warsaw, Nottingham, Cambridge, Naples and Madrid have equipment and experience in measurements of spin-dependent transport. Theorists from Pozna, Naples and Prague are experts in electronic transport, noise and current induced switching effects in magnetic nanostructures. The consortium plans to design and fabricate spin filters and detectors - fundamental elements for spintronics devices. The project includes a proposal to construct new types of spin filters with high efficiency based on the local Zeeman barrier in quantum point contacts and on the Stern-Gerlach effect. A recently discovered spin Hall effect (SHE) is very promising for new types of spin detectors. For spin detection SPINTRA proposes to fabricate and study various SHE devices, which might have the ability to electrically generate large degree of spin- polarizations in spatially small-confined regions. The project is also devoted to basic studies of the coherent spin- dependent transport in magnetic nanodevices, taking into account spin-flip processes, charge fluctuations and many body effects. The Nottingham team, together with other SPINTRA partners, will perform extensive studies of enhanced tunneling anisotropic magnetoresistance (TAMR) in various single electron devices. The recently discovered large TAMR effect is very promising for applications in spintronics. The project SPINTRA also includes investigations of current-induced dynamics and switching effects in various magnetic nanostructures, e.g., in systems with domain walls, metallic nanopilars, tunnel junctions, and single electron devices. These issues are of importance for applications in magnetically controlled logic and low-power non-volatile memories. Aims and objectives of the Individual Project contribution to the CRP: A key objective of the joint SPINTRA research project is to study and exploit spin-polarized transport in semiconductor hetero- and nanostructures for fabrication of spin filters and detectors as basic elements for spintronic devices. One possible approach is to use quantum point contacts as spin barriers for electrons in magnetic fields [1], resulting in a spin filtering element [2,3]. For usual semiconductors such as GaAs, however, the electron g-factors and thus, the spin splitting of the Landau levels is rather small (e.g., |g| = 0.43 for GaAs). Therefore, rather high magnetic fields are required for such devices, impeding the realization of hybrid spin filter structures in which the magnetic fields are produced by the stray fields of ferromagnets integrated on the semiconductor structures. One possible solution to this problem is to exploit narrow gap semiconductors that generally exhibit large g-factors and for which the Zeeman splitting thus becomes sufficiently large already a reasonable magnetic fields. The IV-VI semiconductors are particularly well suited in this respect not only because the electron and hole g-factors are very large but also due to the high carrier mobilities (up to 2 x 106 cm2/Vsec at 4 K [4]) achieved in epitaxial layers. This results in long mean free paths of the electrons and holes at low temperatures and results from the effective screening of charged impurities by the huge dielectric constant. As a result of the many valley band structure of the IVVI compounds, the g-factor is a tensor with components gl and gt along and parallel to the <111> directions. For PbTe, e.g., the effective g-factor in the longitudinal direction is as large as 66 [5] and therefore, the electron spin splitting amounts to up to 70 % of the Landau level splitting with the magnetic field is applied along the main valley axis. As a result, a spin splitting of 28 meV [5] and thus completely spin polarized electron currents can be achieved already at a magnetic field as small as 1 Tesla [6]. A particular feature of PbTe is the possibility for introduction of magnetic impurities into the lattice, forming diluted magnetic semiconductors in which the electronic properties are strongly affected by the exchange interactions. Introduction of Mn in PbTe even at small concentrations of around 1% thus leads to a strong g-factor enhancement up to values of 110, as found by magneto-optical studies [7]. As a consequence, spin polarized currents should be induced already at even smaller magnetic fields, i.e., a 10 meV spin splitting of the electron states in PbMnTe can be achieved at magnetic fields as low as 180 mT. This will strongly facilitate the control of the spin currents by ferromagnet hybrid structures integrated onto the semiconductor devices that is one of the main objectives of the joint research program. To exploit the favourable properties of the IV-VI compounds, the objective of the project part IP4 at the University of Linz is to fabricate IV-VI based heterostructures and quantum wells with high mobility using molecular beam epitaxy. The goal is to produce high mobility 2D electron gas structures that can be further process into spin filtering devices. For control of the spin currents and energy level population, these structures will contain epitaxial gates above or below the electron channels [2,3,7].