27-08-2014, 12:21 PM
SPINTRONICS
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Introduction:-
SPINTRONICS is made of two words Spin and electronics. It refers to the study of the role played by the electron spin in solid state physics, possible devices that specifically exploit spin properties instead of or in addition to charge degree of freedom for example spin relaxation and spin transport in metal and semiconductors are of fundamental research interest not only for being basic solid state physics issues, but also for the already demonstrated potential these phenomena have in electronic technology. The prototype device that is already in use in industry as a read head and memory storage cell is the Giant Magneto Resistive (GMR) sandwich structure which consists of alternating ferromagnetic and nonmagnetic metal layers. Depending on the relative orientation of the magnetizations in the magnetic layers the device resistance changes from small to large. This is also called magneto resistance. It is used to sense changes in magnetic field. Recent efforts in GMR technology have also involved magnetic tunnel junction devices where the tunneling current depends on spin orientations of the electrodes.
History of Spintronics-
In a pioneering work, Mott (1936a, b) provided a basis for our understanding of spin-polarized transport. Mott sought an explanation for an unusual behavior of resistance in ferromagnetic metals. He realized that at sufficiently low temperatures, where magnon scattering becomes vanishingly small, electrons of majority and minority spin, with magnetic moment parallel and anti parallel to the magnetization of a ferromagnet, respectively, do not mix in the scattering processes. The conductivity can then be expressed as the sum of two independent and unequal parts for two different spin projections–the current in ferromagnets is spin polarized. This is also known as the two-current model and has been extended by Campbell et al. (1967); Fert and Campbell (1968). It continues, in its modifications, to provide an explanation for various magnetoresistive phenomena (Valet and Fert, 1993).
Tunneling measurements played a key role in early experimental work on spin-polarized transport. Studying N/F/N junctions, where N was a nonmagnetic7 metal and F was
1) Spin Injection & optical orientation-
Many materials in their ferromagnetic state can have a substantial degree of equilibrium carrier spin polarization. However, as illustrated, this alone is usually not sufficient for spintronic applications, which typically require current flow or manipulation of the nonequilibrium spin (polarization).The importance of generating nonequilibrium spin is not limited to device applications; it can also be used as a sensitive spectroscopic tool to study a wide variety of fundamental properties ranging from spin-orbit and hyperfine interactions to the pairing symmetry of high temperature superconductors and the creation of spin-polarized beams to measure parity violation in high energy physics (Pierce and Celotta, 1984).
Nonequilibrium spin is the result of some source of pumping arising from transport, optical, or resonance methods. Once the pumping is turned off the spin will return to its equilibrium value. While for most applications it is desirable to have long spin relaxation times, it has been demonstrated that short spin relaxation times are useful in the implementation of fast switching (Nishikawa, 1995).
Electrical spin injection, an example of a transport method for generating nonequilibrium spin, has already been realized experimentally by Clark and Feher (1963), who drove a direct current through a sample of In Sb in the presence of constant applied magnetic field. The principle was based on the Feher effect, 18 in which the hyperfine coupling between the electron and nuclear spins, together with different temperatures representing electron velocity and electron spin populations, is responsible for the dynamical nuclear polarization (Slichter, 1989).
Magnetic Bipolar transistor-
The magnetic bipolar transistor (MBT) is a bipolar transistor with spin-split carrier bands and, in general, an injected spin. A related device structure was already proposed by Gregg et al. (1997) in a push for silicon-based spintronics. In this proposal (also called SPICE for spin-polarized injection current emitter) the semiconductors have no equilibrium spin, while the spin source is provided by a ferromagnetic spin injector attached to the emitter, and another ferromagnetic metal, a spin detector, is attached to the base/collector junction to modulate the current flow. In both configurations the aim is to control current amplification by spin and magnetic field.
A scheme of a particular MBT is shown in Fig. 32. Such a three-terminal device can be thought of as consisting of two magnetic p-n junctions connected in scheme of an n-p-n magnetic bipolar transistor with magnetic base (B), nonmagnetic emitter (E), and collector ©. Conduction and valence bands are separated by the energy gap Eg. The conduction band has a spin splitting 2q_, leading to equilibrium spin polarization PB0 = tanh〖(q/KBT)〗. Carriers and depletion regions are represented as in Fig. 30. In the so called forward active regime, where the transistor can amplify currents, the E-B junction is forward biased (here with voltage VBE > 0 lowering the built-in potential Vbi), while the B-E junction is reverse biased (VBC < 0). The directions of the current flows are indicated. Electrons flow from E to B, where they either recombine with holes (dashed lines) or continue to be swept by the electric field in the B-E depletion layer towards C. Holes form mostly the base current, IB, flowing to the emitter. The current amplification β = IC/IB can be controlled by PB0 as well as by the nonequilibrium spin in E. D also apply to an MBT in order to provide a sufficient equilibrium polarization in a magnetic base PB0. While nonmagnetic, the emitter has a nonequilibrium polarization dPE from a spin source, similar to the magnetic diode case in Fig. 30. Only the spin polarization of electrons is assumed. Applying the generalized Shockley theory to include spin effects (Fabian et al., 2002b), a theory of MBT was developed. Later, simplified schemes of MBT [not including the effect of nonequilibrium spin (dPE = 0)] were also considered by Flatt´e(2003) and Lebedeva and Kuivalainen (2003).
The current amplification (gain) β = IC/IB (see Fig. 32) is typically ~ 100 in practical transistors. This ratio depends on many factors, such as the doping densities, carrier lifetimes, diffusion coefficients, and structure geometry. In an MBT β also depends on the spin splitting 2q? (see Fig. 32) and the nonequilibrium polarization dPE. This additional dependence of β in an MBT is called magneto amplification. An important prediction is that the nonequilibrium spin can be injected at low bias all the way from the emitter, through the base, to the collector in order to make possible an effective control of β by dPE.
Conclusion-
We have reviewed selected topics on spintronics, emphasizing both the fundamental aspects of spin dynamics, transport, and relaxation, and the potential applications. While the current push for spintronics is driven by the prospect of technological applications, the fundamental spin physics, which has a longstanding tradition in the solid-state community, is by itself exciting and worth pursuing. Furthermore, even though many proposed spintronic device schemes may turn out to be impractical in the end, their importance lies in stimulating interesting experimental and theoretical research.
There are many challenges and open questions to be tackled by future research, in particular a robust spin injection into silicon. While GaAs is of great technological importance, the control of spin in silicon would raise hopes for seamless integration of spintronics with the current information technology. In addition, the small magnitude of the spin-orbit interaction and the absence of inversion symmetry lead to relatively long room-temperature spin lifetimes, relaxing some constraints on the operational length and time scales. Important materials advances have been realized in improving the compatibility of Si structures (Sieg et al., 1998) suggesting a possibility that the existing control of spin in GaAs or in III-V ferromagnetic semiconductors might be extended to Si.
Future progress in spin-polarized transport will be largely driven by the materials advances. In the context of semiconductors, considering all-semiconductor structures rather than the hybrid structures with metallic ferromagnets will depend on the improvements in ferromagnetic semiconductors, for example, whether they can achieve higher mobility, higher Curie temperature,128 and a simple fabrication of high quality interfaces with nonmagnetic materials. What is missing, even in the currently available materials, is a systematic understanding of the effects of magnetic interfaces and materials inhomogeneities on spin-polarized transport. A comprehensive transport calculation in the actual devices with realistic electronic structure of the studied materials would provide valuable insights into both the spin polarization being measured and how it is reduced from the moment it was generated.
Spin relaxation and spin dephasing of conduction electrons is a rather field, with the basic principles well understood. What is needed are accurate band-structure derived calculations of spin relaxation times, in both metals and semiconductors. The same can be said of g factor, calculation of which from first principles is a nontrivial task that has not been accomplished even for the elemental metals. An important and still debated issue is spin relaxation and decoherence of localized or confined electrons, where the hyperfine-interaction mechanism dominates. Furthermore, single-spin relaxation and decoherence, and their relation to the ensemble spin dephasing, need to be pursued further in the context of quantum computing. A first step towards understanding single spin relaxation is the recent experiment of Hanson et al. (2003b) in a one-electron quantum dot.