Spin current, injection

FIGURE 6.9. I-V curves for devices spun from DCB and THF. Although both devices have essentially the same polymer film thickness, the device using DCB as spin-coating solvent has significantly higher current injection than that using THF. 

Magetoresistivity-based sensors, which are based on a resistivity change induced by a magnetic field, have found wide application in industry. However, such sensors are usually fabricated of metals and hence it is difficult to integrate them with conventional semiconductor devices. To develop magetoresistive sensors in semiconductors, one of the key problems is how to effectively inject spin-polarized electrons into paramagnetic semiconductors. The spin Hall effect (SHE), which involves a spin-current transverse to an applied electric field, " potentially provides a new way to resolve this issue. 

Many of the CC theoretical predictions, such as control of atomic and molecular processes via N versus M photon transitions [137], have been tested and demonstrated experimentally [138-146]. CC methods have also proved to be valid in the context of solid-state systems. In particular, it was shown that excitation by N and M multiphoton processes, having opposite parities, leads to symmetry breaking and the generation of DC electric currents [147-151]. These predictions have been confirmed experimentally in a number of semiconductors [152-155]. Similar techniques were shown to lead to the control of phonon emission [156] or injection of spin currents [157]. 

R. Bhat, J. Sipe, Optically injected spin currents in semiconductors, Phys. Rev. Lett. 85... 

Currem field characteristics measured wiih conjugated polymers sandwiched between an indium-tin oxide (ITO) anode and an aluminum cathode are usually hole dominated and are, consequently, appropriate for testing injection/lransport models for the case of unipolar current How. Data shown in Figure 12-1 refer to injection-limited currents recorded on typically 100 nm thick spin-coated films of derivatives of poly(y d/"fi-phenylenevinylene) (PPV) and a planarized poly(/ /" -pheny-leue) employing a Keilhley source measure unit. The polymers were ... 

Fig. 26. General reaction scheme for dye sensitized hole injection in organic crystals. The scheme shows the excitation of the adsorbed dye ( ... ) in the first row at the left hand side. Through electron transfer the "reduced dye-hole pair" (2D. . , 2C+) is generated with its singlet and triplet spin state (2. row). The dissociated "reduced dye-hole pair is shown in the third row, and finally the hole contributing to the injection current in the fourth row. The regeneration of the dye molecules can be seen on the left hand side of the third row and the generation of sensitized delayed crystal fluorescence in the first row on the right hand side. Details are explained in Section IV 4 of the text...

The differential resistance of mesoscopic ferromagnet/superconductor junctions shows a number of features associated with the injection of spin-polarized carriers into the superconductor. In particular, large peaks are observed at currents corresponding to the superconducting gap voltage. These peaks are... 

In very pure nonpolar dielectric liquids, electron injection currents at very sharp tips follow the Fowler-Nordheim voltage dependence (Halpem and Gomer, 1969), just as is the case in solid insulators, and in a gas, as described before. In a study of the electrochemical behavior of CNT cathodes (Krivenko et al., 2007) direct experimental proof was found of electron emission into the liquid hexamethylphosphortriamide, which was chosen because it is a convenient solvent for the visualization of solvated electrons at room temperature the solution will show an intense blue coloration upon the presence of solvated electrons. Electron spin resonance showed prove of a free electron. Electrogenerated (as opposed to photogenerated) solvated electrons have been used in the synthesis of L-histidinol (Beltra et al., 2005), albeit that in that work the electrons were generated electrochemically from a solution of LiCl in EtNH2, which is a solvent that is easier to handle than liquid ammonia (boiling points at atmospheric pressure are 17 °C and -33.34 °C, respectively). 

When large current densities (10+7 A/cm2) cross CPP spin valves, spin transfer effects start to be observed. When a spin polarized DC current flows across the CPP device, both well defined spin wave modes, and chaotic spin wave generation has been observed as a function of the injected DC current [65], These spin-wave modes lead to excess magnetic noise. These effects were first predicted by Berger [66] and Slonczewski [67], and have been now experimentally observed by several groups. At low frequencies, spin transfer effects also affect the CPP device transfer curve [68], and can be used to induce free layer switching in magnetoresistive devices. 

As a result, nearly perfect interfaces between the ferromagnetic material and the semiconductor are not a prerequisite for efficient spin injection. It is for example possible to insert a non-magnetic seed layer between the ferromagnetic base layer and the semiconductor collector. Since hot electrons retain their spin moment while traversing the thin non-magnetic layer this will not drastically reduce the spin polarization of the injected current. Finally, since electron injection is ballistic in SVT and MTT devices the spin injection efficiency is not fundamentally limited by a substantial conductivity mismatch between metals and semiconductors [161, 162], The latter is the case in diffusive ferromagnetic metal/semiconductor contacts [163],... 

The second spin transistor design that is addressed uses a spin injecting current emitter, hence its acronym SPICE [167, 168], It uses semiconductors in its construction, like the SVT and MTT unlike them, however, its operation also requires spin-dependent transport in the semiconductor itself and thereby arise a small family of additional materials problems which are discussed below. 

The spin transistor as represented in Fig. 34 is a vertical spin transistor. Figure 35 is a schematic picture of a lateral spin transistor as originally proposed by Datta and Das [179]. In this case, a iron emitter injects spins into a 2D electron gas. A Schottky gate can rotate the spin polarization by the Rashba effect, and another iron analyzer detects transmitted spin polarized current. 

FIGURE 10.13. Dependence of the emission intensity on injection current of one-, two-, and three-layer LEDs. A spin-cast PDHF film was used for the single-layer device and vapor-deposited films were used for the two- and three-layer devices. (From Ref. 31.)... 

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