Electrical Spin Injection in a Ferromagnetic / Tunnel Barrier/ Semiconductor Heterostructure

We demonstrate experimentally the electrical ballistic electron spin injection from a ferromagnetic metal / tunnel barrier contact into a semiconductor III-V heterostructure. We introduce the Oblique Hanle Effect technique for reliable optical measurement of the degree of injected spin polarization. In a CoFe / Al2O3 / GaAs / (Al,Ga)As heterostructure we observed injected spin polarization in excess of 8 % at 80K.Comment: 5 pages, 4 figure

Highly efficient room temperature spin injection in a metal-insulator-semiconductor light emitting diode

We demonstrate highly efficient spin injection at low and room temperature in an AlGaAs/GaAs semiconductor heterostructure from a CoFe/AlOx tunnel spin injector. We use a double-step oxide deposition for the fabrication of a pinhole-free AlOx tunnel barrier. The measurements of the circular polarization of the electroluminescence in the Oblique Hanle Effect geometry reveal injected spin polarizations of at least 24% at 80K and 12% at room temperature

An innovative and viable routre for the realization of ultra-thin supercapacitor electrodes assembled with carbon nanotubes

Electrochemical Double Layer Capacitors (EDLC), also known as supercapacitors, have been fabricated using Single Walled Carbon Nanotubes (SWCNTs) as active material for electrode assembling. In particular a new way of fabrication of ultra-thin electrodes (≤25 m) directly formed on the separator has been proposed, and a prototype of EDLC has been realized and tested. For such devices the specific capacitance is in the range 40–45 F/g and the internal resistances in the range 6–8 ·cm2, at current density of 2 mA·cm−2. Keywords: Carbon Nanotube, Supercapacito

Spin diffusion and injection in semiconductor structures: Electric field effects

In semiconductor spintronic devices, the semiconductor is usually lightly doped and nondegenerate, and moderate electric fields can dominate the carrier motion. We recently derived a drift-diffusion equation for spin polarization in the semiconductors by consistently taking into account electric-field effects and nondegenerate electron statistics and identified a high-field diffusive regime which has no analogue in metals. Here spin injection from a ferromagnet (FM) into a nonmagnetic semiconductor (NS) is extensively studied by applying this spin drift-diffusion equation to several typical injection structures such as FM/NS, FM/NS/FM, and FM/NS/NS structures. We find that in the high-field regime spin injection from a ferromagnet into a semiconductor is enhanced by several orders of magnitude. For injection structures with interfacial barriers, the electric field further enhances spin injection considerably. In FM/NS/FM structures high electric fields destroy the symmetry between the two magnets at low fields, where both magnets are equally important for spin injection, and spin injection becomes locally determined by the magnet from which carriers flow into the semiconductor. The field-induced spin injection enhancement should also be insensitive to the presence of a highly doped nonmagnetic semiconductor (NS$^+$) at the FM interface, thus FM/NS$^+$/NS structures should also manifest efficient spin injection at high fields. Furthermore, high fields substantially reduce the magnetoresistance observable in a recent experiment on spin injection from magnetic semiconductors

Spin injection into a ballistic semiconductor microstructure

A theory of spin injection across a ballistic ferromagnet-semiconductor-ferromagnet junction is developed for the Boltzmann regime. Spin injection coefficient $\gamma$ is suppressed by the Sharvin resistance of the semiconductor $r_N^*=(h/e^2)(\pi^2/S_N)$, where $S_N$ is the Fermi-surface cross-section. It competes with the diffusion resistances of the ferromagnets $r_F$, and $\gamma\sim r_F/r_N^*\ll 1$ in the absence of contact barriers. Efficient spin injection can be ensured by contact barriers. Explicit formulae for the junction resistance and the spin-valve effect are presented.Comment: 5 pages, 2 column REVTeX. Explicit prescription relating the results of the ballistic and diffusive theories of spin injection is added. To this end, some notations are changed. Three references added, typos correcte

Spintronics: Fundamentals and applications

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes from the published versio

Nanocomposite Si/diamond layers: room temperature visible-light emitting systems

Visible-light emitting nanocomposite Si/diamond polycrystalline layers, produced by means of a hybrid CVD/powder-flowing technique, have been investigated by atomic force microscopy (AFM),reflection high energy electron diffraction (RHEED), Raman spectroscopy, photoluminescence(PL), and electron spin resonance (ESR). The room temperature emission of the layers at 1.7 eV and 2.0-2.4 eV is dominated by the optical properties of the inserted Si nanoparticles (mean diameter 3.3 +/- 0.5 nm). The density of the ESR active centers is N = (4 +/- 2) x 10(19) cm(-3) with g = 2.0025 (T = 300 K). A study of the temperature-dependent changes in the nature and localization of paramagnetic centers has been carried out by comparing the ESR signals taken at 300 K, 30 K, and 4.2 K. The ESR results suggest that the active centers originate from the dangling bonds induced in sp(3)-coordinated C atoms by insertion of the Si nanoparticles. The g-factor anisotropy, detected at 4.2 K, is consistent with the peculiarities of dipole-dipole interactions in structures with low dimensionality

Electron spin resonance study of poly(ortho-anisidine)/single-walled carbon nanotube composite films: spin dynamics and effects of physisorption processes

Poly(ortho-anisidine) (POA)/single-walled carbon nanotubes (SWCNTs) composite films have been prepared by means of oxidative potentiostatic electropolymerization of the monomer (ortho-anisidine) in the presence of purified SWCNTs. The magnetic properties of the obtained films have been investigated by electron spin resonance (ESR) (ν ∼= 9.4GHz; temperature range 10–300K) and compared with those of pure POA films in the same oxidation state. The similarity of the ESR signals and the dependence on temperature of the ESR line features suggest a polaron origin of the paramagnetic states. A strong increase of the ESR integral intensity has been detected for composite samples kept for 72 h in hydrogen atmosphere at room temperature. Under the same conditions, a less pronounced increase in the intensity has been detected in the case of pure polymer films. The kinetics of the hydrogen adsorption–desorption processes in the nanocomposite samples has been investigated and interpreted in the frame of an H-physisorption mechanism. The comparison between the H-induced effects in POA and nanocomposite POA/SWCNTs films has been carried out taking into account the structure of the samples and the presence of defects

Superparamagnetic resonance of Ni nanoparticles in single-walled carbon nanotubes systems

The paper has been also selected for the publication on Virtual J on Nanoscale Science and Technology (2007) vol 15