Electrical spin injection from Fe into AlxGa1−xAs quantum well heterostructures is demonstrated in small (< 500 Oe) in-plane magnetic fields. The measurement is sensitive only to the component of the spin that precesses about the internal magnetic field in the semiconductor. This field is much larger than the applied field and depends strongly on the injection current density. Details of the observed hysteresis in the spin injection signal are reproduced in a model that incorporates the magnetocrystalline anisotropy of the epitaxial Fe film, spin relaxation in the semiconductor, and the dynamical polarization of nuclei by the injected spins.PACS numbers: 72.25. Hg, 72.25.Rb, 76.60.Jx The injection of spin from a conventional ferromagnetic metal into a semiconductor is a prerequisite for several proposed magneto-electronic devices [1]. Although spin transport across the ferromagnet-semiconductor (FM-S) interface has recently been demonstrated [2,3,4,5], most injection experiments on metallic FM-S systems have required relatively large magnetic fields, in excess of several kilogauss, to produce a spin component perpendicular to the FM-S interface. The most useful properties of typical ferromagnetic thin films, however, such as low-field switching and hysteresis, can be exploited only by coupling to the in-plane component of the magnetization [6]. In the case of metallic FM-S structures, in-plane coupling has been observed only as a small change in transport properties [2] or using optically pumped carriers [7,8].In this Letter we report a demonstration of electrical spin injection in FM-S heterostructures using small (< 500 Oe) in-plane magnetic fields. We measure only the component of the spin that precesses after injection into the semiconductor using electroluminescence polarization (ELP) as a detection technique [6,9]. The effective magnetic field inducing the precession depends strongly on the electrical bias conditions and is dramatically enhanced at the highest injection current densities. The origin of the hysteresis in the spin polarization signal is magnetization reversal in the ferromagnet, but the magnitude and shape of the observed loops depend on the effective field in the semiconductor. Modeling based on the results of optical pumping experiments demonstrates that the origin of the large effective field is dynamical nuclear polarization due to the spin-polarized current injected from the ferromagnet [10]. This approach to dynamical nuclear polarization in semiconductors is a simple alternative to the use of optical pumping or high magnetic fields as sources of spin-polarized electrons [11,12].We report results from two heterostructures with different quantum well (QW) spin detectors. The samples are grown by molecular beam epitaxy on p + GaAs (100) substrates and consist of p-Al x Ga 1−x As/QW/n-

We have studied hyperfine interactions between spin-polarized electrons and lattice nuclei in Al0.1Ga0.9As/GaAs quantum well (QW) heterostructures. The spin-polarized electrons are electrically injected into the semiconductor heterostructure from a metallic ferromagnet across a Schottky tunnel barrier. The spin-polarized electron current dynamically polarizes the nuclei in the QW, and the polarized nuclei in turn alter the electron spin dynamics. The steady-state electron spin is detected via the circular polarization of the emitted electroluminescence. The nuclear polarization and electron spin dynamics are accurately modeled using the formalism of optical orientation in GaAs. The nuclear spin polarization in the QW is found to depend strongly on the electron spin polarization in the QW, but only weakly on the electron density in the QW. We are able to observe nuclear magnetic resonance (NMR) at low applied magnetic fields on the order of a few hundred Oe by electrically modulating the spin injected into the QW. The electrically driven NMR demonstrates explicitly the existence of a Knight field felt by the nuclei due to the electron spin.

Epitaxial ferromagnetic metal -semiconductor heterostructures are investigated using polarization-dependent electroabsorption measurements on GaAs p-type and n-type Schottky diodes with embedded In 1-x Ga x As quantum wells. We have conducted studies as a function of photon energy, bias voltage, magnetic field, and excitation geometry.For optical pumping with circularly polarized light at energies above the band edge of GaAs, photocurrents with spin polarizations on the order of 1 % flow from the semiconductor to the ferromagnet under reverse bias. For optical pumping at normal incidence, this polarization may be enhanced significantly by resonant excitation at the quantum well ground-state. Measurements in a side-pumping geometry, in which the ferromagnet can be saturated in very low magnetic fields, show hysteresis that is also consistent with spin-dependent transport. Magneto-optical effects that influence these measurements are discussed.

Motivated by the anticipated advantageous performance of diamond kinoform refractive lenses for synchrotron X-ray radiation studies, this report focuses on progress in designing, nanofabricating and testing of their focusing performance. The method involves using lift-off and plasma etching to reproduce a planar definition of numerically determined kinoform refractive optics. Tests of the focusing action of a diamond kinoform refractive lens at the APS 8-ID-I beamline demonstrate angular control of the focal spot.

Epoxy based nanocomposites using graphene oxide (GO) sheets dispersed multi-walled carbon nanotubes (CNTs) as combination fillers were prepared using an in situ polymerization technique. A remarkable synergetic effect was observed between CNTs and GO sheets which improved the mechanical properties of the epoxy. It was confirmed by optical and field-emission scanning electron microscopy (FESEM) images that the dispersion of CNTs in epoxy matrix can be significantly improved by adding GO sheets. The overall mechanical properties of CNT-GO/epoxy composites were greatly enhanced with only adding 0.04 wt% (percent by weight) CNTs and 0.2 wt% GO sheets. Moreover, the fatigue and creep rupture lives of pure epoxy was also significantly increased by the addition of GO dispersed CNTs. Approximately a 950% improvement in fatigue life, and 400% improvement in creep rupture life were observed at the applied stress levels tested.