R. Farshchi

Optical communication of spin information between light emitting diodes

For the full implementation of spintronic circuits, it is necessary to transmit spin information from one device to another. Electrons in semiconductors often suffer from high spin relaxation rates, making electrical transport of spin information highly inefficient. Here, we propose optical transport of spin information as an alternative. We demonstrate that the spin information associated with electrons injected from Co2FeSi and Fe layers into the quantum wells of spin light emitting diodes (spin-LEDs) can be transported optically in the form of circularly polarized light and deciphered electrically via the magnetic field dependence of the photocurrent in a distant detector spin-LED.

All-electrical spin injection and detection in the Co2FeSi/GaAs hybrid system in the local and non-local configuration

We demonstrate the electrical injection and detection of spin-polarized electrons in the Co2FeSi/GaAs hybrid system using lateral transport structures. Spin valve signatures and characteristic Hanle curves are observed both in the non-local and the local configuration. The comparatively large magnitude of the local spin valve signal and the high signal-to-noise ratio are attributed to the large spin polarization at the Fermi energy of Co2FeSi in the well-ordered L21 phase.

Magnetocrystalline anisotropy and magnetization reversal in Ga1- xMnxP synthesized by ion implantation and pulsed-laser melting

We report the observation of ferromagnetic resonance (FMR) and the determination of the magnetocrystalline anisotropy in (100)-oriented single-crystalline thin film samples of GaMnP with x=0.042. The contributions to the magnetic anisotropy were determined by measuring the angular- and the temperature-dependencies of the FMR resonance fields and by superconducting quantum interference device magnetometry. The largest contribution to the anisotropy is a uniaxial component perpendicular to the film plane; however, a negative contribution from cubic anisotropy is also found. Additional in-plane uniaxial components are observed at low temperatures, which lift the degeneracy between the in-plane [011] and [01-1] directions as well as between the in-plane [010] and [001] directions. Near T=5K, the easy magnetization axis is close to the in-plane [01-1] direction. All anisotropy parameters decrease with increasing temperature and disappear above the Curie temperature T_C. A consistent picture of the magnetic anisotropy of ferromagnetic GaMnP emerges from the FMR and magnetometry data. The latter can be successfully modeled when both coherent magnetization rotation and magnetic domain nucleation are considered.

Compositional tuning of ferromagnetism in Ga1-xMnxP

We report the magnetic and transport properties of Ga{sub 1-x}Mn{sub x}P synthesized via ion implantation followed by pulsed laser melting over a range of x, namely 0.018 to 0.042. Like Ga{sub 1-x}Mn{sub x}As, Ga{sub 1-x}Mn{sub x}P displays a monotonic increase of the ferromagnetic Curie temperature with x associated with the hole-mediated ferromagnetic phase while thermal annealing above 300 C leads to a quenching of ferromagnetism that is accompanied by a reduction of the substitutional fraction of Mn. However, contrary to observations in Ga{sub 1-x}Mn{sub x}As, Ga{sub 1-x}Mn{sub x}P is non-metallic over the entire composition range. At the lower temperatures over which the films are ferromagnetic, hole transport occurs via hopping conduction in a Mn-derived band; at higher temperatures it arises from holes in the valence band which are thermally excited across an energy gap that shrinks with x.

Suppression of hole-mediated ferromagnetism in Ga1−xMnxP by hydrogen

We report the passivation of the Mn acceptors in Ga1−xMnxP upon exposure to a hydrogen plasma. The as-grown films are nonmetallic and ferromagnetic with a Curie temperature of TC=55 K. After hydrogenation the sample resistivity increases by approximately three orders of magnitude at room temperature and six orders of magnitude at 25 K. Furthermore, the hydrogenated samples are paramagnetic, which is evidenced by a magnetization curve at 5 K that is best described by a Brillouin function with g=2 and J=5/2 expected for Mn atoms in the 3d5 configuration. Upon annealing, partial depassivation and a recovery of ferromagnetism are observed. These observations unambiguously demonstrate that the ferromagnetism in Ga1−xMnxP is carrier-mediated similar to Ga1−xMnxAs.

Nonlocal, local, and extraction spin valves based on ferromagnet/semiconductor hybrid structures consisting of the Heusler alloy Co2FeSi on GaAs

We demonstrate electrical injection, extraction and detection of spin polarization in a lateral transport structure consisting of ferromagnetic Co2FeSi stripes on a nonmagnetic n-GaAs transport channel. For the lateral transport based on electrical spin injection, the characteristic spin-valve signatures are observed both in the nonlocal and local configurations. The comparatively large magnitude of the local spin valve signal and the high signal-to-noise ratio are attributed to the large spin polarization at the Fermi energy of Co2FeSi in the well-ordered L21 phase. Furthermore, we elucidate a device concept in which the basic building block consists of a local spin valve utilizing spin extraction instead of injection at the ferromagnetic stripes for its fundamental operation principle. An extended device comprises an array of such extraction-spin valves in which the spin polarization in the transport channel results from a cascade of spin extraction events. Such a multiple-extraction spin valve acts as a nonvolatile reconfigurable current divider in which a single electrical output corresponds to a particular magnetization configuration of the entire stripe array (2m-1 electrical output levels for m ferromagnetic contacts). We discuss potential implementations of this concept for spintronic memory circuits and for sources of highly spin-polarized drift currents.