Both electrical injection and detection of spin-polarized electrons are demonstrated in a single wafer allsemiconductor GaAs-based lateral spintronic device, employing p + -͑Ga, Mn͒As/ n + -GaAs ferromagnetic Esaki diodes as spin aligning contacts. Spin-dependent phenomena, such as spin precession and spin-valve effect, are observed in nonlocal signal and the measurements reveal the unusual origin of the latter in the investigated devices. The conversion of spin-polarized holes into spin-polarized electrons via Esaki tunneling leaves its mark in a bias dependence of the spin-injection efficiency, which at maximum reaches the value of 50%.
We report here on anisotropy of spin polarization obtained in lateral all-semiconductor, all-electrical spin injection devices employing p + -͑Ga, Mn͒As/ n + -GaAs Esaki diode structures as spin aligning contacts, resulting from the dependence of the efficiency of spin tunneling on the orientation of spins with respect to different crystallographic directions. We observed an in-plane anisotropy of 8% in the case of spins oriented either along ͓110͔ or ͓110͔ direction and 25% anisotropy between in-plane and perpendicular-to-plane orientations of spins. © 2009 American Institute of Physics. ͓doi:10.1063/1.3247187͔Realizing functional devices based on spins in semiconductors requires efficient spin injection of spin-polarized electrons into semiconducting material and subsequent detection of the resulting spin accumulation.1 One of the very promising materials to be utilized as a source of spinpolarized carriers in such devices is the III-V ferromagnetic semiconductor ͑Ga,Mn͒As that can be grown epitaxially on GaAs-based heterostructures.2 Employing an Esaki-Zener diode structure ͑Ga, Mn͒As/ n + -GaAs allows using p-type ͑Ga,Mn͒As material as an efficient source of spin-polarized electrons.3-5 Under small negative bias electrons from the valence band of ͑Ga,Mn͒As can tunnel to the conduction band of GaAs, and spin injection takes place. Under positive bias tunneling in reverse direction occurs, and spin polarization in GaAs is established due to extraction of majority spins from GaAs. As was shown experimentally and supported theoretically, the spin polarization can be relatively high at low biases and then decreases with increasing bias. 6,7 In a recent report 8 we have presented a lateral device based on ͑Ga,Mn͒As/GaAs structure with a successful implementation of an efficient all-electrical spin injection and detection scheme. We have measured a relatively high spin injection efficiency P of Ϸ50% for low bias currents ͉I͉ Յ 10 A. In this study we test such a device for the dependence of the obtained spin injection efficiency on the orientation of injected spins. As recently discovered, the tunnel resistance in structures with a single ͑Ga,Mn͒As ferromagnetic layer depends on the relative orientation of magnetization in that layer with respect to the direction of crystallographic axes leading to an effect called tunneling anisotropic magnetoresistance ͑TAMR͒.9 By analogy, the anisotropy in related polarization of tunneling current could be described as tunneling anisotropic spin polarization ͑TASP͒.
We report here on a local spin valve effect observed unambiguously in lateral all-semiconductor all-electrical spin injection devices, employing p+ −(Ga,Mn)As/n+ −GaAs Esaki diode structures as spin aligning contacts. We discuss the observed local spin-valve signal as a result of the interplay between spin-transport-related contribution and the tunneling anisotropic magnetoresistance of the magnetic contacts. The magnitude of the spin-related magnetoresistance change is equal to 30 Ω which is twice the magnitude of the measured non-local signal
Spin injection from Fe(001) and (Ga,Mn)As(001) into n-GaAs(001) was investigated using a method which provides two-dimensional cross-sectional images of the spin polarization in GaAs. While the distribution of the spin polarization below the injecting contact is nearly uniform for (Ga,Mn)As, a strong confinement near the contact edge is observed for Fe and FeCo. The spin polarization in GaAs changes sign when the injected current is reversed. Multiple sign reversals as a function of bias voltage as reported previously for Fe injectors are not observed with (Ga,Mn)As and Fe contacts grown on clean n þþ ÀGaAs in agreement with earlier results for an epitaxial FeCo injector.
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