Regensburg 2010 – wissenschaftliches Programm
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MA: Fachverband Magnetismus
MA 6: Magnetic Thin Films I (Heusler Alloys)
MA 6.10: Vortrag
Montag, 22. März 2010, 16:45–17:00, H10
Disorder-induced sign reversal of spin polarization in Co2FeSi — •Pawel Bruski1, Oliver Brandt1, Steve Erwin2, Rouin Farshchi1, Klaus-Jürgen Friedland1, Jens Herfort1, and Manfred Ramsteiner1 — 1Paul-Drude-Institut für festkörperelektronik, 10117 Berlin — 2Center for Computational Materials Science, Naval Research Laboratory, Washington, DC 20375, USA
The ferromagnetic Heusler alloy Co2FeSi is closely lattice matched to GaAs and is predicted to be halfmetallic, meaning electrons at the Fermi level are 100% spin-polarized. We investigated spin light emitting GaAs/(Al,Ga)As diodes (spin-LEDs) with Co2FeSi injection layers grown by molecular-beam epitaxy at different substrate temperatures TS. An opposite sign of the electroluminescence polarization (ca. 20%), i.e., spin polarization of injected electrons has been observed for spin-LEDs grown at TS=100 ∘C and TS=280 ∘C, respectively. Previous structural studies revealed that the partially disordered B2 phase of Co2FeSi dominates near the interface for lower TS while the fully ordered L21 phase dominates for higher TS. Consequently, the experimentally observed sign reversal is attributed to a different spin orientation dominating at the Fermi energy of the two different Co2FeSi phases. This conclusion is supported by first-principles calculations of the density of states by the LDA+U method. For intermediate TS, Co2FeSi layers consist of a spatially inhomogeneous distribution of the L21 and the B2 phases compensating each other by the injection of oppositely spin-polarized electrons. Consequently, the corresponding spin-LEDs exhibit a comparatively small total spin-injection efficiency.