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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 T_S. An opposite sign of the electroluminescence polarization (ca. 20%), i.e., spin polarization of injected electrons has been observed for spin-LEDs grown at T_S=100 ^∘C and T_S=280 ^∘C, respectively. Previous structural studies revealed that the partially disordered B2 phase of Co₂FeSi dominates near the interface for lower T_S while the fully ordered L2₁ phase dominates for higher T_S. 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 T_S, 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.