Disorder-induced reversal of spin polarization in the Heusler alloy Co2FeSi

We study the spin polarization in the conduction band of Co2FeSi layers with a different degree of structural order. The injected spin polarization in Co2FeSi/(Al,Ga)As spin light-emitting diodes as well as the planar Hall effect measured for the Co2FeSi injectors exhibit a sign reversal between injectors crystallized in the ordered L2(1) phase and the Fe-Si disordered B2 phase. These results are explained by a disorder-induced change in the spin polarization at the Fermi energy of Co2FeSi. Support for the occurrence of such a striking change in the electronic band structure is obtained by first principles calculations.