Spin Injection Experiments from Half-Metallic Ferromagnets into Semiconductors: The Case of NiMnSb and (Ga,Mn)As

Abstract

Materials with 100% conduction electron spin polarization are possible candidates for electrical spin injection into non-magnetic semiconductors, as they do not suffer from the conduction mismatch problem encountered by ferromagnetic metals with incomplete spin polarization. However, I show that truly half-metallic contacts are extremely challenging to fabricate, and will in many cases suffer from other problems such as the presence of a Schottky barrier that prevents the injection of any carriers at all. The suggested solution is to insert a tunnel barrier to accommodate both the Schottky barrier and any deviations from a true half-metallic nature. The largest spin injection we have obtained using NiMnSb is (6% for polycristalline films on AlO x tunnel barriers, and 2.2% or less for epitaxial single crystalline films. This is much below what is obtained using traditional contacts such as (Co)Fe, and suggests that the experimental NiMnSb/GaAs interfaces have a low spin polarization. On the other hand, using a Zener tunnel junction to convert polarized holes into polarized electrons, we have obtained 80% spin injection from the ferromagnetic semiconductor (Ga,Mn)As into non-magnetic GaAs, showing that this material possesses a very high spin polarization.