Direct experimental comparison between the electrical spin injection in a semiconductor and in a metal

The electrical injection of spin polarized electrons in a semiconductor can be achieved in principle by driving a current from a ferromagnetic metal, where current is known to be significantly spin polarized, into the semicon ductor or normal metal via ohmic conduction. For detection a second ferroma gnet can be used as drain. In this paper we address the issue of the effici ency of such an approach to spin injection. For this purpose, we made submi cron multiterminal lateral spin valve junctions, with NiFe ferromagnetic el ectrodes. The ferromagnets were making good ohmic contact either to a two-d imensional electron gas (2DEG) channel, or to a Cu channel. In the all-meta l case we observe a clear spin accumulation signal. Due to spurious magneto resistive contribution of the ferromagnetic electrodes, this could only be detected in a non-local geometry. Despite all our efforts, we have not been able to observe spin injection in the semiconductor. We show that both res ults are in quantitative agreement with the theoretical predictions based o n conductivity mismatch arguments. (C) 2001 Published by Elsevier Science B .V.