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 semiconductor or normal metal via ohmic conduction. For detection a second ferromagnet can be used as drain. In this paper we address the issue of the efficiency of such an approach to spin injection. For this purpose, we made submicron multiterminal lateral spin valve junctions, with NiFe ferromagnetic electrodes. The ferromagnets were making good ohmic contact either to a two-dimensional electron gas (2DEG) channel, or to a Cu channel. In the all-metal case we observe a clear spin accumulation signal. Due to spurious magnetoresistive 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 results are in quantitative agreement with the theoretical predictions based on conductivity mismatch arguments.