In a recent Letter Hammar et al. claim the observation of injection of a spin-polarized current in a two-dimensional electron gas (2DEG). This is an important observation, since, despite considerable effort of several groups, all attempts to realize spin-injection into a 2DEG using purely electrical measurements have failed sofar. However, in my opinion the claim made is not correct, and the observed behaviour can be explained by a combination of a magneto resistance (Hall) effect (e.g. generated by the fringe magnetic fields present at the edges of the ferromagnetic electrode), with a {\it spin-independent} rectification effect due to the presence of a metal- semiconductor junction.
@article{arxiv.cond-mat/0001187,
title = {Comment on "Observation of Spin Injection at a Ferromagnet-Semiconductor Interface, by P.R. Hammar et al},
author = {B. J. van Wees},
journal= {arXiv preprint arXiv:cond-mat/0001187},
year = {2009}
}