  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.

Condensed Matter