Using free electron approximation, we calculated the spin dependent tunnel conductance of ballistic ferromagnet / tunnel barrier / two-dimensional electron gas (FM/I/2DEG) junctions and FM/I/2DEG/I/FM double junctions for different barrier strengths. We find that a tunnel barrier improves spin injection considerably. For sufficiently strong barriers, it is predicted that the tunnel conductance ratio between spin up and spin down channels is, in first approximation, equal to the ratio between their Fermi velocities in the FM. For single junctions, this results in a significant current polarization $(\sim 10\%).$ This corresponds to a relative resistance change of several percent between parallel and antiparallel magnetization of the two FM electrodes, respectively, for the double junction. In the weak barrier regime, the magnitude and sign of the current polarization are strongly dependent on the (controllable) electron density in the 2DEG.

• Received 30 December 2000

DOI:https://doi.org/10.1103/PhysRevB.64.161307