Spin injection and relaxation in ferromagnet-semiconductor heterostructures

We present a complete description of spin injection and detection in\nFe/Al_xGa_{1-x}As/GaAs heterostructures for temperatures from 2 to 295 K.\nMeasurements of the steady-state spin polarization in the semiconductor\nindicate three temperature regimes for spin transport and relaxation. At\ntemperatures below 70 K, spin-polarized electrons injected into quantum well\nstructures form excitons, and the spin polarization in the quantum well depends\nstrongly on the electrical bias conditions. At intermediate temperatures, the\nspin polarization is determined primarily by the spin relaxation rate for free\nelectrons in the quantum well. This process is slow relative to the excitonic\nspin relaxation rate at lower temperatures and is responsible for a broad\nmaximum in the spin polarization between 100 and 200 K. The spin injection\nefficiency of the Fe/Al_xGa_{1-x}As Schottky barrier decreases at higher\ntemperatures, although a steady-state spin polarization of at least 6 % is\nobserved at 295 K.\n