Postgrowth thermal annealing of a CoFe/ MgO͑100͒ tunnel spin injector grown on a GaAs/ AlGaAs quantum well structure results in a significantly increased spin injection efficiency as inferred from the polarization of heavy-hole electroluminescence from a quantum well optical detector. The as-deposited sample displayed an initial polarization at 100 K of 43%, which was increased to 52% after a 1 h anneal at 300°C, and finally to 55% after a second 1 h anneal at 340°C. The polarization remained

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... changed upon further annealing to temperatures as high as 400°C. These results show that tunnel spin injectors based on CoFe/ MgO are robust with high thermal stability, making them useful for device applications.