Spin-injection efficiency of MgO-based tunnel spin injectors

The efficient injection of spin-polarized electrons from a ferromagnetic contact into a semiconductor is an important ingredient of many proposed spintronic devices. Such injection has been demonstrated by analyzing the circular polarization of photons that are generated after recombination of the injected electrons with holes in a quantum-well layer inside the semiconductor [1-4]. Recently, the spin-injection efficiency has been improved dramatically by inserting a MgO tunnel barrier between the ferromagnetic contact and the semiconductor [5]. Although the measured circular polarization of the electroluminescence is directly proportional to the spin polarization of the recombining electrons, the circular polarization of the luminescence does not directly relate to the polarization of the injected electron spins. This is because the recombination time of the electrons in the quantum well can be much longer than the lifetime of the spin, such that the spin has decayed already when the electron recombines.

We have measured separately the spin lifetime and the recombination time in the quantum well detector of a MgO-based spin injection device. Comparing these times with the circular polarization of the electroluminescence, the spin polarization immediately after the injection process can be determined. We find a spin polarization of about 70%, which is independent of temperature [6]. This value is larger than the spin polarization at the Fermi energy inside the ferromagnetic metal, emphasizing the spin-selective property of the MgO tunnel barrier.

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