Spin injection into magnetic alloys using optical spin excitation technique

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The purpose of this study is to examine the spin filtering effect of spin polarized electrons excited in GaAs with illumination of circularly polarized light at the ferromagnetic metal/GaAs interfaces.Several types of samples, e.g., Fe/GaAs, Fe/MgO/GaAs, Fe_3O_4/GaAs, were grown using magnetron sputtering and reactive MBE techniques. These samples were illuminated with right or left circularly polarized light with a wavelength of 800-850 nm from sample normal direction through the magnetic layer, thereby helicity dependent photocurrent was detected in a magnetic field normal to the film plane. The helicity dependent photocurrent showed clear field dependence, corresponding to the magnetization process of the magnetic layer. The bias dependence of the helicity dependent photocurrent was also measured in a magnetic field sufficient to saturate the magnetization. Magnetic circular dichroism was predominant for reverse bias for Fe/GaAs samples, while a significant feature was observed at a forward bias. We attribute this characteristic feature in the bias dependence of helicity dependent photocurrent to spin filtering effect at the Fe/GaAs interface. The efficiency of the spin filtering, however, is limited to a value to a few %. Also, any significant enhancement in the efficiency was not obtained by inserting an MgO tunneling barrier at the interface, presumably due to the roughness of the Mg0 layer and the presence of pin holes. For Fe_3O_4/GaAs samples, we observed a similar feature in the bias dependence of the helicity dependent photocurret at zero bias.From these combined results, we conclude that magnetic metal layer can act as a spin filter and work as a spin detector, although the efficiency is small. Atomically flat layers are necessary to improve the spin detection efficiency.