Strong increase of the effective polarization of the tunnel current inFe/AlOx/Aljunctions with decreasing Fe layer thickness

The voltage dependence of the magnetoresistance ratio in a three-terminal device that consists of an ${\mathrm{F}\mathrm{e}/\mathrm{A}\mathrm{l}\mathrm{O}}_x$ emitter, an Al/Fe/Au base, and an n-type Si collector, was measured while changing the thickness of an Fe layer in the emitter. The ratio increased with decreasing the Fe thickness and was as large as 100% for a thickness of 0.8 nm. The results were analyzed by a phenomenological model that took into account the spin-polarized tunneling of electrons from the emitter into the base as well as the spin-dependent hot electron transport in the base. The spin polarization of electrons injected from the 0.8-nm Fe was estimated to be about 40% even at the injection voltage of 1 V. It was suggested that the large polarization was caused by the suppression of spin-flip scattering for electrons tunneling from the two-dimensional Fe electrodes.