Oscillatory spin polarization and magneto-optical Kerr effect in Fe$_{3}$O$_{4}$ thin films on GaAs(001)

Magnetite is an attractive material for spin injection and detection, because the theory predicts completely negative spin polarization at the Fermi level at room temperature. We fabricated high quality Fe$_{3}$O$_{4}$ films on GaAs (001) by molecular beam epitaxy. The Fermi level spin polarization of the Fe$_{3}$O$_{4}$ film was probed using the ultrafast optical measurement of ferromagnetic proximity polarization (FPP). The systematic thickness dependence of FPP and MOKE were measured on wedged Fe$_{3}$O$_{4}$ films on GaAs(001), and similar oscillatory and sign reversing behaviors were observed even though the two measurements rely on different mechanisms (spin dependent electron reflection for FPP, and optical transitions for MOKE). Quantum confinement of the t$_{2g}$ states near the Fermi level provides an explanation for the similar thickness dependences of the FPP and MOKE oscillations.