  We have studied in-gap states in epitaxial CoFe2O4(111), which potentially
acts as a perfect spin filter, grown on a Al2O3(111)/Si(111) structure by using
ellipsometry, Fe L2,3-edge x-ray absorption spectroscopy (XAS), and Fe
L2,3-edge resonant inelastic x-ray scattering (RIXS), and revealed the relation
between the in-gap states and chemical defects due to the Fe2+ cations at the
octahedral sites (Fe2+ (Oh) cations). The ellipsometry measurements showed the
indirect band gap of 1.24 eV for the CoFe2O4 layer and the Fe L2,3-edge XAS
confirmed the characteristic photon energy for the preferential excitation of
the Fe2+ (Oh) cations. In the Fe L3-edge RIXS spectra, a band-gap excitation
and an excitation whose energy is smaller than the band-gap energy (Eg = 1.24
eV) of CoF2O4, which we refer to as "below-band-gap excitation (BBGE)"
hereafter, were observed. The intensity of the BBGE was strengthened at the
preferential excitation energy of the Fe2+ (Oh) cations. In addition, the
intensity of the BBGE was significantly increased when the thickness of the
CoFe2O4 layer was decreased from 11 to 1.4 nm, which coincides with the
increase in the site occupancy of the Fe2+ (Oh) cations with decreasing the
thickness. These results indicate that the BBGE comes from the in-gap states of
the Fe2+ (Oh) cations whose density increases near the heterointerface on the
bottom Al2O3 layer. We have demonstrated that RIXS measurements and analyses in
combination with ellipsometry and XAS are effective to provide an insight into
in-gap states in thin-film oxide heterostructures.
