X-ray photoelectron spectra were recorded with a Kratos AXIS Supra spectrometer equipped with a monochromatic source of Al Kα (1486.7 eV) working at 120 W with a base pressure of 10 -9 Torr; the survey spectra were recorded at pass energy of 160 eV with a step size of 1000 meV, while the high resolution spectra were registered at pass energy of 40 eV with a step size of 100 meV. Due to the magnetic nature of the ferrite samples, the magnetic lenses were disabled and operated under the