Origin of magnetically dead layers in spinel ferrites MFe2O4 grown on Al2O3: Effects of postdeposition annealing studied by XMCD

We study the electronic and magnetic states of as-grown and annealed MFe2O4(111)/Al2O3(111) (M = Co, Ni) thin films with various thicknesses grown on Si(111) substrates with the gamma-Al2O3(111) buffer layers by using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD), to investigate magnetically dead layers in these films. Although the magnetically dead layers in the as-grown samples are formed near the interface with the Al2O3 buffer layer, we reveal that ferrimagnetic order is partially recovered by postdeposition annealing at 973 K for 48 h in air. By analyzing the line shapes of the XAS and XMCD spectra, we conclude that, in the dead layers, there are a significant number of vacancies at the Td sites of the spinel structure, which may be the microscopic origin of the degraded ferrimagnetic order in the MFe2O4 thin films.