Origin of magnetically dead layers in spinel ferrites $M{\text{Fe}}_2{\text{O}}_4$ grown on ${\text{Al}}_2{\text{O}}_3$: Effects of post-deposition annealing studied by XMCD

We study the electronic and magnetic states of as-grown and annealed $M{\text{Fe}}_2{\text{O}}_4$(111)/${\text{Al}}_2{\text{O}}_3$(111) ($M=\text{Co, Ni}$) thin films with various thicknesses grown on Si(111) substrates with the $\gamma$-${\text{Al}}_2{\text{O}}_3$(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 ${\text{Al}}_2{\text{O}}_3$ buffer layer, we reveal that ferrimagnetic order is partially recovered by post-deposition annealing at 973 K for 48 hours 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 $T_d$ sites of the spinel structure, which may be the microscopic origin of the degraded ferrimagnetic order in the $M{\text{Fe}}_2{\text{O}}_4$(111) thin films.