Electronic structure and magnetic properties of magnetically dead layers in epitaxial CoFe2O4/Al2O3/Si(111) films studied by x-ray magnetic circular dichroism

Epitaxial $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}/\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ bilayers are expected to be highly efficient spin injectors into Si owing to the spin filter effect of $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$. To exploit the full potential of this system, understanding the microscopic origin of magnetically dead layers at the $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}/\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ interface is necessary. In this paper, we study the cation distribution, electronic structures, and the magnetic properties of $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}(111)$ layers with various thicknesses (thickness $d=1.4$, 2.3, 4, and 11 nm) in the epitaxial $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}(111)/\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}(111)/\mathrm{Si}(111)$ structures using soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) combined with cluster-model calculation. The magnetization of $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}$ measured by XMCD gradually decreases with decreasing thickness $d$, and finally, a magnetically dead layer is clearly detected at $d=1.4\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. The magnetically dead layer has frustration of magnetic interactions, which is revealed from comparison between the magnetizations at 300 and 6 K. From analysis using configuration-interaction cluster-model calculation, the decrease of $d$ leads to a decrease in the inverse-to-normal spinel structure ratio and also a decrease in the average valence of Fe at the octahedral sites. These results strongly indicate that the magnetically dead layer at the $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}/\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ interface originates from various complex networks of superexchange interactions through the change in the cation distribution and electronic structure. Furthermore, from comparison of the magnetic properties between $d=1.4$ and 2.3 nm, it is found that the ferrimagnetic order of the magnetically dead layer at the $\mathrm{CoF}{\mathrm{e}}_{2}{\mathrm{O}}_{4}/\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ interface is partially restored by increasing the thickness from $d=1.4$ to 2.3 nm.