Electronic structure and magnetic properties of magnetically dead layers in epitaxial CoFe_2O_4/Al_2O_3/Si(111) films studied by x-ray magnetic circular dichroism

摘要

Epitaxial CoFe_2O_4/Al_2O_3 bilayers are expected to be highly efficient spin injectors into Si owing to the spin filter effect of CoFe_2O_4. To exploit the full potential of this system, understanding the microscopic origin of magnetically dead layers at the CoFe_2O_4/Al_2O_3 interface is necessary. In this paper, we study the cation distribution, electronic structures, and the magnetic properties of CoFe_2O_4(111) layers with various thicknesses (thickness d = 1.4, 2.3, 4, and 11 nm) in the epitaxial CoFe_2O_4(111)/Al_2O_3(111)/Si(111) structures using soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) combined with clustermodel calculation. The magnetization of CoFe_2O_4 measured by XMCD gradually decreases with decreasing thickness d, and finally, a magnetically dead layer is clearly detected at d = 1.4 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 CoFe_2O_4/Al_2O_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 CoFe_2O_4/Al_2O_3 interface is partially restored by increasing the thickness from d = 1.4 to 2.3 nm.