Reduced magnetocrystalline anisotropy of CoFe$_2$O$_4$ thin films studied by angle-dependent x-ray magnetic circular dichroism

TL;DR

This study investigates how the magnetocrystalline anisotropy of CoFe$_2$O$_4$ thin films decreases at the interface with Al$_2$O$_3$, revealing a reduction linked to fewer Co$^{2+}$ ions at specific lattice sites, using angle-dependent XMCD.

Contribution

It demonstrates the reduction of magnetocrystalline anisotropy in thin CoFe$_2$O$_4$ films and links it to changes in Co$^{2+}$ ion occupancy at lattice sites.

Findings

Thinner CoFe$_2$O$_4$ films exhibit significantly reduced MCA.

Reduction in MCA is associated with fewer Co$^{2+}$ ions at the $O_h$ site.

Magnetic properties are degraded at the interface with Al$_2$O$_3$.

Abstract

Spinel-type CoFe$_2$O$_4$ is a ferrimagnetic insulator with the N\'eel temperature exceeding 790 K, and shows a strong cubic magnetocrystalline anisotropy (MCA) in bulk materials. However, when a CoFe$_2$O$_4$ film is grown on other materials, its magnetic properties are degraded so that so-called magnetically dead layers are expected to be formed in the interfacial region. We investigate how the magnetic anisotropy of CoFe$_2$O$_4$ is modified at the interface of CoFe$_2$O$_4$/Al$_2$O$_3$ bilayers grown on Si(111) using x-ray magnetic circular dichroism (XMCD). We find that the thinner CoFe$_2$O$_4$ films have significantly smaller MCA values than bulk materials. The reduction of MCA is explained by the reduced number of Co$^{2+}$ ions at the $O_h$ site reported by a previous study [Y. K. Wakabayashi $\textit{et al.}$, Phys. Rev. B $\textbf{96}$, 104410 (2017)].