Enhancement of Exchange Bias and Perpendicular Magnetic Anisotropy in CoO/Co Multilayer Thin Films by Tuning the Alumina Template Nanohole Size

The interest in magnetic nanostructures exhibiting perpendicular magnetic anisotropy and exchange bias (EB) effect has increased in recent years owing to their applications in a new generation of spintronic devices that combine several functionalities. We present a nanofabrication process used to induce a significant out-of-plane component of the magnetic easy axis and EB. In this study, 30 nm thick CoO/Co multilayers were deposited on nanostructured alumina templates with a broad range of pore diameters, 34 nm <= D-p <= 96 nm, maintaining the hexagonal lattice parameter at 107 nm. Increase of the exchange bias field (H-EB) and the coercivity (H-C) (12 times and 27 times, respectively) was observed in the nanostructured films compared to the non-patterned film. The marked dependence of H-EB and H-C with antidot hole diameters pinpoints an in-plane to out-of-plane changeover of the magnetic anisotropy at a nanohole diameter of similar to 75 nm. Micromagnetic simulation shows the existence of antiferromagnetic layers that generate an exceptional magnetic configuration around the holes, named as antivortex-state. This configuration induces extra high-energy superdomain walls for edge-to-edge distance >27 nm and high-energy stripe magnetic domains below 27 nm, which could play an important role in the change of the magnetic easy axis towards the perpendicular direction.

Automated summary

The interest in magnetic nanostructures with perpendicular magnetic anisotropy and exchange bias has grown due to their potential applications in spintronic devices. In this study, a nanofabrication process was used to induce a significant out-of-plane component of the magnetic easy axis and exchange bias in CoO/Co multilayers deposited on nanostructured alumina templates. The increase in exchange bias and coercivity in the nanostructured films compared to non-patterned films indicates a change in magnetic anisotropy from in-plane to out-of-plane.