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 ex-change bias 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 perpendicular magnetic anisotropy and exchange bias. 30-nm-thick CoO/Co multilayers were deposited on nanostructured alumina templates with a broad range of pore diameters, 34 nm ≤ Dp ≤ 96 nm, while maintaining the hexagonal lattice parameter at 107 nm. Increase of both the exchange bias field (HEB) and the coercivity (HC) (12 times and 27 times, respectively) was ob-served in the nanostructured films compared to the non-patterned film. The marked dependence of HEB and HC with antidot hole diameters pinpoints to an in-plane to out-of-plane changeover of the magnetic anisotropy at a nanohole diameter of ∼ 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 is responsible of inducing extra high-energy superdomain walls for samples with edge-to-edge distance (W) >> 27 nm and high-energy stripe magnetic domains for W < 27 nm, responsible of the perpendicular magnetic signal of the samples.