Tuning Nanohole Sizes in Ni Hexagonal Antidot Arrays: Large Perpendicular Magnetic Anisotropy for Spintronic Applications

Abstract

Perpendicular magnetic anisotropy (PMA) in transition metal thin films offers a pathway for enabling the interesting physics of nanomagnetism and developing a wide range of spintronics applications. We demonstrate a simple method to obtain Ni thin films with PMA by depositing them onto nanoporous anodic alumina membranes (NPAAMs), with different pore diameters varying in the range between 32 ± 2 and 93 ± 1 nm. Thus, several sets of Ni antidot arrays thin films have been fabricated with different hole diameters, 35 nm ≤ d ≤ 89 nm, and fixed interhole distances, Dint, around 103 ± 2 nm but reducing the edge-to-edge separation between adjacent antidots, (W = Dint – d), and in two different situations, by considering that W is well above or below the layer thickness, t, of the thin film. The crossover from the in-plane magnetization to out of plane magnetization in a ferromagnetic thin film has been achieved by modifying only the nanopore size of the patterned anodic alumina template and the experimental results were supported by micromagnetic simulations performed with Mumax3 code. A dramatic change in the coercivity, HC, dependence with d and W parameters has been observed with a critical nanohole diameter, dc, at which the appearance of the perpendicular magnetization is observed. The decreasing of the in-plane coercivity for samples with d > 75 nm is due to the weakened of the in-plane magnetic anisotropy and the rising of the out of plane component. The effective perpendicular magnetic anisotropy energy density for Ni antidot thin film with d = 90 nm and t = 20 nm is around 1.44 erg/cm2, larger than that obtained by traditional approaches for Ni films with PMA (0.03–0.2 erg/cm2). These findings point toward antidot thin films as novel routes to engineer the magnetic behavior of ferromagnetic metal with large PMA, which might entail a milestone for future applications in bit patterned magneto-optic perpendicular recording media and spintronic devices.