Thickness gradient related magnetic anisotropy of wedged Co nanocluster assemblies deposited on glass plates

We prove the existence of uniaxial magnetic anisotropy (UMA) directly related to thickness gradients in a material system comprised of planar assemblies of Co nanoclusters (diameter∼2.5 nm), grown by pulsed laser deposition (PLD) on bare glass substrates. PLD provides a widespread coating that takes place with a decreasing thickness along the radial directions of a glass plate faced perpendicular to the ablation jet, yielding a wedge-like shape of the film thickness profile. Reflection and transmission magneto-optical effects enabled us to study magnetization vector as a function of the film thickness and location, revealing in-plane magnetization with a well-defined UMA along the in-plane direction parallel to the thickness gradient only in high-gradient films. The dependence of anisotropy energy on film thickness has been analyzed within a phenomenological model, accounting for the surface and bulk contributions to the effective magnetic anisotropy energy (MAE). According to that, below a threshold thickness, the volume contribution is low and Nèel-like interface contribution to MAE is negligible, as well as the null magneto-crystalline contributions of such a disorder cluster system, leaving only potential surface terms left. MAE is thus tentatively explained with the extra cluster terraces related to high-gradient thickness. Additionally, the magnetization along the in-plane hard axis or thickness gradient direction (TGD) includes a minor out-of-plane component noticed as a Faraday rotation in transmission geometry. We speculate it may arise from geometrical frustration of moments occurred in the hard axis magnetization of the network of interconnected assemblies.