Manifold role of a Ta adhesion layer in pressure-tuned Co/Pt multilayers

In this study, we systematically compare the impact of a 1.5-nm-thin Ta adhesion layer on the structural and magnetic behavior of Co/Pt multilayers (MLs) sputter-deposited at a low and a high Ar gas pressure. Our focus is a comprehensive structural analysis, centered on real-space cross-sectional scanning transmission electron microscopy (STEM) images, which are then correlated with complementary, reciprocal-space measurements from x-ray reflectometry (XRR) and x-ray diffraction (XRD). Drawing on this comparison and supplemented by atomic force microscopy (AFM), we develop a simplified model to fit the XRR data. Fits and data agree excellently, while the extracted density profiles match well with the STEM data. The structural features are then correlated with the magnetic ones, as determined by magnetic reversal loops and magnetic force microscopy (MFM) images. We find considerably better texture with the adhesion layer, as well as vastly different structural features at the higher deposition pressure between the different underlayers. At the lower pressure, the influence of the underlayer is much less pronounced. Magnetically, the Ta layer results in a marked improvement in the effective perpendicular magnetic anisotropy (PMA), increasing it up to an impressive 340% at the higher pressure, while remaining comparable at the lower one. We attribute the changes in structure and magnetism to two different factors: (1) The adhesion layer stops the implantation of Pt atoms into the substrate, improving layer smoothness and texture. (2) The momentum transfer to the film is reduced at higher sputter pressures, which reduces intermixing between Co and Pt, but increases overall roughness. Combining both, the roughness remains small enough to enable well-defined grains with sharper Co/Pt interfaces and lower intragrain curvature, resulting in the increased PMA. Our study provides a practical approach for controlling anisotropy in sputter-deposited ML systems containing heavy atom species, utilizing different combinations of adhesion layer and sputter pressure.