Electrodeposition and microstructural characterization of CoPt ultrathin films with perpendicular magnetic anisotropy

• CoPt film with perpendicular magnetic anisotropy fabricated by electrodeposition. • Magnetic anisotropy origins analysis based on structural characterization. • Large perpendicular anisotropy due to in-plane crystal lattice expansion. In this study, we fabricated few-nanometer-thick and extremely smooth CoPt thin films with perpendicular magnetic anisotropy (PMA) via additive-assisted electrodeposition, aiming to apply it as the recording layer in a 3D domain-wall motion memory (3D-DWMM) composed of alternating ultrathin layers with different magnetic anisotropies. By adding 0.1 mmol/dm 3 saccharin sodium to the bath, we fabricated a 3 nm-thick Co 62 Pt 38 film exhibiting high PMA ( K u eff ≈ 0.2 MJ/m 3 ) and an exceptionally smooth surface, whose roughness was less than one-tenth of the film thickness. Minimal saccharin addition ensured negligible sulfur incorporation, maintaining the magnetic properties. To understand the origin of the PMA, we analyzed the microstructures of the films in detail. The easy axes of magnetization of CoPt were confirmed to be oriented out-of-plane, indicating that crystal magnetic anisotropy contributes to PMA. Additionally, the CoPt crystals were stretched in-plane, suggesting that crystal distortion could enhance PMA. Preferential Co–Co bonds in the in-plane direction were also confirmed, which may further contribute to PMA. This study provides a novel method for saccharin-controlled electrodeposition of extremely smooth CoPt ultrathin films and offers crucial insights into the origin of PMA in these deposited films, thereby significantly contributing to the development of 3D-DWMM.