Tunable magnetic properties in compositionally graded L10-FePt thin films

The tetragonally ordered L10-FePt alloy exhibits strong magnetocrystalline anisotropy and high coercivity values, making it a model platform for high-density data storage and magnetic nanoparticles applications. However, the tunability of its magnetic properties through structural and compositional design remains a challenge. Here, we present a novel approach of fabricating compositionally graded L10-FePt thin films via thermally activated solid-state reactions of Fe/Pt bilayers grown on MgO(001). Upon annealing, vacancy-mediated migration and grain boundary diffusion drive the transformation into a structurally coherent columnar grain structure, consisting of epitaxial domains and misoriented regions. Nanoscale compositional analysis reveals a Pt concentration gradient, with near-stoichiometric L10-FePt in the upper film and a progressively Fe-rich profile near the FePt/MgO interface, while maintaining chemical ordering and tetragonality. This self-organized gradient has profound consequences, as the Fe-enriched interfacial region enhances Fe–Fe exchange stiffness, suppressing out-of-plane anisotropy and coercivity while promoting in-plane magnetization at room temperature. Magnetization reversal occurs through softened hysteresis and complex domain configurations, as confirmed by micromagnetic simulations. These results establish chemical grading as a powerful design strategy to modulate anisotropy, coercivity and domain behavior in L10-FePt, unlocking new opportunities for materials tailored for advanced spintronic and data storage applications.