Structural and magnetic properties of quad-interface ferromagnetic multilayers for magnetic tunnel junction free layers

The performance of the free layer in a magnetic tunnel junction (MTJ) is critical for optimizing magnetic random access memory devices. In this study, we systematically investigate a quad-interface free-layer structure, MgO/B-FM/W/T-FM/MgO/B-FM/W/T-FM/MgO, focusing on the modulation of key magnetic properties, including saturation magnetization (Ms), magnetic dead layer thickness (tdl), perpendicular magnetic anisotropy (PMA), and interfacial anisotropy energy constant (Ki), by varying the thicknesses of the ferromagnetic (FM) and tungsten (W) layers, the annealing process, FM composition, and interfacial symmetry. Our results demonstrate that the insertion of the W layer is pivotal in achieving the transition from IPM (in-plane magnetization) to PMA. Moreover, the W layer thickness strongly influences interfacial atomic interdiffusion; as the W thickness increases, Ms decreases. The annealing process effectively suppresses dead layer growth, further enhancing magnetic performance. Comparative analysis of FeB and CoFeB systems reveals that FeB exhibits superior PMA, Ki, and Ms, owing to its more favorable 3d orbital half-filling and better lattice matching with MgO. An extremely large PMA value of 1.5 erg/cm2 has been achieved by the quad-interface free-layer structure. Finally, investigations varying the thickness ratio between the top (T-FM) and bottom (B-FM) layers highlight significant interfacial asymmetry, showing that interfacial loss in the T-FM layer is lower than in the B-FM layer. Overall, this study provides crucial experimental evidence and theoretical insight for structural engineering and comprehensive performance optimization of multi-interface MTJs.