Competition between spin–orbit coupling and intersublattice exchange revealed in Gd-doped Dy-TM ferrimagnets

Ultrafast demagnetization in rare-earth–transition-metal (RE–TM) ferrimagnets is governed by angular-momentum transfer among coupled magnetic sublattices and the lattice. However, how spin–orbit coupling (SOC)-mediated dissipation competes with intersublattice exchange remains unclear. Here, we examine this competition in Gd-doped Dy–Co and Dy–Fe ferrimagnetic films using time-resolved magneto-optical Kerr effect spectroscopy. By substituting Gd (orbital quantum number L = 0) for Dy (L = 5), we systematically suppress the 4f-SOC-mediated dissipation channel while largely preserving the 3d–4f exchange coupling framework. The Co-based films exhibit single-step demagnetization whose timescale lengthens with Gd doping, reflecting the governing role of strong 3d SOC. In contrast, the Fe-based series exhibits pronounced two-step demagnetization, with the slow-component timescale reaching a maximum at an intermediate Gd content. The experimental findings are well reproduced by a constrained four-temperature model. This work provides a physical framework for understanding how the relative strengths of SOC and 3d–4f exchange coupling govern the dynamical visibility of RE-mediated angular-momentum pathways in the investigated Gd-doped Dy–TM alloys, providing a possible route for engineering ultrafast magnetization control in next-generation spintronic devices.