Laser-induced ultrafast demagnetization dynamics of ${\mathrm{Tb}}_x({\mathrm{Dy}}_{0.2}{\mathrm{Co}}_{0.8}{)}_{1-x}$ ferrimagnetic alloy films with differing $\mathrm{Tb}$ compositions were systematically investigated by time-resolved magneto-optical Kerr effect measurements. In low-$\mathrm{Tb}$-doped compositions ($\mathrm{Co}$ rich), the one-stage demagnetization process is dominated by the spin-orbit coupling of the $\mathrm{Co}$ 3d spins. However, for high-$\mathrm{Tb}$-doped compositions ($\mathrm{Tb}$-$\mathrm{Dy}$ rich), a distinct discontinuous two-stage demagnetization emerges. Specifically, the second-stage demagnetization rate $1/{\tau }_2$ decreases monotonically with increasing x, and is dominated by intersublattice exchange coupling. A modified four-temperature model elucidates novel competing mechanisms underlying the demagnetization processes in ${\mathrm{Tb}}_x({\mathrm{Dy}}_{0.2}{\mathrm{Co}}_{0.8}{)}_{1-x}$ alloy thin films, and confirms the critical role of intersublattice exchange coupling in dominating the two-stage demagnetization process. These findings demonstrate that 3d-4f intersublattice coupling engineering—particularly through dual-rare-earth–single-transition-metal configurations—serves as a fundamental design paradigm for controlling ultrafast magnetic switching in rare-earth–transition-metal systems, providing new insights for developing next-generation high-speed spintronic memory devices.

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• Light-induced magnetic effects
• Magneto-optical effect
• Spin-orbit coupling
• Spintronics
• Ultrafast demagnetization
• Ferrimagnets
• Magnetic multilayers
• Transition-metal rare-earth alloys