At terahertz drive frequencies, magnetization dynamics enter an inertial regime beyond the scope of standard Landau-Lifshitz-Gilbert (LLG) theory. We reveal a nutational damping torque comprising a bulk term and an interfacial contribution that describes spin emission into adjacent metals. The interfacial term generates nutational spin pumping: a dc spin current that scales quadratically with the drive frequency. Using a scattering approach, we relate this effect to microscopic material parameters and predict pronounced enhancements in ultrathin films and low Fermi-energy conductors. Nutational spin pumping can be detected directly via the inverse spin Hall effect or indirectly via thickness-dependent renormalization of nutational damping.

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• Magnetization dynamics
• Spin pumping
• Landau-Lifschitz-Gilbert equation