Despite intense experimental effort, theoretical proposals, and modeling approaches, a lack of consensus exists about the intrinsic mechanisms driving ultrafast magnetization dynamics in $3d$ ferromagnets. In this Letter, we find evidence of an electron-phonon mediated spin flip as the driving mechanism for the ultrafast magnetization dynamics in all three $3d$ ferromagnets: nickel, iron, and cobalt. We use a microscopic three-temperature model with parameters calculated from first principles, which has been validated by a direct comparison to the electron and lattice dynamics extracted from previous experiments. By a direct comparison to the experimentally measured magnetization dynamics for different laser fluences, we determine the spin-flip probability of each material. We find robust evidence that relatively small values of the spin-flip probability enable ultrafast demagnetization in all three $3d$ ferromagnets.

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• Demagnetization
• Magnetism
• Spin dynamics
• Ultrafast demagnetization
• Ultrafast magnetic effects
• Ultrafast magnetization dynamics
• Magnetic systems
• Methods in magnetism