The ultrafast dynamics of $\alpha$-MnAs magnetization and atomic structure are studied by the time-resolved magneto-optical Kerr effect and time-resolved x-ray diffraction after photoexcitation. In the basal plane of the hexagonal unit cell, Mn atomic disorder increases on a timescale of 400 fs, comparable to the laser-induced demagnetization timescale and significantly faster than As disorder (700 fs). This differentiation of the Mn and As relaxation times is observed in the ferromagnetic phase but disappears in the paramagnetic phase. These results strongly support recent density-functional theory calculations predicting that the magnetostructural phase transition of MnAs is driven by a change in nature of the dominant interactions between Mn $3d$ electrons, from magnetic exchange to bonding interactions.

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• Lattice dynamics
• Magnetic phase transitions
• Ultrafast magnetic effects
• Magneto-optical Kerr effect
• X-ray diffraction