Exchange striction determines how fast antiferromagnetic insulators demagnetize
Antiferromagnets combine terahertz spin dynamics with insensitivity to stray fields, and how quickly their order can be manipulated sets the speed limit on device operation. Femtosecond optical pulses demagnetize antiferromagnetic insulators on timescales that span picoseconds to nanoseconds across compounds, and no material parameter is known that accounts for the spread or predicts where a new compound will fall. In a compensated antiferromagnet, no angular momentum needs to leave the spin system, so the rate is set by energy flow from the lattice into the spins. Time-resolved second-harmonic generation experiments show that Cr2O3 demagnetizes within 2 ps once the lattice is driven above the Neel temperature, two orders of magnitude faster than the structurally similar FeBO3. First-principles calculations trace the disparity to exchange striction: short Cr-Cr contacts make the exchange coupling tenfold more sensitive to atomic displacements and widen the phase space for phonon decay into magnon pairs. Spin-lattice simulations with ab initio parameters reproduce the order of magnitude of the measured ratio. The derivative of the exchange coupling with respect to the ionic displacement thus emerges as a computable parameter that predicts how fast an insulating antiferromagnet can be demagnetized. The results advance our understanding of ultrafast control in insulating antiferromagnets, and suggest a practical pathway to screen candidate materials for thermally assisted antiferromagnetic memory before synthesis.