  In Mott insulators the evolution of antiferromagnetic order to
superconducting or charge-density-wave-like states upon chemical doping
underpins the control of quantum phases. Photo-doping can induce similar
transitions on the ultrafast timescale, however the response of the spin system
has remained elusive. Here, we use 4D-ultrafast optical spectroscopy to extract
quantitative magnetic dynamics in the spin-orbit coupled Mott insulator
Sr3Ir2O7. We demonstrate that light can non-thermally melt long-range spin
order. At low fluences magnetic order recovers within 1 ps despite
demagnetization of roughly 50%. However, high fluences induce a crossover to a
long-lived demagnetized state without increasing the lattice temperature. We
show that the generation of photo-induced spin defects enables a mechanism that
stabilizes the demagnetized state which could help expose new transient phases.
