  Measuring how the magnetic correlations throughout the Brillouin zone evolve
in a Mott insulator as charges are introduced dramatically improved our
understanding of the pseudogap, non-Fermi liquids and high $T_C$
superconductivity. Recently, photoexcitation has been used to induce similarly
exotic states transiently. However, understanding how these states emerge has
been limited because of a lack of available probes of magnetic correlations in
the time domain, which hinders further investigation of how light can be used
to control the properties of solids. Here we implement magnetic resonant
inelastic X-ray scattering at a free electron laser, and directly determine the
magnetization dynamics after photo-doping the Mott insulator Sr$_2$IrO$_4$. We
find that the non-equilibrium state 2~ps after the excitation has strongly
suppressed long-range magnetic order, but hosts photo-carriers that induce
strong, non-thermal magnetic correlations. The magnetism recovers its
two-dimensional (2D) in-plane N\'eel correlations on a timescale of a few ps,
while the three-dimensional (3D) long-range magnetic order restores over a far
longer, fluence-dependent timescale of a few hundred ps. The dramatic
difference in these two timescales, implies that characterizing the
dimensionality of magnetic correlations will be vital in our efforts to
understand ultrafast magnetic dynamics.
