  Although ultrafast manipulation of magnetism holds great promise for new
physical phenomena and applications, targeting specific states is held back by
our limited understanding of how magnetic correlations evolve on ultrafast
timescales. Using ultrafast resonant inelastic x-ray scattering we demonstrate
that femtosecond laser pulses can excite transient magnons at large wavevectors
in gapped antiferromagnets, and that they persist for several picoseconds which
is opposite to what is observed in nearly gapless magnets. Our work suggests
that materials with isotropic magnetic interactions are preferred to achieve
rapid manipulation of magnetism.
