  We investigate how fast and how effective photocarrier excitation can modify
the exchange interaction $J_\mathrm{ex}$ in the prototype Mott-Hubbard
insulator. We demonstrate an ultrafast quenching of $J_\mathrm{ex}$ both by
evaluating exchange integrals from a time-dependent response formalism and by
explicitly simulating laser-induced spin precession in an antiferromagnet that
is canted by an external magnetic field. In both cases, the electron dynamics
is obtained from nonequilibrium dynamical mean-field theory. We find that the
modified $J_\mathrm{ex}$ emerges already within a few electron hopping times
after the pulse, with a reduction that is comparable to the effect of chemical
doping.

Strongly Correlated Electrons