  Using a combination of first-principles and magnetization-dynamics
calculations, we study the effect of the intense optical excitation of phonons
on the magnetic behavior in insulating magnetic materials. Taking the
prototypical magnetoelectric \CrO\ as our model system, we show that excitation
of a polar mode at 17 THz causes a pronounced modification of the magnetic
exchange interactions through a change in the average Cr-Cr distance. In
particular, the quasi-static deformation induced by nonlinear phononic coupling
yields a structure with a modified magnetic state, which persists for the
duration of the phonon excitation. In addition, our time-dependent
magnetization dynamics computations show that systematic modulation of the
magnetic exchange interaction by the phonon excitation modifies the
magnetization dynamics. This temporal modulation of the magnetic exchange
interaction strengths using phonons provides a new route to creating
non-equilibrium magnetic states and suggests new avenues for fast manipulation
of spin arrangements and dynamics.
