Probing ultrafast photo-induced dynamics of the exchange energy in a Heisenberg antiferromagnet

Manipulating the macroscopic phases of solids using ultrashort light pulses has resulted in spectacular phenomena, including metal-insulator transitions(1-3), superconductivity(4) and subpicosecond modification of magnetic order(5). The development of this research area strongly depends on the understanding and optical control of fundamental interactions in condensed matter, in particular the exchange interaction. However, disentangling the timescales relevant for the contributions of the exchange interaction and spin dynamics to the exchange energy, E-exr is a challenge. Here, we introduce femtosecond stimulated Raman scattering to unravel the ultrafast photoinduced dynamics of magnetic excitations at the edge of the Brillouin zone. We find that femtosecond laser excitation of the antiferromagnet KNiF3 triggers a spectral shift of the two-magnon line, the energy of which is proportional to E-ex. By unravelling the photo-induced modification of the twomagnon line frequency from a dominating nonlinear optical effect, we find that E-ex is increased by the electromagnetic stimulus.