Spatiotemporal migration of antiferromagnetic domain walls in Sr 2 IrO 4

By laser pump-probe time-resolved coherent magnetic x-ray diffraction imaging, we have measured the migration velocity of antiferromagnetic domain walls in the Mott insulator Sr 2 IrO 4 at 100 K. Coherent diffraction patterns, recorded at the 106 pure-magnetic Bragg peak, showed fringes, which responded distinctly to the excitation laser. These patterns could not be inverted with current phasing algorithms, but could be modeled as four domains with phase shifts consistent with the antiferromagnetic structure. During the laser-induced demagnetization, we observed these domains and their domain walls moving at 3 × 10 6 m / s , significantly faster than acoustic velocities. This is understood to arise from a purely electronic spin contribution to the magnetic structure without any role for coupling to the crystal lattice.