  X-ray free electron lasers (XFEL) create femtosecond X-ray pulses with high
brightness and high longitudinal coherence allowing to extend X-ray
spectroscopy and scattering techniques into the ultrafast time-domain. These
X-rays are a powerful probe for studying coherent quasiparticle excitations in
condensed matter triggered by an impulsive optical laser pump. However, unlike
coherent phonons, other quasiparticles have been rarely observed due to small
signal changes and lack of standards for the identification. Here, we exploit
resonant magnetic X-ray diffraction using an XFEL to visualize a photoexcited
coherent magnon in space and time. Large intensity oscillations in
antiferromagnetic and ferromagnetic Bragg reflections from precessing moment
are observed in a multiferroic Y-type hexaferrite. The precession trajectory
reveals that a large, long-lived, photoinduced magnetic-field changes the net
magnetization substantially through the large-amplitude of the magnon. This
work demonstrates an efficient XFEL probe for the coherent magnon in the
spotlight for opto-spintronics application.
