4D Visualization of a Nonthermal Coherent Magnon in a Laser Heated Lattice by an X‐ray Free Electron Laser

Ultrafast optical manipulation of magnetic phenomena is an exciting achievement of mankind, expanding one's horizon of knowledge toward the functional nonequilibrium states. The dynamics acting on an extremely short timescale push the detection limits that reveal fascinating light–matter interactions for nonthermal creation of effective magnetic fields. While some cases are benchmarked by emergent transient behaviors, otherwise identifying the nonthermal effects remains challenging. Here, a femtosecond time‐resolved resonant magnetic X‐ray diffraction experiment is introduced, which uses an X‐ray free‐electron laser (XFEL) to distinguish between the effective field and the photoinduced thermal effect. It is observed that a multiferroic Y‐type hexaferrite exhibits magnetic Bragg peak intensity oscillations manifesting entangled antiferromagnetic (AFM) and ferromagnetic (FM) Fourier components of a coherent AFM magnon. The magnon trajectory constructed in 3D space and time domains is decisive to evince ultrafast field formation preceding the lattice thermalization. A remarkable impact of photoexcitation across the electronic bandgap is directly unraveled, amplifying the photomagnetic coupling that is one of the highest among AFM dielectrics. Leveraging the above‐bandgap photoexcitation, this energy‐efficient optical process further suggests a novel photomagnetic control of ferroelectricity in multiferroics. Femtosecond time‐resolved resonant magnetic X‐ray diffraction experiment using an X‐ray free electron laser visualizes a light‐induced coherent magnon in a multiferroic. The magnon trajectory in time and 3D space unveils the photomagnetic coupling largely enhanced by above‐bandgap photoexcitation, distinguished from a laser‐heated magnetic behavior. This energy‐efficient photoexcitation process suggests the potential for a novel photomagnetic control of ferroelectricity in multiferroics.