4D Visualization of a Nonthermal Coherent Magnon in the 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 towards the functional nonequilibrium states. The dynamics acting on an extremely short timescale pushes 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, we introduce femtosecond time-resolved resonant magnetic X-ray diffraction experiment using an X-ray free electron laser to distinguish between the effective field and the photoinduced thermal effect. We observe that a multiferroic Y-type hexaferrite exhibits magnetic Bragg peak intensity oscillations manifesting entangled antiferromagnetic and ferromagnetic Fourier components of a coherent antiferromagnetic magnon. The magnon trajectory constructed in three-dimensional space and time domains is decisive to evince ultrafast field formation preceding the lattice thermalization. We directly unravel a remarkable impact of photoexcitation across the electronic band gap, amplifying the photomagnetic coupling that is one of the highest among antiferromagnetic dielectrics. Leveraging the above-band-gap photoexcitation, this energy-efficient optical process further suggests a novel photomagnetic control of ferroelectricity in multiferroics. This article is protected by copyright. All rights reserved.