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.

中文翻译：

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通过 X 射线自由电子激光器对激光加热晶格中的非热相干磁振子进行 4D 可视化

对磁现象的超快光学操纵是人类一项令人兴奋的成就，它将人类的知识视野扩展到功能性非平衡态。在极短的时间尺度上作用的动力学突破了检测极限，揭示了令人着迷的光与物质相互作用，以非热产生有效磁场。虽然某些情况以突发瞬态行为为基准，但识别非热效应仍然具有挑战性。这里介绍一种飞秒时间分辨共振磁X射线衍射实验，该实验使用X射线自由电子激光器（XFEL）来区分有效场和光致热效应。据观察，多铁性 Y 型六角形铁氧体表现出磁布拉格峰值强度振荡，表现出相干 AFM 磁振子的纠缠反铁磁 (AFM) 和铁磁 (FM) 傅立叶分量。在 3D 空间和时间域中构建的磁振子轨迹对于证明晶格热化之前的超快场形成具有决定性作用。光激发对电子带隙的显着影响被直接揭示，放大了 AFM 电介质中最高的光磁耦合之一。利用上述带隙光激发，这种节能光学过程进一步提出了多铁性材料中铁电性的新型光磁控制。