Dornes C, Acremann Y, Savoini M, Kubli M, Neugebauer M J, Abreu E, Huber L, Lantz G, Vaz C A F, Lemke H, Bothschafter E M, Porer M, Esposito V, Rettig L, Buzzi M, Alberca A, Windsor Y W, Beaud P, Staub U, Zhu Diling, Song Sanghoon, Glownia J M, Johnson S L
Institute for Quantum Electronics, Physics Department, ETH Zurich, Zurich, Switzerland.
Laboratory for Solid State Physics, Physics Department, ETH Zurich, Zurich, Switzerland.
Nature. 2019 Jan;565(7738):209-212. doi: 10.1038/s41586-018-0822-7. Epub 2019 Jan 2.
The Einstein-de Haas effect was originally observed in a landmark experiment demonstrating that the angular momentum associated with aligned electron spins in a ferromagnet can be converted to mechanical angular momentum by reversing the direction of magnetization using an external magnetic field. A related problem concerns the timescale of this angular momentum transfer. Experiments have established that intense photoexcitation in several metallic ferromagnets leads to a drop in magnetization on a timescale shorter than 100 femtoseconds-a phenomenon called ultrafast demagnetization. Although the microscopic mechanism for this process has been hotly debated, the key question of where the angular momentum goes on these femtosecond timescales remains unanswered. Here we use femtosecond time-resolved X-ray diffraction to show that most of the angular momentum lost from the spin system upon laser-induced demagnetization of ferromagnetic iron is transferred to the lattice on sub-picosecond timescales, launching a transverse strain wave that propagates from the surface into the bulk. By fitting a simple model of the X-ray data to simulations and optical data, we estimate that the angular momentum transfer occurs on a timescale of 200 femtoseconds and corresponds to 80 per cent of the angular momentum that is lost from the spin system. Our results show that interaction with the lattice has an essential role in the process of ultrafast demagnetization in this system.
爱因斯坦-德哈斯效应最初是在一项具有里程碑意义的实验中观察到的，该实验表明，通过使用外部磁场反转磁化方向，铁磁体中与排列的电子自旋相关的角动量可以转换为机械角动量。一个相关的问题涉及这种角动量转移的时间尺度。实验已经证实，在几种金属铁磁体中进行强烈的光激发会导致磁化强度在短于100飞秒的时间尺度上下降——这一现象被称为超快退磁。尽管这一过程的微观机制一直存在激烈争论，但在这些飞秒时间尺度上角动量去向的关键问题仍然没有答案。在这里，我们使用飞秒时间分辨X射线衍射来表明，在铁磁体铁的激光诱导退磁过程中，自旋系统损失的大部分角动量在亚皮秒时间尺度上转移到晶格上，引发一个从表面传播到体相的横向应变波。通过将X射线数据的一个简单模型与模拟和光学数据进行拟合，我们估计角动量转移发生在200飞秒的时间尺度上，并且相当于从自旋系统损失的角动量的80%。我们的结果表明，与晶格的相互作用在该系统超快退磁过程中起着至关重要的作用。
