During the last two decades, a variety of models have been developed to explain the ultrafast quenching of magnetization following femtosecond optical excitation. These models can be classified into two broad categories, relying either on a local or a non-local transfer of angular momentum. The acquisition of the magnetic depth profiles with femtosecond resolution, using time-resolved x-ray resonant magnetic reflectivity, can distinguish local and non-local effects. Here, we demonstrate the feasibility of this technique in a pump–probe geometry using a custom-built reflectometer at the FLASH2 free-electron laser (FEL). Although FLASH2 is limited to the production of photons with a fundamental wavelength of 4 nm ($\simeq 310\hspace{0.17em}\text{eV}$), we were able to probe close to the Fe L₃ edge ($706.8\hspace{0.17em}\text{eV}$) of a magnetic thin film employing the third harmonic of the FEL. Our approach allows us to extract structural and magnetic asymmetry signals revealing two dynamics on different time scales which underpin a non-homogeneous loss of magnetization and a significant dilation of 2 Å of the layer thickness followed by oscillations. Future analysis of the data will pave the way to a full quantitative description of the transient magnetic depth profile combining femtosecond with nanometer resolution, which will provide further insight into the microscopic mechanisms underlying ultrafast demagnetization.

中文翻译：

---

利用时间分辨 X 射线共振磁反射率进行超快磁深度分析


在过去的二十年中，已经开发了多种模型来解释飞秒光激发后磁化的超快淬灭。这些模型可以分为两大类，依赖于角动量的局部或非局部转移。使用时间分辨 X 射线共振磁反射率采集飞秒分辨率的磁深度剖面，可以区分局部和非局部效应。在这里，我们使用 FLASH2 自由电子激光器 (FEL) 上的定制反射计演示了该技术在泵浦探针几何结构中的可行性。尽管 FLASH2 仅限于产生基本波长为 4 nm 的光子（$<span\; data-immersive-translate-walked="49834e83-22c0-466f-919a-4cfc6141bc82">\simeq </span>\mathrm{<span\; data-immersive-translate-walked="49834e83-22c0-466f-919a-4cfc6141bc82">310</span>}\text{<span data-immersive-translate-walked="49834e83-22c0-466f-919a-4cfc6141bc82">电子伏特</span>}$），我们能够探测靠近 Fe L ₃边缘（$\mathrm{<span\; data-immersive-translate-walked="49834e83-22c0-466f-919a-4cfc6141bc82">706.8</span>}\text{<span data-immersive-translate-walked="49834e83-22c0-466f-919a-4cfc6141bc82">电子伏特</span>}$）采用 FEL 三次谐波的磁性薄膜。我们的方法使我们能够提取结构和磁不对称信号，揭示不同时间尺度上的两种动态，这些动态导致磁化强度的非均匀损失和层厚度显着膨胀 2 Å，随后出现振荡。未来对数据的分析将为飞秒与纳米分辨率相结合的瞬态磁深度剖面的全面定量描述铺平道路，这将进一步深入了解超快退磁的微观机制。