Ultrashort magnon pulses and phonon-magnon interactions

研究纳米结构中的超快动力学对于克服现代数据记录和通信技术中的速度限制至关重要。虽然超短激光和声学刺激在调制光学性质和磁序方面变得相当普遍，但在纳米尺度上激发超短磁振子脉冲的可能性仍未被探索。这个项目的目的是开发一种产生超短磁振子脉冲的实验技术，这种脉冲可以潜在地应用于未来自旋电子器件的磁化操作。此外，这项研究对于揭示声子-磁振子相互作用的机制具有重要意义。为了揭示超快磁声学，我将对不同铁磁材料的厚膜进行光学泵浦-探测实验。磁化动力学将通过时间分辨磁光克尔效应(MOKE)进行监测。由于厚铁磁薄膜的光吸收分布本质上是不均匀的，超短激光脉冲的光激励导致产生皮秒声应变脉冲。同时，激光感应加热导致了磁晶的空间局域化退磁和磁晶各向异性的变化，从而改变了有效磁场在热穿透深度内的方向。这一效应驱动了相干磁化动力学，它可以表示为一组自旋波。在我的研究中，我想要研究自旋波包的传播及其对自旋波色散的依赖，系统地改变外部磁场的方向和大小。此外，使用几个泵浦脉冲的序列，它们之间的延迟可调，应该可以激发更短的磁振子脉冲。对不同的铁磁材料(Ni、Co、Fe及其合金)的实验将有助于揭示超快时间尺度上自旋波的产生和传播以及声子-磁振子相互作用的材料特性。

Investigation of ultrafast dynamics in nanostructures is crucial to overcome the speed limits in modern data recording and telecommunication technologies. Whereas ultrashort laser and acoustic stimuli become rather common to modulate optical properties and magnetic order, possibility to excite ultrashort magnon pulses at the nano-scale remains unexplored. The aim of this project is to develop an experimental technique to generate ultrashort magnon pulses, which can be potentially applied to manipulate magnetization in future spintronic devices. In addition to that, this study is important to reveal mechanisms of phonon-magnon interactions.In order to shed light on ultrafast magnetoacoustics, I will perform optical pump-probe experiments on thick films of different ferromagnetic materials. Magnetization dynamics will be monitored by means of the time-resolved magneto-optical Kerr effect (MOKE). Due to essentially non-uniform light absorption profile in thick ferromagnetic films, optical excitation with ultrashort laser pulses results in the generation of picosecond acoustic strain pulses. At the same time the laser-induced heating leads to the spatially localized demagnetization and changes in the magnetocrystalline anisotropy thereby altering the direction of the effective magnetic field within the heat penetration depth. This effect drives coherent magnetization dynamics which can be represented as a packet of spin waves. In my study I want to investigate propagation of the spin wave packet and its dependence on the spin wave dispersion, systematically varying the direction and magnitude of the external magnetic field. Furthermore, using trains of several pump pulses with adjustable delay between them, it should be possible to excite shorter magnon pulses. Experiments on different ferromagnetic materials (Ni, Co, Fe and their alloys) will help to reveal the material-specific properties of the spin wave generation and propagation as well as the phonon-magnon interactions at ultrafast time scales.