Ballistic electron-driven magnetization dynamics induced by femtosecond laser excitation

铁磁体中的电子激发可以触发超快自旋动力学，在磁记录的速度增加中具有潜在的应用。该项目研究超快磁化动力学，这是由弹道热电子驱动的金属层。在铁磁体中，这些电子通过自旋相关散射引起磁化强度M的绝对值的变化。如果电子是自旋极化的，则在贵金属和铁磁体的界面处的散射导致自旋转移力矩，从而改变M的方向。为了揭示潜在的机制，我们计划研究模型系统，首先实现了层的Au与其大弹道平均自由程和Fe作为巡回铁磁体。稍后我们将比较不同的系统。该项目旨在（i）理解超快退磁和（ii）飞秒全光产生自旋转移力矩效应。发射的动力学将在时间分辨实验中由磁光探测。我们将分析激发态下的电子-电子、电子-磁振子和电子-声子散射等基本过程。我们的方法避免了通过层堆叠的净电流和由此产生的磁奥斯特场，因为弹道热电子的传播被保持平衡的传导电子屏蔽。在第一个资助期研究的结构包括在MgO（001）上外延生长的10 - 100 nm厚的Fe/Au和Fe/Au/Fe堆叠。外Fe或Au层的光激发和在邻近透明衬底的Fe层处通过（非）线性光学器件在后泵浦前探测方案中的检测，促进了所提出的研究。我们的方法导致非平衡电子驱动的自旋动力学，这有可能克服声子介导的角动量转移在目前采用的计划和应用中的平衡作用的限制。

Electronic excitations in a ferromagnet can trigger ultrafast spin dynamics with potential applications in a speed increase in magnetic recording. The project investigates ultrafast magnetization dynamics, which is driven in metallic layers by ballistic hot electrons. In a ferromagnet these electrons induce a change in the absolute value of the magnetization M through spin-dependent scattering. If the electrons are spin-polarized, scattering at the interface of a noble metal and a ferromagnet results in spin-transfer torque and hence modifies the direction of M. To reveal the underlying mechanisms, we plan to study model systems, which at first realized by layers of Au with its large ballistic mean free path and Fe as an itinerant ferromagnet. Later we will compare different systems. The project aims at (i) understanding of ultrafast demagnetization and (ii) femtosecond all-optical generation of spin transfer torque effects. The launched dynamics will be probed by magneto-optics in a time-resolved experiment. We will analyze the elementary processes like electron-electron, electron-magnon, and electron-phonon scattering in the excited state. Our approach avoids a net electrical current through the layer stack and the resulting magnetic Oersted fields, because the propagation of ballistic hot electrons is screened by conduction electrons remaining in equilibrium. The structures investigated in the first funding period consist of a 10 - 100 nm thick stack of Fe/Au and Fe/Au/Fe which is grown epitaxially on MgO(001). Optical excitation of the outer Fe or Au layer and detection at the Fe layer adjacent to the transparent substrate by (non-) linear optics in a back-pump front-probe scheme, facilitates the proposed study. Our approach leads the way to non-equilibrium electron driven spin dynamics, which has the potential to overcome limitations of phonon-mediated angular momentum transfer acting in equilibrium in currently employed schemes and applications.