Identification and control of ultrafast spin dynamics in ferromagnetic solids by tailored fields

分析了在强电磁场驱动下外延过渡金属铁磁薄膜和界面处的自旋动力学，旨在通过优化和定制可见和红外光谱区的驱动强场来控制动力学。在我们的合作理论和实验项目的框架内，我们对飞秒（fs）激光诱导退磁进行了完全的时间依赖密度泛函理论研究，我们利用这种现象来研究电子电荷和自旋动力学，特别强调自旋依赖的电荷转移，自旋轨道相互作用以及与核运动的耦合。同时，我们利用时间分辨线性和非线性磁光进行了实验研究。这使我们能够分析由于自旋轨道耦合、声子和磁激励下的电子散射以及外源电磁场诱导的自旋转移和输运效应引起的动态自旋依赖效应，这些效应使铁磁体远离平衡状态。在第一个资助期，由于我们协调的实验-理论努力，我们已经确定了Co/Cu（001）超快速退磁中的自旋依赖电荷转移和自旋轨道耦合介导的自旋翻转。在此基础上，我们将设计激光脉冲来优化控制自旋动力学。这些最佳脉冲将用于我们的实验工作，以进一步验证和开发。基于我们新开发的远程物理ansatz，我们将开发一个核动力学代码，它与已经存在的自旋-电荷动力学代码完全耦合（http://elk.sourceforge.net）。在第一个资助期，我们已经证明量子最优控制可以与后一种代码接口，从而可以定制激光脉冲。进一步的裁剪将通过对目标函数采用实验约束来完成。在实验部分，我们将利用非共线光参量放大产生的低于15 fs的脉冲进一步提高时间分辨率，并利用光参量啁啾脉冲放大将驱动激光场的强度提高到10^15 W/cm^2。通过微观过程和理论上的最佳脉冲设计识别实验观察到的特征，从而使脉冲整形能够操纵和控制自旋动力学，使其朝着定制的强电磁场驱动的期望响应方向发展。

We analyze spin dynamics in epitaxial transition metal ferromagnetic films and at interfaces driven by intense electromagnetic fields and aim at controlling the dynamics by optimizing and tailoring the driving intense fields in the visible and infrared spectral region. In the framework of our collaborative theoretical and experimental project we perform a fully ab-initio time-dependent density functional theory investigation of femtosecond (fs) laser induced demagnetization, a phenomenon which we exploit in order to investigate the electronic charge and spin dynamics with particular emphasis on spin-dependent charge transfer, spin-orbit interaction and coupling to nuclear motion. Simultaneously, we carry out an experimental investigation employing fs time-resolved linear and non-linear magneto-optics. This allows us to analyze dynamic spin-dependent effects due to spin-orbit coupling, electronic scattering with phononic and magnetic excitations, and spin transfer and transport effects induced by the externally applied electromagnetic fields, which drive the ferromagnet far out of equilibrium. In the first funding period, we have identified spin-dependent charge transfer and spin-orbit coupling mediated spin flips in the ultrafast demagnetization of Co/Cu(001) due to our coordinated experimental-theoretical effort. On this basis, we will design laser pulses to optimally control the spin-dynamics. These optimal pulses will then be used in our experimental effort for further validation and development. Based on our newly developed ansatz for long-range physics, we will develop a nuclear dynamics code, which is fully coupled to the already existing spin-charge-dynamics code (http://elk.sourceforge.net). In the first funding period, we have demonstrated that quantum optimal control can be interfaced to the latter code such that laser pulses can be tailored. Further tailoring will be done by employing experimental constraints on the target functionals. Within the experimental part we will further improve the time resolution using sub 15 fs pulses generated by non-collinear optical parametric amplification and increase the intensity of driving laser fields up to 10^15 W/cm^2 employing optical parametric chirped pulse amplification. Identification of experimentally observed signatures with microscopic processes and the theoretical optimal pulse design will thus enable pulse shaping to manipulate and control the spin dynamics towards a desired response driven by tailored intense electromagnetic fields.