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

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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.