Ultrafast demagnetization of ferromagnetic thin films after pulsed laser excitation is a fascinating and technologically promising phenomenon yet still not well understood. We combine time-, angle-, and energy-resolved two-photon photoemission with spin-resolved electron detection.
Image-potential states act as “observer states” for the magnetization dynamics even on an ultrafast timescale. A particularly suitable method to study the dynamics of these unoccupied states is two-photon photoemission (2PPE). With a spin-polarized low energy electron diffration (SP-LEED) detector we additionally achieve spin-resolution, which makes it possible to distinguish between majority- and minority-electron dynamics.

Recent result: Hot spots in the bandstructure of fcc cobalt

In measurements from the valence bands of epitaxial fcc cobalt on Cu(001) linear magnetic dichroism is observed. With image-potential states as spectator states we identify initial bulk and surface states with minority spin-character as the source for dichroic intensities and apparent dichroic lifetimes. The excellent agreement with ab initio fully relativistic calculations of the cobalt fcc bandstructure allows us to precisely determine spin-orbit hybridization points close to the Fermi level. These spin hot-spots enhance spin-flip scattering by several orders of magnitude and are therefore assumed to be crucial in ultrafast demagnetization.

Phys. Rev. Lett. 101, 066402 (2008).

Publications 2005-2008:

• A. B. Schmidt, M. Pickel, M. Wiemhöfer, M. Donath, and M. Weinelt. Spin-Dependent Electron Dynamics in Front of a Ferromagnetic Surface. Phys. Rev. Lett. 95, 107402 (2005).
• M. Pickel, M. Donath, A.B. Schmidt, and M.Weinelt, A two-photon photoemission study of spin-dependent electron dynamics, Surf. Sci. 600, 4176 (2006).
• M. Donath, C. Math, M. Pickel, A.B. Schmidt, and M.Weinelt, Realization of a spin-polarized two-dimensional electron gas via image-potential-induced surface states, Surf. Sci. 601, 5701 (2007).
• A. B. Schmidt, M. Pickel, M. Donath, and M. Weinelt, Ultrafast Spin-Dependent Electron Dynamics at the Surface of Ferromagnetic Thin Films, JMMM 310, 2330 (2007).
• M. Pickel, Image-potential states as a sensor for magnetism, PhD thesis, Freie Universität Berlin, (2007).
• A. B. Schmidt, Spin-dependent electron dynamics in front of ferromagnetic surfaces, PhD thesis, Freie Universität Berlin, (2007).
• M. Weinelt, A. B. Schmidt, M. Pickel, and M. Donath, Spin-polarized image-potential-state electrons as ultrafast magnetic sensor in front of ferromagnetic surfaces, Progr. Surf. Sci. 82, 388 (2007).
• A. Melnikov, H. Prima-Garcia, M. Lisowski, T. Gießel, R. Weber, R. Schmidt, C. Gahl,
  U. Bovensiepen, and M. Weinelt, Non-equilibrium magnetization dynamics of Gadolinium studied by magnetic linear dichroism in time-resolved 4f core-level photoemission, Phys. Rev. Lett. 100, 107202 (2008).
• A. B. Schmidt, M. Pickel, T. Allmers, M. Budke, J. Braun, M .Weinelt, and M Donath,
  Surface electronic structure of fcc Co films: a combined spin-resolved one- and two-photon-photoemission study, J. Phys. D: Appl. Phys. 41, 164003 (2008).
• M. Pickel, A. B. Schmidt, F. Giesen, J. Braun, J. Minar, H. Ebert, M. Donath, and M.
  Weinelt, Spin-Orbit Hybridization Points in the Face-Centered-Cubic Cobalt Band Structure, Phys. Rev. Lett. 101, 066402 (2008).