Ultrafast magnetization dynamics and their signature in the transient electronic band structure

Femtomagnetism is an established and active research field in thin film and surface magnetism, which promises magnetic writing speeds three orders of magnitude faster than current technology. Investigations in this area comprise the spin and magnetization dynamics in ferro- and ferrimagnetic samples initiated with a moderate femtosecond laser pulse.  In the first two picoseconds after optical excitation the electronic system and the underlying lattice and spin subsystems of the ferromagnet are not in equilibrium. It remains controversial, which microscopic processes are responsible for the change of the magnetization within a few hundred femtoseoconds: scattering among electrons, phonons and magnons, and/or spin-transport? On which timescale do the band structure and spin polarization change and how does this affect the demagnetization dynamics.
To approach these problems we perform time- and angle-resolved photoemission with a high-order harmonics VUV source. We have studied ultrafast demagnetization for the local-moment ferromagnets Gadolinium and Terbium. In the lanthanide metals equilibration of the excited state could involve more than one timescale, because the optical excitation occurs in the valence band but the magnetic moment is dominated by the localized 4f electrons. Following excitation by a femtosecond infrared pulse, we directly map the transient exchange splitting of the Gd and Tb valence bands. Simultaneously we record the magnetic linear dichroism of the 4f photoemission line. This allows us to compare the magnetization dynamics of 4f core and 5d6s valence electrons in one measurement and develop a detailed picture of the magnetization dynamics in the lanthanides.