Ultrafast Magnetization Dynamics

There is nowadays much experimental evidence that the excitation of a ferromagnetic solid or film with an ultrashort laser pulse leads to demagnetization on a time scale of a few hundred femtoseconds (Figure 1). However, the microscopic processes responsible for such ultrafast changes in the macroscopic magnetism of the ferromagnet have not been unambiguously identified yet. The absorption of an ultrashort laser pulse produces a transient hot-electron population between the Fermi and the vacuum level of the ferromagnet, which eventually thermalizes to a Fermi distribution through electron-electron interactions on a femtosecond time scale. The temperature of this quasi-equilibrium system is then reduced through electron-phonon interactions, which dominate on the low picosecond time-scale. It is still an open question how to describe the spin dynamics within this framework (see Figure 2). A complete understanding of such processes would be extremely helpful for technical applications like ultrafast magnetization-switching schemes for read-write processes. In fact, the control of elementary processes sets the fundamental limits on laser assisted data manipulation in the respective materials. Most of the recent experimental approaches applied in this field employed time-resolved magneto-optical strategies. Such experiments access the ensemble spin dynamics taking place in the femtosecond time scale. On the other hand, spin- and time-resolved photoemission addresses the dynamics of single electron spin-flip processes which are responsible – in a microscopic view – for the observed changes in the samples overall magnetization. Bringing together these two experimental methods has, therefore, the potential for an efficient access to the microscopic view of the ultrafast dynamics of macroscopic magnetism. In our group, time-resolved magneto-optical Kerr effect (TR-MOKE) and spin-, energy- and time-resolved two-photon photoemission (SETR-2PPE) are used to address respectively the macroscopic and microscopic dynamical response of the ferromagnet induced by ultrashort optical excitation. Comparing the results obtained with these two complementary experimental techniques, we can make conclusions about the microscopic mechanisms leading to the observed ultrafast demagnetization. At present we are mostly working on thin films of the 3d-ferromagnets Fe, Ni and Co and on thin films of Heusler alloys.