The interaction of a femtosecond electromagnetic pulse with the electron spin in a ferromagnetic metal has been the object of intense investigations, both theoretical and experimental, during the past fifteen years. The main effect that has been observed – though not yet fully elucidated – is the quick loss of magnetization following the excitation by a femtoseconde laser pulse.
New experiments carried out at the IPCMS [J.-Y. Bigot et al., Nature Physics 5, 515 (2009)] have now given a new twist to this problem, with promising future developments, both theoretical and experimental. These experiments have shown the existence of a coherent coupling between a femtosecond laser pulse and the magnetization of a ferromagnetic thin film. The underlying mechanism is thought to involve a form of spin-orbit coupling (SOC) that goes beyond the usual one due to the ion electric field. In order to properly describe this additional SOC, one must take into account the material polarization induced by the laser pulse, which interacts coherently with the spins. This coherent mechanism is clearly distinguished from the incoherent ultrafast demagnetization associated with the thermalization of the spins.
The theoretical description of these coherent effects is still lacking and will be mandatory in order to gain a sound understanding of ultrafast laser-spin interactions. The electromagnetic field associated with a femtosecond laser pulse is strong enough to significantly perturb the electronic charges and spins in condensed matter systems, so that relativistic effects – both in the dielectric and magnetic responses – become important. Given the intensity of the fields involved, nonlinear effects are also expected to play a considerable role.
On a timescale of a few picoseconds, the spin dynamics and related thermal demagnetization is fairly well understood in terms of the spin-phonon interaction, which is responsible for the damping of the precession of the magnetization in ferromagnetic materials . On a shorter time scale (≈10 fs) the spins couple coherently to the polarization field induced by the laser: this stage is less thoroughly understood and is the object of the present proposal. Although some evidence of coherent behaviour was obtained in past experiments carried out at the IPCMS, much more accurate results are expected in the near future thanks to new attosecond laser facilities to be built in the framework of the Equipex project UNION. The intensity of the pulses will also be higher in order to boost the coherent coupling.
Experiments will be carried out, for instance, on Garnet thin films or even spin-polarized electron wave packets, for which our theoretical models should be directly applicable. In the proposed simulations, we will be able to monitor the evolution of the magnetization both in space and in time and to relate the loss of magnetization to various effects, including spin-orbit coupling, electron-phonon coupling, and exchanges between spin and orbital angular momentum. This information should help to clarify the fundamental physics issues underpinning the planned experiments.