Understanding the Microscopic Mechanisms of the Ultrafast Demagnetization in Transition Metals

After the absorption of an ultrashort laser pulse in ferromagnetic transition metals the magnetic order breaks down on a time scale of a few hundred femtoseconds. This ultrafast demagnetization effect has the potential to drive new technological developments of controlling and manipulating the magnetization on a subpicosecond time-scale. Understanding the origin of the ultrafast demagnetization is a question of fundamental importance not only in basic science but also in order to control the specifics of the effect. In particular, the understanding of the angular momentum transfer between the spin system and the lattice still represents a key challenge in this ultrafast phenomenon.

The goal of our project is to develop a many-body electronic theory in order to elucidate the microscopic physics of the ultrafast magnetization dynamics in itinerant ferromagnets which follows a femtosecond laser-pulse absorption. The relevant valence 3d and 4p electrons are described in terms of a multiband model Hamiltonian which includes Coulomb interactions, interatomic hybridizations, spin-orbit interactions, as well as the coupling to time-dependent external electric fields on the same footing. Importantly, the proposed theory takes into account from the start electron correlations and the local magnetic degrees of freedom such as spin fluctuations. Exact numerical time evolutions are performed for small ferromagnetic transition-metal clusters, which allow us to study the interplay between the electronic correlations, responsible for the local magnetic moment formation, the spin-orbit mixing which triggers angular momentum transfer, and the electronic hoppings during the magnetization dynamics. Our results reveal that the femtosecond demagnetization can be explained in terms of a three-step mechanism: (i) The laser pulse creates electron-hole pairs. This opens the way for (ii) the SOC, yielding local angular-momentum transfer from the spins to the electronic orbits with a characteristic time scale of 10 fs. However, angular momentum is not accumulated in the orbital angular momentum, since iii) it is quenched by the motion of electrons in the lattice. This takes place on a much shorter time scale of only 1 fs.

Figure: Characterization of the dynamical spin state after the laser-pulse absorption. The red curve shows that the average spin magnetization decreases on a time scale of around 100 femtoseconds, whereas the local magnetic moments given by the blue curve remain remarkably stable. The ultrafast demagnetization can be explained by a rapid decrease of nearest-neighbor spin correlations (green curve) and an increase of local spin fluctuations (violet curve).

Reference: W. Töws and G.M. Pastor, Phys. Rev. Lett. 115, 217204 (2015)