Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization

We propose a semiclassical model for femtosecond laser-induced demagnetization due to spin-polarized excited electron diffusion in the superdiffusive regime. Our approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly, as well as the presence of several metal films in the sample. Solving the derived transport equation numerically we show that this mechanism accounts for the experimentally observed demagnetization within 200 fs in Ni, without the need to invoke any angular momentum dissipation channel.

Figure 1

Sketch of the superdiffusive processes caused by laser excitation. Different mean free paths for majority and minority spin carriers are shown and also the generation of a cascade of electrons after an inelastic scattering (elastic scatterings are not shown for simplicity). The inset shows the geometry for the calculation of the electron flux term in the continuity equation.

Figure 2

Calculated spatial magnetization profile of Ni at three times caused by laser excitation (at $t=0\mathrm{fs}$). The resulting magnetization profile is given by the full curve, the initial one by the dotted curve. The magnetization profile without electric field correction is given by the dashed curve. The surface of the film is at 0 nm depth, the Ni film extends up to 15 nm depth, the remaining is the Al film.

Figure 3

Computed laser-induced demagnetization in Ni. The shaded area shows where the theoretical result is expected to be (depending on the inelastic lifetime). For comparison we also show recent experimental XMCD data [2]. The used time structure of the laser pulse (in a.u.) is depicted by the red solid line.