Domain wall dynamics due to femtosecond laser-induced superdiffusive spin transport

Manipulation of magnetic domain walls via a helicity-independent laser pulse has recently been experimentally demonstrated and various physical mechanisms leading to domain wall dynamics have been discussed. Spin-dependent superdiffusive transport of hot electrons has been identified as one of the possible ways to affect a magnetic domain wall. Here, we develop a model based on superdiffusive spin-dependent transport to study the laser-induced transport of hot electrons through a smooth magnetic domain wall. We show that the spin transfer between neighboring domains can enhance ultrafast demagnetization in the domain wall. More importantly, our calculations reveal that when the laser pulse is properly focused onto the vicinity of the domain wall, it can excite sufficiently strong spin currents to generate a spin-transfer torque that can rapidly move the magnetic domain wall by several nanometers in several hundred femtoseconds, leading to a huge nonequilibrium domain wall velocity.

Figure 1

Sketch of a head-to-head magnetic domain wall and the used coordination system. The wave vector of the incident laser beam is aligned with the $y$ axis.

Figure 2

Superdiffusive transport through a narrow magnetic domain wall. (a) Dashed (blue) line shows out-of-plane component of magnetization in the vicinity of a narrow magnetic domain wall in the equilibrium ($t=-\infty$). Solid (red) line shows the out-of-plane magnetic component $300\mathrm{fs}$ after the maximum of the laser pulse. (b) Time variation of Gaussian laser pulse intensity with peak at time $t=0$ and FWHM $35\mathrm{fs}$. (c) Corresponding time variation of the spin fluxes calculated for different distances (2, 6, and $10\mathrm{nm}$) from the domain wall.

Figure 3

Laser-induced demagnetization in the vicinity of the domain wall calculated for various DW widths. (a) Spatial dependence of magnetization taken $1\mathrm{ps}$ after the pulse. (b) Demagnetization in the DW center, $z=0$, taken $1\mathrm{ps}$ after the pulse as a function of the DW width. (c) Time variation of Gaussian laser pulse intensity with peak at time $t=0$ and FWHM $35\mathrm{fs}$. (d) Time dependence of magnetization in the DW center after the laser pulse, for various DW widths. The lines correspond to those shown in (a).

Figure 4

Spatial and temporal dependence of the transverse spin current in the neighborhood of a domain wall in the units of $[M_0/M\left(z\right)][\hslash /\left(2e\right)]{\mathrm{fs}}^{-1}$. The domain wall center is located at $z=0$ and its width is $\mathrm{\Delta }=10\mathrm{nm}$. The excitation region is restricted to the area between the white dashed lines. (a) Time variation of Gaussian laser pulse intensity with peak at time $t=0$ and FWHM $35\mathrm{fs}$. (b) The excitation region is symmetric with respect to the position of the domain wall center. (c) and (d) The excitation region is asymmetric with respect to the position of the domain wall center. In case (d) the borderline of the excitation region passes through the domain wall center.

Figure 5

Spatial and temporal dependence of a laser-induced charge current in the neighborhood of the domain wall, given in units of particles per $\mathrm{fs}$. The parameters of the calculations are the same as in Fig. 4. Panel (a) shows the time variation of the Gaussian laser-pulse intensity.

Figure 6

Laser-induced dynamics of the domain wall. (a) Time variation of Gaussian laser pulse intensity with peak at time $t=0$ and FWHM $35\mathrm{fs}$. (b) Time dependence of the domain wall position after the laser pulse for DW width $\mathrm{\Delta }=10\mathrm{nm}$ calculated for different positions of the excitation region. (c) DW displacement taken $1\mathrm{ps}$ after the DW motion for different DW widths as a function of the excitation region position.