Feedback Effect during Ultrafast Demagnetization Dynamics in Ferromagnets

Motivated by the recent controversy about the importance of spin-flip scattering for ultrafast demagnetization in ferromagnets, we study the spin-dependent electron dynamics based on a dynamical Elliott-Yafet mechanism. The key improvement to earlier approaches is the use of a modified Stoner model with a dynamic exchange splitting between majority and minority bands. In the framework of our microscopic model, we find a novel feedback effect between the time-dependent exchange splitting and the spin-flip scattering. This feedback effect allows us to reproduce important properties of the demagnetization dynamics quantitatively. Our results demonstrate that in general Elliott-Yafet spin-flip scattering needs to be taken into account to obtain a microscopic picture of demagnetization dynamics.

Figure 1

Schematic plot of the occupation probability (electron distribution times density of states) in (left) and out of (right) quasiequilibrium. The feedback effect is indicated by gray arrows. Inset: equilibrium magnetization $M(T)$ computed by Eq. (3) for nickel, normalized to the value at $T=0\mathrm{K}$. The experimental data (red dots) are taken from Ref. 37.

Figure 2

Transient magnetization dynamics after ultrashort laser irradiation ($F=1.4F_0$) for different cases: the role of thermalized electrons and secondary electrons as well as of a transient exchange splitting.

Figure 3

Transient magnetization dynamics after ultrashort laser irradiation for different laser fluences. Experimental results, obtained by time-resolved MOKE measurements, are taken from Ref. 32.

Figure 4

Magnetization dynamics after ultrashort laser irradiation at a constant fluence ($F=1.4F_0$) and various ambient temperatures $T=80–480\mathrm{K}$.