Tuning the Laser-induced Ultrafast Demagnetization of Transition Metals

The role of the initial excitation in the ultrafast demagnetization process of ferromagnetic transition metals represents an important question, as a detailed understanding may lead to possibilities of controlling the spin dynamics by tuning the laser-pulse characteristics. In this project we address the questions how the efficiency of the demagnetization process depends on the degree of excitation of the ferromagnet, how an initial transfer of angular momentum upon laser absorption of different circularly and linearly polarized light may affect the subsequent dynamics, and whether any specific spin dynamics takes place while the laser field is active? In this way, the possibilities of tuning the ultrafast demagnetization are quantified. To these aims, we consider a many-body electronic theory in which the dynamics of the electronic translational, orbital and spin degrees of freedom, as well as their coupling to the external electric field, are described quantum mechanically and on the same footing.

The time evolution during and after the pulse absorption is determined exactly by performing numerical Lanczos propagations on a small cluster model with parameters appropriate for Ni. The most relevant laser parameters, namely, the fluence, wave length, polarization, and pulse duration are varied systematically. The results show that reasonable changes in these parameters do not affect the ultrafast demagnetization dynamics qualitatively and have only a minor influence on the demagnetization timescale. In contrast, our model predicts that the degree of demagnetization correlates well with the average number of electrons excited by the laser or average number of absorbed photons, which can be tuned by varying the fluence, spectral distribution and polarization of the laser pulse.

Figure: Time dependence of the average (a) spin moment Sz and (b) orbital moment Lz following the excitation with a laser pulse having linear (σ = 0), right circular (σ = +) or left circular (σ = −) polarization. The initial polarization-dependent differences in Lz are washed away rapidly after the end of the pulse due to the very effective electron hoppings throughout the lattice. In addition, the inset in (a) shows the degree of demagnetization as a function of the average number of absorbed photons nph. This demonstrates that the polarization dependence of the degree of demagnetization basically results from the corresponding different absorption cross sections.

Reference: W. Töws and G. M. Pastor, Phys. Rev. B 100, 024402 (2019)