Uncovering the role of the density of states in controlling ultrafast spin dynamics

Strong ultrafast demagnetization due to the intraband transitions

“…Figure 2 shows that the inclusion of intraband transitions to the QLE (23) dramatically enhances the demagnetization from 1%-5% in (a) to 50%-90% in (b). This is in accordance with the experiments where moderate fluences (∼10 mJ cm −2 ) were sufficient to demagnetize ferromagnets more than 50% [35,[68][69][70]. Previous ab − initio studies using the velocity gauge, based either on the TDDFT [28][29][30][31]33] or the time-dependent Liouville density functional theory [34,71], were unable to achieve demagnetization of ferromagnets greater than 10% when using the experimental fluence, and that is most likely because they did not explicitly include the intraband transitions in their calculations.…”

“…observation in figure 3 is that the hcp Co crystal is least efficient in converting the absorbed energy into demagnetization. The cutoff energy of hcp Co (1.5 eV) is near resonant with the driving laser (1.6 eV), which explains the largest energy absorption of hcp Co in figure 3(e), but the resulting demagnetization of hcp Co in figure 3(b) is the smallest of all three samples, consistent with the experimental observation of pure Ni, Fe and Co films in [70]. It is fascinating to find such diverse responses from different ferromagnets under the same laser pulses in figure 3, reflecting on their underlying band structures.…”

“…2 (ωA 0 ) 2 = 0.587 mJ cm −2 and 14.7 mJ cm −2 , respectively, comparable to the fluence used in a recent experiment (0.5-15 mJ cm −2 ) of [70]. The electric field amplitude is E 0 = ωA 0 .…”

“…Finally in figure 6, we plot the amount of maximum demagnetization in each ferromagnetic sample predicted by the QLE for various incident fluences. Also shown for comparisons are experimental results [2,4,18,25,35,[68][69][70][73][74][75][76] and the TDDFT calculations by the ELK software in [28]. Direct comparisons with experiments are difficult because measurements of magnetization are influenced by the thickness of crystals [35] as well as the temperature [68].…”

Ultrafast spin dynamics: complementing theoretical analyses by quantum-information measures

“…21 and 36, the interface acts as a 'source' of spin-polarized currents in the sample. More precisely, the magnetization change in the Cu region is initiated at the interface by a backflow mechanism [36,37]; it then spreads out into the Cu region via a spin-polarized current with rapidly decreasing magnitude. As a result, the Cu magnetization exhibits a peak at t ≈ 15 fs, i. e. slightly after the laser pulse.…”