Ultrafast demagnetization of ferromagnetic transition metals: The role of the Coulomb interaction

The Elliott-Yafet (EY) mechanism is arguably the most promising candidate to explain the light-induced ultrafast demagnetization dynamics in ferromagnetic transition metals on time scales on the order of 100 fs. So far, only electron-phonon (or impurity) scattering has been analyzed as the scattering process needed to account for the demagnetization. We show that an EY-like mechanism based on electron-electron scattering has the potential to explain time-resolved magneto-optical Kerr effect measurements on thin magnetic Co and Ni films, without reference to a “phononic spin bath.”

Figure 1

(Color online) Band structure ${ϵ}_{\vec{k}}^{\mu }$ of (a) Ni and (b) Co for majority (solid lines) and minority (dashed lines) electrons in $\Gamma \text{−}X$ direction (Ref. 20). The Fermi energy $E_{\text{F}}$ is set to zero. The arrows indicate a typical ultrafast demagnetization scenario: electrons are excited by an ultrashort laser pulse (vertical light gray arrows), which does not change the total magnetization. They relax via intraband and interband scattering (gray arrows). The latter scattering process leads to depolarization of the electrons.

Figure 2

(Color online) Dynamical distribution functions $n_{\vec{k}}^{\mu }$ for four Ni bands (cf. Fig. 1). Electrons are excited from (a) bands ${\Delta }_2^{\uparrow }$ and ${\Delta }_5^{\uparrow }$ into (b) band ${\Delta }_1^{\uparrow }$. Ultrafast demagnetization occurs by scattering into bands (c) ${\Delta }_2^{\downarrow }$ and (d) ${\Delta }_5^{\downarrow }$.

Figure 3

(Color online) Normalized TR-MOKE rotation (gray curve) and calculated magnetization dynamics for Ni (blue curve) and Co (red curve). Assuming the same laser fluence, the choice of parameters ${\alpha }_{\text{Co}}=0.15$ and ${\alpha }_{\text{Ni}}=0.3$ yields good agreement between theory and experiment. The demagnetization times are $T_{\text{Co}}=215\text{fs}$ and $T_{\text{Ni}}=200\text{fs}$. In the calculation, the stronger quenching of the magnetization for Ni is due to band-structure effects.