Many-Body Theory of Ultrafast Demagnetization and Angular Momentum Transfer in Ferromagnetic Transition Metals

Exact calculated time evolutions in the framework of a many-electron model of itinerant magnetism provide new insights into the laser-induced ultrafast demagnetization observed in ferromagnetic (FM) transition metal thin films. The interplay between local spin-orbit interactions and interatomic hopping is shown to be at the origin of the observed postexcitation breakdown of FM correlations between highly stable local magnetic moments. The mechanism behind spin- and angular-momentum transfer is revealed from a microscopic perspective by rigorously complying with all fundamental conservation laws. An energy-resolved analysis of the time evolution shows that the efficiency of the demagnetization process reaches almost 100% in the excited states.

Figure 1

Time dependence of the spin magnetization $2S_z$ following a 5 fs laser-pulse excitation for an equilateral triangle having $N_e=4$ valence electrons and different SOC strengths $\xi$. The inset shows the demagnetization time ${\tau }_{\mathrm{dm}}$ as a function of $\xi$.

Figure 2

Time dependence of the average $d$-electron local spin moment $⟨({\overrightarrow{s}}_i^d{)}^2{⟩}^{1/2}$, NN spin-spin correlation function $⟨{\overrightarrow{s}}_i^d·{\overrightarrow{s}}_j^d⟩$, and off-plane spin and orbital angular momentum $S_z$ and $L_z$.

Figure 3

Time dependence of the spin magnetization $2S_z^{(\alpha )}$ in the excited-state manifolds corresponding to $\alpha =0$, 1, and 2 photon absorptions ($\hslash \omega =1.55\mathrm{eV}$). The average $S_z=\sum _{\alpha }{w}_{\alpha }{S}_z^{(\alpha )}$ is given by the dashed curve. The inset shows the spectral distribution $D_{\mathrm{\Psi }}(ϵ)$ of $\mathrm{\Psi }(t)$ after the laser-pulse passage ($t\ge 3{\tau }_p=15\mathrm{fs}$). The weights $w_{\alpha }$ of the different spectral parts of $\mathrm{\Psi }$ are indicated.