Ultrafast Spin Dynamics: The Effect of Colored Noise

Recent experimental results have pushed the limits of magnetization dynamics to pico- and femtosecond time scales. This ultrafast dynamics occurs in extreme conditions of strong and rapid fields and high temperatures. This situation requires a new description of magnetization dynamics, taking into account that the electron correlation time could be of the order of the inverse spin frequency. For this case we introduce a thermodynamically correct phenomenological Landau-Lifshitz-Miyasaki-Seki approach. We demonstrate the effect of the noise correlation time on the ultrafast demagnetization rate.

Figure 1

Equilibrium magnetization as a function of temperature for a system of $\mathcal{N}={32}^3$ interacting spins, integrating the LLMS equation with different correlation times and integrating the LL equation. The inset shows distribution functions for a noninteracting spin system modeled within the LLMS approach for different values of the reduced field $\zeta ={\mu }_{s}H/k_{B}T$ ($\gamma H{\tau }_c=1.76$) and correlation time ${\tau }_c=10\mathrm{fs}$. The solid line in the inset represents the Boltzmann distribution.

Figure 2

Longitudinal relaxation time (normalized to the uncorrelated case) as a function of the correlation time for various temperatures calculated within the LLMS approach. The inset shows longitudinal relaxation for various correlation times and $T=600\mathrm{K}$.

Figure 3

Transverse relaxation time (normalized to the uncorrelated case) as a function of the correlation time for various temperatures calculated within the LLMS approach. The inset shows transverse relaxation for two temperatures $T=5\mathrm{K}$ and $T=600\mathrm{K}$ and ${\tau }_c=10\mathrm{fs}$.

Figure 4

Laser-induced demagnetization as a function of time for various noise correlation times ${\tau }_c$ modeled within LLMS and LL approaches.