Connecting the timescales in picosecond remagnetization experiments

In femtosecond demagnetization experiments, one gains access to the elementary relaxation mechanisms of a magnetically ordered spin system on a time scale of $100\mathrm{fs}$. Following these experiments, we report a combined micromagnetic and experimental study that connects the different regimes known from all-optical pump-probe experiments by employing a simple micromagnetic model. We identify spin-wave packets on the nanometer scale that contribute to the remagnetization process on the intermediate time scale between single-spin relaxation and collective precession.

Figure 1

(Color online) Micromagnetic simulation for a Ni film with $t=50\mathrm{nm}$ and 55% demagnetization. The evolution of the remagnetization is shown after demagnetization by a fs-laser pulse. The demagnetization profile mirrors the penetration profile of the laser light that decays exponentially on a length of $25\mathrm{nm}$.

Figure 2

(Color online) Left side: time-resolved Kerr rotation for a $50\mathrm{nm}$ thick Ni film after demagnetization with increasing pump-pulse fluence. Right side: micromagnetic simulation of the relaxation of the magnetization for a $50\mathrm{nm}$ thick Ni film with increasing demagnetization. The dotted line is an analysis using a double exponential function. The spectra are normalized and vertically shifted for comparability.

Figure 3

(Color online) Cut through the micromagnetic simulation for the Ni film with $t=50\mathrm{nm}$ and 55% demagnetization. On the left, the evolution spin wave emission from the excited area is shown in real space. On the right, the corresponding Fourier transform is shown as a function of the spatial frequency.