Slow recovery of the magnetisation after a sub-picosecond heat pulseN. Kazantseva1, U. Nowak1, R. W. Chantrell1, J. Hohlfeld2 and A. Rebei2
1 Department of Physics, University of York - York, YO10 5DD, UK
2 Seagate Research - 1251 Waterfront Place, Pittsburgh, PA, 15222, USA
received 30 July 2007; accepted in final form 12 November 2007; published January 2008
published online 10 December 2007
The response of a magnetic spin system to pulsed laser heating on time scales in the picosecond regime is investigated using an atomic level classical spin Hamiltonian, the dynamics of which are based on the stochastic Landau-Lifshitz-Gilbert equation. It is found that the ferro- to paramagnetic phase transition can occur in less than one picosecond, in agreement with published experimental data. Calculating the spin temperature via the internal energy of the spin system we find that the system does not necessarily fully demagnetize even for spin temperatures above the Curie temperature. Our findings suggest that the spin system is far from thermal equilibrium so that the concept of a spin temperature has to be questioned on the time scale of picoseconds. Most importantly, the time for recovery of the magnetization can vary by orders of magnitude depending on the magnetic state after heating, a prediction which is verified by supporting pump-probe experiments.
75.40.Gb - Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scalilng, etc.).
75.40.Mg - Numerical simulation studies.
75.75.+a - Magnetic properties of nanostructures.
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