Ultrafast manipulation of magnetic anisotropy in a uniaxial intermetallic heterostructure TbCo2/FeCo

We study experimentally and theoretically the dynamics of spin relaxation motion excited by a femtosecond pulse in the TbCo2/FeCo multilayer structures with different ratios of TbCo2 to FeCo thicknesses r(d) = d(TbCo)(2)/d(FeCo). The main attribute of the structure is in-plane magnetic anisotropy that is artificially induced during sputtering under a DC magnetic field. The optical pump-probe method revealed strongly damped high-frequency oscillations of the dynamical Kerr rotation angle, followed by its slow relaxation to the initial state. Modeling experimental results using the Landau-Lifshitz-Gilbert (LLG) equation showed that the observed entire dynamics is due to destruction and restoration of magnetic anisotropy rather than to demagnetization. For the pumping fluence of 7 mJ cm(-2), the maximal photo-induced disruption of the anisotropy field is about 14% for the sample with r(d) = 1 and decreases when r(d) increases. The anisotropy relaxation is a three-stage process: the ultrafast one occurs within several picoseconds, and the slow one occurs on a nanosecond time scale. The Gilbert damping in the multilayers is found to be one order of magnitude higher than that in the constituent monolayers.