Ultrafast magnetization dynamics in an epitaxial Ni_(54.3)Mn_(31.9)Sn_(13.8) Heusler-alloy film close to the Curie temperature

摘要

The influence of the amplitude of an external magnetic field (H) and femtosecond laser pulse fluence (F) on ultrafast magnetization dynamics has been investigated in a ferromagnetic Ni54.3Mn31.9Sn13.8 Heusler-alloy film using the time-resolved magneto-optical Kerr effect. A large slowing down of the demagnetization process was observed and characteristic parameters of magnetization precession were determined for a wide range of H and F values. Long demagnetization times of the order of hundreds of picoseconds have been found and explained as a result of the Curie temperature (T-C) proximity in the alloy film studied. Effective magnetic anisotropy field (H-k(eff)) and Gilbert damping parameter dependencies were determined. A significant reduction of the precession frequency versus F of the uniform Kittel mode was found. A strong decrease of H-k(eff) with F was well simulated in the frame of an extended version of the microscopic three-temperature model (eM3TM), and explained by the T-C proximity effect. The estimated low values of the eM3TM model parameters, the demagnetization rate and electron-lattice coupling constant, appeared essential to explain the slowing down effect of demagnetization. Precession amplitude dependencies were explained by a phenomenological approach taking into account H-k(eff) and changes of the equilibrium magnetization angles induced by pump-pulse excitation.