Ultrafast magnetization dynamics in an epitaxialNi54.3Mn31.9Sn13.8Heusler-alloy film close to the Curie temperature

Abstract

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 ${\mathrm{Ni}}_{54.3}{\mathrm{Mn}}_{31.9}{\mathrm{Sn}}_{13.8}$ 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 (${T}_{\mathrm{C}}$) proximity in the alloy film studied. Effective magnetic anisotropy field (${H}_{\text{k}}^{\text{eff}}$) 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 ${H}_{\text{k}}^{\text{eff}}$ with $F$ was well simulated in the frame of an extended version of the microscopic three-temperature model (eM3TM), and explained by the ${T}_{\mathrm{C}}$ 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 ${H}_{\text{k}}^{\text{eff}}$ and changes of the equilibrium magnetization angles induced by pump-pulse excitation.