The study of ultrafast magnetization reversal across magnetization compensation temperature in GdFeCo film induced by femtosecond laser pulses

The subpicosecond magnetization reversal is observed by a femtosecond (fs) laser ultrafast heating of the GdFeCo film across magnetization compensation temperature, under the saturated magnetic field. Then the film is induced by fs laser without external magnetic field, and the curve of magnetization dynamics also shows magnetization reversal crossing zero magnetization. Such result shows that the ultrafast magnetization reversal in GdFeCo system induced by ultrafast heating the film across magnetization compensation temperature does not originate from the extrinsic magnetic field. To study the origin of the magnetization reversal and reveal the role of the extrinsic magnetic field and the fs-laser pump, the external field dependence and the pump fluence dependence of magnetization dynamics are measured. The external field dependence result shows that the curves of ultrafast magnetization reversal almost overlap in the initial several picoseconds (ps) and then the nucleation and growth of the reversed domains are accelerated with increasing external fields. And the pump fluence dependence result shows that the magnetization reversal occurs only when the pump fluence is intense enough and the magnetization reversal increases with the pump fluence. Therefore the ultrafast magnetization reversal during the initial several picoseconds may originate from the intense-enough ultrafast heating of fs laser, and the extrinsic magnetic field is responsible for the later domain formation and domain-wall motion.