Investigation of ultrafast spin dynamics in a Ni thin film

Optically induced ultrafast demagnetization has been studied in a polycrystalline nickel thin film by means of a magneto-optical pump-probe technique. The time and magnetic field dependence of the effect have been explored by measuring changes in the reflectivity, and the rotation and ellipticity associated with the linear magneto-optical Kerr effect. We find that, contrary to an earlier report, there is no significant difference in the time dependence of the rotation and ellipticity signals. Furthermore, we observe dynamic hysteresis loops with strange shapes which we believe result from a slow magnetic reorientation induced by average heating effects. These observations emphasize the importance of studying ultrafast demagnetization at saturation.