We report a systematic measurement of the electron-phonon thermalization time scale as a function of the sample base temperature in Ni using femtosecond electron diffraction. A strong temperature dependence of this time scale has been observed in the vicinity of the Curie point. By assuming that ultrafast magnetism completes well before the electron-phonon thermalization, this correlation between thermalization time and sample base temperature fits a modified three-temperature model describing the energy transfer among charge, spin, and phonon subsystems. The results indicate that charges and spins can be characterized by a unified temperature during ultrafast lattice heating.

• Received 11 January 2010

DOI:https://doi.org/10.1103/PhysRevB.81.220301