Role of critical spin fluctuations in ultrafast demagnetization of transition-metal rare-earth alloys

Ultrafast magnetization dynamics induced by femtosecond laser pulses have been measured in ferrimagnetic Co0.8Gd0.2, Co0.74Tb0.26, and Co0.86Tb0.14 alloys. Using element sensitivity of x-ray magnetic circular dichroism at the Co L₃, Tb M₅, and Gd M₅ edges, we see that the demagnetization dynamics is element dependent. We show that a thermalization time as fast as 280 ± 30 fs is observed for the rare earth in the alloy when the excited-state temperature is below the compensation temperature. It is limited to 500 ± 100 fs when the excited-state temperature is below the Curie temperature (TC). Therefore, for transition-metal rare-earth alloys, we propose that critical spin fluctuations in the vicinity of TC reduce the demagnetization rates of the 4f electrons, whereas far from TC the limited demagnetization rates should be avoided.