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 L-3, Tb M-5, and Gd M-5 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 (T-C). Therefore, for transition-metal rare-earth alloys, we propose that critical spin fluctuations in the vicinity of T-C reduce the demagnetization rates of the 4f electrons, whereas far from T-C the limited demagnetization rates should be avoided.