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

TLDR

The study proposes that critical spin fluctuations near the Curie temperature reduce 4f electron demagnetization rates in transition‑metal rare‑earth alloys, whereas far from Tc these rates should be avoided. Ultrafast magnetization dynamics induced by femtosecond laser pulses were measured in ferrimagnetic Co₀.₈Gd₀.₂, Co₀.₇₄Tb₀.₂₆, and Co₀.₈₆Tb₀.₁₄ alloys, using element‑specific X‑ray magnetic circular dichroism at the Co L₃, Tb M₅, and Gd M₅ edges to reveal element‑dependent demagnetization dynamics. The rare‑earth sublattice exhibited a thermalization time as fast as 280 ± 30 fs when the excited‑state temperature was below the compensation temperature, but this was limited to 500 ± 100 fs when the temperature was below the Curie temperature.

Abstract

Ultrafast magnetization dynamics induced by femtosecond laser pulses have been measured in ferrimagnetic Co${}_{0.8}$Gd${}_{0.2}$, Co${}_{0.74}$Tb${}_{0.26}$, and Co${}_{0.86}$Tb${}_{0.14}$ 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 $\ifmmode\pm\else\textpm\fi{}$ 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 $\ifmmode\pm\else\textpm\fi{}$ 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 4$f$ electrons, whereas far from ${T}_{C}$ the limited demagnetization rates should be avoided.