Capturing the coupled dynamics of the charge, spin and lattice degrees of freedom through the ferromagnetic phase transition using time-resolved spectroscopies

The coupled interactions of the charge, spin and lattice give rise to intriguing phenomena in quantum materials. Because electrons respond faster than the lattice, ultrafast techniques can uncover which coupled interactions are dominant in a particular material. Here we investigate the ferromagnetic-to-paramagnetic phase transition in nickel using both time-resolved photoemission spectroscopy and magneto-optical Kerr spectroscopy and make several surprising findings. First, the spin system can absorb the laser energy within 20 fs after laser excitation. This is evidenced by a divergence in the transient heat capacity of the spin system as the electron temperature approaches the Curie temperature. Second, we observe a longer and universal timescale of 180 fs in the collapse of the exchange splitting or demagnetization. Moreover, there is a strong fluence dependence of both the phase transition itself and the recovery, which can proceed via two distinct pathways, with timescales of 540 fs and 70 ps, that are attributed to ultrafast spin-spin relaxation and slower timescale phonon coupling, respectively. Our results uncover the clear time sequence of the multiple dynamics and suggest that critical behavior dominates the ultrafast phase transition in nickel.