Estimation of transient thermal state in the Py disk after femtosecond laser pulse excitation

Since the silicon nitride was almost transparent to the laser wavelength used in our work, upon femtosecond laser pulse excitation, the optical pulse energy was first absorbed by the Py disk, resulting in a rapid rise of the electron temperature and subsequently equilibrating with the spin and lattice on a time scale below 1 ps through the electron-electron and electron-lattice couplings (42, 55). After the femtosecond laser pulse–induced locally equilibrated electron, spin, and lattice system, we described the transient thermal state of the Py disk using a simple single-temperature model in the picosecond time range (42). The relative heat capacities of the Py and silicon nitride layer were taken into account in the model, and the thermal conductivity (~25 W/mK) of the Py was taken from (56), while the thermal boundary resistance between the two layers was estimated from (57). At a laser fluence of 12 mJ/cm2 (above the threshold), the Py disk was initially heated up to a transient peak temperature exceeding the Curie point and then the system cooled down rapidly through the local equilibration between the Py disk and the silicon nitride substrate with a cooling rate of up to ~1012 K/s, followed by subsequent thermal diffusion through the substrate. On longer time scales, the slower lateral heat diffusion across the substrate ultimately brought the system back to room temperature.

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