Photon Energy-Dependent Ultrafast Magnetization Dynamics in Magnetic Heterostructures

Laser-induced ultrafast demagnetization has traditionally been understood as a process in which photon absorption excites nonequilibrium electrons, leading to demagnetization followed by magnetization recovery. It has been widely assumed that the time scale of these dynamics is governed primarily by the total absorbed laser power rather than the specific photon energy. In this study, we present evidence that challenges this assumption. By employing spintronic terahertz emission spectroscopy in magnetic heterostructures, we reveal that the time scales of both demagnetization and recovery systematically vary with laser wavelength, i.e., photon energy. Specifically, magnetization dynamics induced by shorter-wavelength optical pulses evolve over significantly longer time scales than those triggered by longer-wavelength infrared pulses, even when the total absorbed laser power remains constant. Our findings suggest that higher-energy (shorter-wavelength) photons enhance magnon excitation, which in turn drives spin transport across the magnet/nonmagnet interface, thereby modifying the magnetization dynamics. These results uncover a previously overlooked influence of laser photon energy on ultrafast magnetization dynamics, offering new opportunities for optical control of spintronic phenomena.