Photon energy-dependent ultrafast magnetization dynamics in magnetic heterostructures

Abstract

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 challenge this assumption by employing spintronic terahertz emission spectroscopy, a technique primarily sensitive to spin transport rather than local magnetization changes. We reveal that the time scales of both demagnetization and recovery systematically vary with laser wavelength. Specifically, magnetization dynamics induced by shorter-wavelength optical pulses evolve over significantly longer time scales than those triggered by longer-wavelength infrared pulses, even at constant absorbed power. Our findings suggest that higher-energy (shorter-wavelength) photons enhance magnon excitation, which drives spin transport across the magnet/nonmagnet interface, thereby modifying the magnetization dynamics. These results highlight the direct influence of photon energy on ultrafast demagnetization, building upon and complementing earlier studies from various perspectives, while also offering new opportunities for optical control of spintronic phenomena.