Coupling of Heat Carriers in Laser-Induced Thermal Transport in Pt/Co and Pt/CoFeB Bilayers

Coupling of electrons with phonons or with magnons plays a key role in various transport processes, including ultrafast demagnetization and spin diffusion in magnetic materials. However, experimental determination of coupling strength is challenging due to the difficulty in separately observing an effective temperature of each type of excitation. In this work, we design simple bilayers, consisting of an optically-thick Pt layer and sub-nm-thick ferromagnetic (FM) layer, i.e., Co and CoFeB. The Pt surface is irradiated with a short laser pulse, and the temperature changes of phonons and magnons are observed separately, via the change in reflectance of the Pt surface (i.e., time-domain thermoreflectance), and via the change in magnetization of the FM layer (i.e., time-resolved magneto-optic Kerr effect). We describe the time-evolution of the effective temperatures of electrons, phonons, and magnons by a three-temperature model. We conclude that magnons in Co thermalize rapidly with electrons in Co and that we can derive an improved value of the electron-phonon coupling of Pt from measurements of Pt/Co bilayers. We observe that magnons in CoFeB equilibrate more slowly than magnons in Co and we extract the electron-magnon coupling of CoFeB from measurements of Pt/CoFeB bilayers.