Energy relaxation dynamics in ferromagnetic Co film with femtosecond transient absorption

Abstract

Femtosecond time-resolved pump-probe technology has been widely used in studying ultrafast dynamics in various materials. In the present study a Co thin film on MgO, as a typical ferromagnetic material, was selected for investigating the ultrafast energy relaxation dynamics related to spin feature of materials. The energy relaxation dynamics of this thin film was measured by femtosecond transient absorption. The results can be well stimulated by a three-temperature model, in which electron, spin and lattice were involved, indicating that spin plays an important role in the energy relaxation. From this model, the hot electron temperature from the transient absorption under different pumping laser energy was calibrated which can reach 638, 724, 841 and 935 K with the energy of 3.2, 4.3, 6.0 and 7.5 J/m2 respectively. In additional, three time constants were obtained by a global analysis of the transient absorption, and attributed to electron–spin (250∼350 fs), electron–lattice (10∼20 ps), and spin–lattice interaction (300∼400 ps), respectively. By analyzing the dynamics of the ferromagnetic material after laser excitation, the rich physics governing ultrafast energy relaxation dynamics are elucidated, and the temperature references for femtosecond laser manipulation of this material are provided.