Ultrafast magnetization dynamics in high perpendicular anisotropy [Co/Pt]n multilayers

We present time-resolved magneto-optical Kerr effect measurements of picosecond laser induced precessional dynamics in out-of-plane magnetized [Co(4 A)/Pt(8 A)](n) multilayer films. A fast precession of magnetization (up to 81 GHz) emerges 6 ps after pumping and decays within a 100 ps time scale. The uniform precessional frequency was studied for varying external magnetic bias field and number of Co/Pt repeats n. The variation of the precession frequency with external bias field is quantitatively understood using the macrospin model of the Landau-Lifshitz-Gilbert equation of motion [Phys. Z. Sowjetunion 8, 153 (1935); Phys. Rev. 100, 1243 (1955)], yielding a large perpendicular anisotropy of up to 1.0x10(7) ergs/cm(3) for our samples. The precession frequency increases sharply with reduced n and almost saturates below n=5, suggesting a commensurate variation of the perpendicular anisotropy. A heavy damping of the precessional motion is observed with increasing n and can be explained by enhanced spin-orbit coupling in ultrathin films and interfacial effects.