Laser-Induced Magnetization Dynamics in Interlayer-Coupled [Ni/Co]₄/Ru/[Co/Ni]₃ Perpendicular Magnetic Films for Information Storage

Rapid manipulation of magnetization orientation by a femtosecond pulse laser is an efficient way for advanced information storage technology. We report a comprehensive study of the interlayer coupling effect on the magnetization dynamics in [Ni/Co]₄/Ru(t_Ru)/[Co/Ni]₃ perpendicular magnetic films by the time-resolved magneto-optical Kerr effect approach. By controlling the antiferromagnetic (AF) interlayer coupling field H_ex and external magnetic field H, we demonstrate distinctly different dynamic behaviors upon laser excitation. First, three kinds of demagnetization process, including nonchange, ultrafast increase, or decrease in Kerr signal, can occur within the short time delay of 2 ps. Second, in the relaxation process, in addition to the normal coherent precession, an additional out-of-phase mode is detected for the AF-coupled samples, whose amplitude and frequency depend also strongly on H_ex and H. The H-dependent frequency curves of both modes are well interpreted with the deduced analytical expression by taking the bilinear and biquadratic coupling into account. Additionally, the magnetic damping of optical mode not only is coupling-dependent but also shows a strong peak at a critical field H_crit, which is found to scale with H_ex. These results provide new insights into the magnetic dynamics in perpendicularly exchange-coupled systems for spintronic applications with ultrafast control of information operation.