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(tRu)/[Co/Ni]3 perpendicular magnetic films by the time-resolved magneto-optical Kerr effect approach. By controlling the antiferromagnetic (AF) interlayer coupling field Hex 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 Hex 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 Hcrit, which is found to scale with Hex. These results provide new insights into the magnetic dynamics in perpendicularly exchange-coupled systems for spintronic applications with ultrafast control of information operation.