Annealing effect on laser-induced magnetization dynamics in Co/Ni-based synthetic antiferromagnets with perpendicular magnetic anisotropy

We report a comprehensive study of annealing treatment modulation on the laser-induced ultrafast magnetic behaviors in antiferromagnetically exchange-coupled [Ni/Co]₄/Ru/[Co/Ni]₃ multilayers with perpendicular magnetic anisotropy (PMA). Magnetic hysteresis loops indicate that the uniaxial PMA field H_keff decreases monotonously with the increase in annealing temperature T_a, but the variation of interlayer coupling field H_ex is rather complicated. Time-resolved magnetic-optical Kerr effect (MOKE) measurements demonstrate that the laser-excited demagnetization and precession process relies significantly on T_a. Upon laser impulsion, the MOKE signal immediately shows a nonchanging transient increase and decrease with H increasing for low T_a, but only the ultrafast decreasing behavior for high T_a. From the subsequent dynamic precession spectra, the optical and acoustic precession modes are identified. By fitting the field-dependent frequency curves via the deduced dispersion relations, both H_keff and H_ex are determined and their variation trends agree well with the results from the static magnetic measurement. Moreover, it is found that the critical field where the ultrafast signal decrease occurs is dependent on the co-effect of H_keff and H_ex, whereas the maximum field at which the optical mode precession disappears shares the same trend as H_ex. The magnetic damping of acoustic mode is seen to increase with T_a due to the increased inhomogeneities. Our findings provide a deep understanding of the magnetic properties in synthetic antiferromagnetic multilayers with high annealing temperatures, which will be helpful for designing advanced spintronic devices.