Significant and Nonmonotonic Dynamic Magnetic Damping in Asymmetric Co - Fe/Ru/Co - Fe Trilayers

Static and dynamic magnetic properties of Co-Fe(10 nm)/Ru(tRu=0-3nm)/Co-Fe(5 nm) asymmetric trilayers are systematically investigated. The interlayer exchange coupling (IEC) strengths of bilinear (J1), biquadratic (J2), and their equivalent (Jeff) terms are determined; they show a weak nonmonotonic behavior with tRu. Interestingly, the magnetic remanence ratio ? of hard axis to easy axis has two distinct peaks; this can reasonably be interpreted by taking the strong J2 term into account. Various pump-laser fluences are utilized to modulate the static IEC during the time-resolved magneto-optical Kerr effect measurements, by which individual magnetization precessions of the two Co-Fe layers are achieved and the effects of dynamic IEC through mutual spin currents are highlighted. With the increase in tRu, the magnetic damping factors of both layers display the same nonmonotonic behavior, which has been mainly ascribed to the spin pumping damping aSP associated with the dynamic IEC. Moreover, it is found that the variation trend of damping difference ?aSP between the two Co-Fe layers is similar to that of Jeff, revealing that the dynamic IEC effect is actually dominated by the static IEC and thereupon a theoretical formula is proposed to describe the correlation between ?aSP and Jeff. These results suggest the feasibility of efficient control of spin pumping damping through controlled IEC, which has great significance for microwave spintronic devices based on asymmetric trilayer structures.