Spin waves in perpendicularly magnetized Co/Ni(111) multilayers in the presence of magnetic domains

Co/Ni(111) multilayers with variable cobalt thickness t(Co) between 0.15 and 0.35 nm and fixed nickel thickness t(Ni) = 0.6 nm were grown on a Pt(1.6 nm)/Ta(3 nm) substrate by dc magnetron sputtering. A strong perpendicular magnetic anisotropy was found both using vibrating sample magnetometry and Brillouin light scattering from thermally excited spin waves. The simultaneous presence of two spin-wave modes, for a range of magnetic fields H applied in plane between approximate to 2.5 and 5 kOe, was connected with the presence of bubble domains, revealed by polar Kerr microscopy. The lower frequency mode (nu approximate to 3 GHz), which exhibits a smooth dependence on the strength of H, was attributed to harmonic oscillations of the domain walls. The higher frequency mode, which displayed the typical field behavior of a film with a perpendicular anisotropy, was interpreted as the superposition of two nearly degenerate modes, associated, respectively, with the in-phase and out-of-phase precession of the spins in the bubble array. The higher frequency mode also displayed an unprecedented, nonmonotonic dependence on cobalt thickness, reflecting the nonmonotonic t(Co) dependence of the effective anisotropy field of the multilayer.