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

Abstract

Co/Ni(111) multilayers with variable cobalt thickness ${t}_{\mathrm{Co}}$ between 0.15 and 0.35 nm and fixed nickel thickness ${t}_{\mathrm{Ni}}=0.6$ 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 $\ensuremath{\approx}$$2.5$ and 5 kOe, was connected with the presence of bubble domains, revealed by polar Kerr microscopy. The lower frequency mode ($\ensuremath{\nu}\ensuremath{\approx}3$ 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 ${t}_{\mathrm{Co}}$ dependence of the effective anisotropy field of the multilayer.