Brillouin Scattering from Surface Acoustic Waves in Ni/Al Multilayer Films

The surface acoustic wave velocities in Ni/Al multilayer films were determined using Brillouin scattering for several modulation periods Λ ranging from 10.7 Å to 84.9 Å. The Rayleigh wave velocity shows a minimum around Λ∼40 Å. In order to obtain more detailed information on the elastic properties of the multilayer system, the effective elastic constants, C 11, C 33, C 44 and C 13, were determined for each multilayer. The C 13 constant increases by about 50% below Λ∼40 Å and the C 33 constant decreases slightly below Λ∼30 Å as Λ decreases. On the other hand, the C 11 constant is independent of Λ. These elastic constants effect the Rayleigh wave velocity only slightly, while the minimum of the C 44 elastic constant is a major cause of the minimum Rayleigh wave velocity.