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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, C11, C33, C44 and C13, were determined for each multilayer. The C13 constant increases by about 50% below Λ∼40 Å and the C33 constant decreases slightly below Λ∼30 Å as Λ decreases. On the other hand, the C11 constant is independent of Λ. These elastic constants effect the Rayleigh wave velocity only slightly, while the minimum of the C44 elastic constant is a major cause of the minimum Rayleigh wave velocity.