Picosecond ultrasonics and Brillouin light scattering studies of the elastic properties of ultrathin multilayers in the composite system permalloy-alumina

C. Rossignol (1) , Bernard Perrin (1) , Bernard Bonello (1) , Ph. Djemia , Ph. Moch , H. Hurdequin

1 LMDH - Laboratoire des Milieux Désordonnés et Hétérogènes

Picosecond ultrasonics and Brillouin light scattering were used to investigate the acoustical properties of periodic (Py-Al2O3) multilayers constituted of ultrathin polycrystalline layers of permalloy (Py corresponding to a Ni80Fe20 alloy) separated by a dielectric spacer of amorphous alumina (Al2O3). Picosecond ultrasonics gives access to the elastic properties of multilayers both through the travel time of acoustic echoes propagating in the whole structure and through the frequencies of localized vibrational modes lying within the gaps induced by the periodicity of the system. These measurements show that the effective longitudinal elastic constant along the bilayer stacking axis, softens from 7% to 40%, with respect to a reference value obtained in thick samples, when the period decreases from 14.8 to 5.3 nm. This strong softening is ascribed to interfacial effects between permalloy and alumina layers. In contrast with the longitudinal elastic constant, the softening of the in-plane shear elastic constant, derived from Brillouin light scattering measurements, does not exceed 10% for the smallest period sample.

Mots clés: Permalloy; multilayers; periodic structures; echo; localised modes; elastic constants; interface phenomena; alumina; ultrasonic propagation

C. Rossignol, Bernard Perrin, Bernard Bonello, Ph. Djemia, Ph. Moch, et al.. Picosecond ultrasonics and Brillouin light scattering studies of the elastic properties of ultrathin multilayers in the composite system permalloy-alumina. Physical Review B: Condensed Matter and Materials Physics (1998-2015), 2004, 70, pp.094102. ⟨10.1103/PhysRevB.70.094102⟩. ⟨hal-00004952⟩