Elastic properties of ultrathin permalloy/alumina multilayer films using picosecond ultrasonics and Brillouin light scattering

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. 80. 5em{0ex}to0. 5em{0ex}5. 30. 3em{0ex}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.