Anomalous Elastic Softening in Superlattice Thin Films Studied by Picosecond-Laser Ultrasounds and Micromechanics Modeling

In this study, we observed anomalous softening in elasticity of Co/Pt superlattice thin films by measuring the out-of-plane elastic constant using the picosecond laser ultrasounds. It showed correlation with the strain energy of Co and Pt layers; as the strain energy increased, the elastic constant was lowered. In Co/Pt superlattice, the strain of each layer becomes more than 0.01 because of 10.7% lattice misfit at the interfaces, which will be partially released by occurrence of noncohesive-bonding regions. We consider that as the strain energy increases, the volume fraction of the noncohesive-bonding region increases, and the elastic constant is lowered. This view is supported by the micromechanics calculation.