Mechanism of Elastic Softening Behavior in a Superlattice

We measured the out-of-plane elastic constants C(33) of a Co/Pt superlattice by picosecond ultrasound, and found that they were closely related to the thickness ratio of Co and Pt layers; C(33) was smaller than the prediction from the bulk values except for a specific thickness ratio. This behavior can be explained by the weak bonding at the interfaces that occurs to reduce the elastic strain energy, not by the interfacial strain. This view explained the relationship among C(33), the elastic strain energy, and perpendicular magnetic anisotropy.