In this study, strong strain dependence of the elastic stiffness is found in polycrystalline Pt films. Epitaxial and polycrystalline Pt films thinner than 100 nm were deposited on monocrystal MgO substrates and resonance frequency of longitudinal standing wave in the film thickness direction was measured by picosecond ultrasound. In epitaxial Pt film, the resonance frequency was comparable to or smaller than the value expected from the elastic stiffness of bulk Pt, indicating softening of

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... ial Pt films, where nanoscale defects were observed: the softening was attributed to the defects. However, in polycrystalline Pt films, resonance frequency increased as the film thickness decreased and when the film thickness was smaller than 20 nm, the value exceeded the predicted value from the bulk stiffness, indicating the stiffening of the Pt film. Furthermore, resonance frequency showed good correlation with the strain. This trend is expected considering the higher elasticity. However, the slope between the resonance frequency and the strain is ten times as large as the value deduced from third-order elastic constant.