CONSISTENT INTERPRETATION OF NEGATIVE POI SSONS RATIO REPORTED IN METALLIC MULTILAYERS - ROLE OF STRESS-FREE LATTICE-PARAMETER

Elastic properties of multilayers with low period thickness show in so me cases anomalies which are generally correlated with structural modi fications in individual layers. In the recent past, several studies ha ve evidenced using X-ray diffraction in-plane and out-of-plane strains with the same sign. Some authors have then proposed in the case of W/ Ni and Nb/Cu metallic superlattices to use a negative Poisson's ratio. This result is surprising because the value of this coefficient in me tals is generally positive. In this article. we introduce a novel inte rpretation mainly based on the experimental determination by sin(2) Ps i method of the reference parameter (stress-free lattice parameter) us ed in strain calculations. Then, we show that the introduction of the bulk parameter instead of stress-free parameter for the reference para meter is an unrealistic assumption in the case of thin films and multi layers (W and Ag/Ni) and may thus lead to wrong results which are then in total disagreement with those obtained by other techniques. Theref ore, the elastic anomaly concerning Poisson's ratio which has been rep orted by some authors in scientific literature do not result from real structure of multilayers but from experimental X-ray; diffraction dat a analysis.