Structural and Elastic Response of Mo/Ni Multilayers to Ion Irradiation
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Structural and Elastic Response of Mo/Ni Multilayers to Ion Irradiation Jérôme Pacaud1, Franck Martin1, Anny Michel1, Christiane Jaouen1, Philippe Djemia2, François Ganot2 1 Laboratoire de Métallurgie Physique, UMR CNRS 6630, Université de Poitiers, Ave Marie et Pierre Curie, BP 30179, 86962 Chasseneuil Cedex, France 2 Laboratoire des Propriétés Mécaniques et Thermodynamiques des Matériaux, UPR CNRS 9001, Université Paris-Nord, Avenue J. B. Clément, 93430 Villetaneuse, France ABSTRACT Mo/Ni multilayers are investigated by x-ray diffraction and Brillouin light scattering before and after ion induced stress relaxation and mixing. Study of the evolution of interplanar distances in both layers as a function of the period exhibits a strong anomaly of the Mo (110) distance (in the growth direction) that can be correlated with the elastic anomaly. The very low interplanar distance in the molybdenum layers found after stress relaxation seems to favor an explanation of this behavior based on the diffusion of Ni in the Mo layers during the growth. INTRODUCTION There has been an important number of studies devoted to the elastic properties of multilayers in the past few years [1]. This comes from the large differences of behavior observed between systems and even for the same system depending on the preparation conditions. The Mo/Ni system has been considered as a model system for studying the relationship between elastic properties and structure of the multilayer due to the large softening of the shear modulus and the good coherency that can be achieved in this bcc/fcc system [2-5]. Different theoretical models have been proposed to explain the anomalous elastic behavior based on different physical mechanisms such as coherency strains [6], surface tension stresses at interfaces [7], deformations related to specific superlattice electronic properties [8-10] and the presence of atomic disorder at the incoherent interfaces [11-12]. By referring to the correlation between the average lattice constant change of the superlattice and the elastic anomalies, many studies tend to conclude that structural effects such as coherency strains or atomic disorder would be the physical mechanisms responsible of the observed anomalies. However, questions concerning the exact nature of the lattice strains are not answered or controversial. The purpose of this work is to increase the knowledge of the Mo/Ni system by a structural study correlated to the elastic properties determination when introducing modifications of the stress state and the structure by ion irradiation. EXPERIMENTAL PROCEDURES Deposition The Mo/Ni multilayers, with period scaling between 1.5 and 140 nm, were grown on Si (001) covered with a thin (3 nm) layer of amorphous SiO2 in a high vacuum computer – controlled NORDIKO-3000 sputtering system equipped with an RF-plasma source and two
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cryogenic pumps. Pressure in the deposition chamber or deposition rates may dramatically change the final state of the deposited material. They were, in the present case, deposited with
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