Elastic Properties of Pure and Hydrogenated Ce/Fe Multilayer Films
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Universitit Gittingen, I. Physikalisches Institut, Bunsenstrasse 9, D-37073 Gdttingen, Germany
ABSTRACT The study of the elastic properties of pure Ce/Fe multilayers throughout the structural phase transition of the Fe layers from crystalline to amorphous reveals striking similarities to the crystal-to-glass transition in bulk metallic materials which is driven by a shear instability of the crystal lattice. The transition is reflected in a pronounced minimum (- -70%) of the flexural modulus of the multilayers as determined by a vibrating-reed technique. Hydrogenation leads to multilayers composed of CeH_2/Fe, with distinctly different moduli depending on the texture of the Fe sublayers. For multilayers with a Fe (111) texture a reorientation transition of the spontaneous magnetization is observed at a well-defined temperature. This transition is accompanied by a relaxation of thermal stresses and a considerable softening (-30%) of the flexural modulus.
INTRODUCTION It is generally accepted that elastic anomalies (often referred to as 'supermodulus effects') observed in a series of artificial metallic multilayers and superlattices are correlated with structural changes resulting from interface effects, but the exact nature of this interplay is not well understood at present. In view of the close correlation between the structural and elastic properties in such layered metallic systems it is particularly interesting to study the elastic response of a multilayer as one of the constituent materials undergoes a structural transition. The Ce/Fe
multilayers investigated here are a remarkable example for a system where a structural and magnetic disorder transition occurs simultaneously.2 Both constituents grow in an amorhous structure below and in a crystalline structure above a critical thickness, which is near 20 A for Fe and near 60 A for Ce. The study of their elastic properties throughout the structural transition is closely related to recent investigations of the crystal-to-glass transition in bulk metallic materials. Such experiments have revealed that this transformation is driven by a mechanical shear 34 instability of the crystal lattice. ' Hydrogenation causes a profound change of the structural and magnetic properties of the multilayers. In particular, a reorientation transition of the spontaneous magnetization occurs from out of plane to in plane for multilayers with a Fe (111) texture. 5' 6 In the present study we have probed the influence correlated to this reorientation on the elastic properties of the multilayer films. EXPERIMENTAL PROCEDURES AND SAMPLE STRUCTURE The multilayers were grown on glass substrates by alternating ion-beam sputter deposition of pure Ce and Fe. Hydrogenation was performed by reactive sputtering in a hydrogen atmosphere (8X10-6 mbar) leading to a superstructure built of CeH_ 2/Fe. Only the Ce sublayers absorb hydrogen, due to a large negative enthalpy of mixing. The stochiometry of the hydride is close to 303
Mat. Res. Soc. Symp. Proc. Vol. 382 @1995 Materials Research Society
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