Shear-Elasticity Anomaly In Ce/Fe Multilayers

Brillouin light-scattering experiments have been performed at room temperature to determine the shear modulus C-44 of Ce/Fe multilayers (in their reference frame) as a function of the modulation wavelength Lambda. The study is complementary to a previous investigation of the flexural modulus E-F of this system in the sense that, while E-F is dominated by the Fe layers, C-44 largely reflects the elasticity of the more compliant Ce component. In the heterostructures, the Ce layers exhibit a laminated two-phase structure composed of low-volume alpha and high-volume gamma-like Ce, with a thickness-dependent relative fraction. Furthermore, structural transitions occur from an amorphous phase to the crystalline phases of bcc Fe and fcc Ce, for Fe abruptly within the entire sublayer volume at a critical thickness near 20 Angstrom, for Ce more gradually with increasing layer thickness starting near 40 Angstrom. These structures and phase transitions are reflected in the variation of C-44 with Lambda. A large portion of this variation can be ascribed to the Lambda-dependent alpha-gamma-like composite structure of the Ce layers. A superposed softening of C-44 is essentially the signature of the amorphous-to-crystalline transition in the Ce sublayers.