A structure investigation of epitaxial Fe–Pt multilayers

Fe/Pt epitaxial multilayers have been studied by cross-sectional transmission electron microscopy and high-resolution transmission electron microscopy (HRTEM). The Fe/Pt multilayers were prepared by sputter deposition on MgO substrates. All samples had the (002)Fe/Pt, plane parallel to the surface of MgO. The nominal bilayer period Λ was varied from 2.5 to 12.1 nm. The multilayers with Λ ≥ 4.4 nm exhibited a Bain orientation relationship between fcc Pt and the tetragonally distorted bcc Fe. Examination of the multilayers with Λ = 12.1 nm revealed the least distorted bcc Fe structure and the best-defined interface between the bcc Fe and fcc Pt layers. The HRTEM images taken from the areas adjacent to the interface revealed that (200)bccFe is rotated 3° away from (200)Pt. This rotation appears to arise in order to accommodate the elastic misfit between the layers. The ratio of the interplanar distance of the (002)bccFe planes relative to that of (110)bcCFe, that is d(oo2)Fe/d(110)Fe, was found to be 1.37 for Λ = 7.6 nm and 1.28 for Λ = 4.4 nm, which are smaller than the value of 1.414 expected for an undistorted bcc structure. The variation in the ratio could be related to a relaxation of the internal stresses developed in the multilayers. For the Fe/Pt multilayers with a nominal value of Λ = 2.5 nm, both the selected area diffraction patterns and the HRTEM images show epitaxial growth of fcc Fe and Pt with a cube-on-cube orientation relationship, that is [010]Fe//[010]pt, (002)Fe//(002)pt, with about 1° deviation. A tetragonal distortion occurred in the Pt layers with the largest lattice constant in the plane of the film. Diffuse diffraction spots and streaking along the film growth direction were observed in all multilayers, probably owing to a combination of both the small dimensions of the layer structure and the formation of a Fe/Pt solid solution close to the interfaces.