Microtwinning in epitaxial nickel-iron films

Abstract

Thin films of permalloy (81% Ni, 19% Fe) grown by evaporation on rock salt cleavage faces have been studied in the electron microscope. Selected area diffraction patterns indicate the presence of microtwinning on all types of {111} planes. The diffratction pattern are also subject to the effects of multiple diffraction, film deformation, and an' effective aperture' in the optical behaviour of the electron microscape when used for selected area diffraction. Electron micrographs reveal a' film' that consists of discrete particles, three-dimensional in character, with irregular shapes suggestive of hemispheres of various sizes which have coalesced, side by side. The irregular shapes fit together like pieces of a jig-saw puzzle to cover the film area. Twinned components can be identified with dark field illumination. The three-dimensional particles seem to be a consequence of isotropic growth of crystal nuclei that are in epitaxial orientation but distributed at random over the substrate surface. Microtwinning appears to be a relief mechanism for reduction of the strain energies which accumulate at the boundaries between growing crystal nuclei.