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The response of epitaxial CoFe2O4 thin films to biaxial compressive stress imposed by MgAl2O4 and SrTiO3 single crystalline substrates is studied using X‐ray diffraction and Raman spectroscopy. It is found that the Poisson ratio ν signals a non‐auxetic behavior and depends on the substrate used. The Raman modes show an increase in frequency when increasing compressive strain by reducing film thickness; this is due to the shrinking of the unit cell volume. Such behavior is in qualitative agreement with recent ab initio calculations, although the measured values are significantly smaller than predictions. In contrast, the measured Poisson ratio is found to be in good agreement with expectations based on general arguments of atomic packing density. Possible guidelines for searching auxetic response in materials with spinel structure are discussed.

In this article, we combine diffuse x-ray scattering with a Monte Carlo simulation method for the determination of the dislocation density in thin heteroepitaxial layers. As a model, we consider GaN epitaxial layers containing threading dislocations perpendicular to the surface. The densities of particular types of threading dislocations following from the comparison of measured and simulated distributions of diffusely scattered x-ray intensity are compared with the dislocation densities determined by etching. A good agreement was found.

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