The Poisson Ratio in CoFe₂O₄ Spinel Thin Films

Abstract 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.