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.

Strain-relaxation and critical thickness of epitaxial La1.85Sr0.15CuO4 films

“…A linear trend for films under in-plane compressive strain and tensile strain up to ∼0.25% is observed, the slope of which, ν * , is equivalent to −2ν/(1 − ν), where ν is the Poisson ratio. 23 We determined a ν ∼ 0.32, which is in good agreement with the value of ν = 0.3 obtained for bulk LSCO and other layered oxides. 24,25 The linearity that we observe for the less strained (thicker) films on LSAT and STO further supports that the variation in the unit cell size from the bulk value in Fig.…”

Pillar shape modulation in epitaxial BiFeO3–CoFe2O4 vertical nanocomposite films

“…The variation in unit cell volume of CFO with thickness agrees with a prior report. 15 Other studies have found an in-plane easy axis when the nanocomposite was thin. 16 Shape modified BFO-CFO nanocomposites were then fabricated consisting of long and short CFO pillars grown sequentially in order to tune the overall magnetic hysteresis.…”

Negative Poisson’s ratios in metal nanoplates

“…W hen stretched, materials with a negative Poisson's ratio become thicker in the direction perpendicular to the original force [1][2][3][4][5][6][7][8][9][10][11] . Materials exhibiting such counterintuitive behaviour, known as auxetics, are of great interest not only because they are rare, but also because of their numerous potential applications in various fields, such as the design of fasteners 12 , prostheses 13 , pizeocomposites 14,15 , filters 16 , earphones 17 , seat cushions 18,19 and superior dampers 20 .…”