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Deleterious effect of rare earth elements substitution on the auxetic behavior of CoFe₂O₄ thin films

CoFe_2−xGd_xO4 (x = 0, 0.2) thin films of about 70 nm thickness were epitaxially grown on (001) MgO substrates using pulsed laser deposition and a controlled O₂/N2 deposition pressure within the 0.01–1 mbar range. Using Resonant elastic X-Ray scattering (REXS) experiments, we bring the proof that the Gd cations are inserted in the spinel cell. As expected, they mostly occupy the octahedral sites and drive the Co cations towards the tetrahedral ones. However, they also surprisingly partly occupy the tetrahedral sites. Despite the increase of the amount of Co in the tetrahedral sites, the insertion of the rare earth element, globally inflating the volume of the cell, precludes the occurrence of the auxetic behavior observed in the undoped samples. The out-of-plane cell parameters indeed never reach values lower than those of the in-plane parameters, fully strained by the MgO substrate. This study brings a new light on the parameters controlling the auxetic behavior in CFO thin films and demonstrates that the location of the Co cations in the tetrahedral sites is not a sufficient condition.