Non-auxetic/auxetic transitions inducing modifications of the magnetic anisotropy in CoFe2O4 thin films

An auxetic behaviour is evidenced in CoFe2O4 thin films grown by pulsed laser deposition on (100) MgO substrates under various O2/N2 pressures. This rare behaviour for an intrinsic material is observed for intermediate oxidation conditions, in between two non auxetic domains delimited by high (>0.05 mbar) and low (<0.03 mbar) O2/N2 deposition pressures. Combining X-ray resonant diffraction and Mössbauer spectroscopy, we experimentally prove that the auxetic behaviour is related to the presence of cobalt ions in the tetrahedral sites. The impact of the structural modifications caused by the various oxidation conditions on the electronic and magnetic properties are studied for the various oxidation domains. Variations as important as a transition from a p-type semiconducting to an insulating behaviour, or from an in-plane to an out-of-plane magnetization are observed when spanning from the lowest (0.01 mbar) to the highest (1 mbar) studied oxidation pressures.