Strain-controlled oxygen content and the cationic electronegativity in LaBaCo2O5.5+δ thin films

LaBaCo2O5.5+δ (LBCO) films were grown on (001)-oriented SrTiO3 (STO), (LaAlO3)0.3(SrAlTaO6)0.7 (LSAT), and LaAlO3 (LAO) substrates by a polymer assisted deposition method. The effects of strain induced by lattice mismatch on magnetic and electrical properties of LBCO films were studied. It is found that both the conductivity and ferromagnetic (FM) transition temperature of LBCO films show an abnormally ascending order of STO < LAO < LSAT. It is suggested that there are three main factors essentially to affect the physical properties of LBCO films, i.e., the oxygen content, cationic ordering, and cationic electronegativity. The tensile strain induced-decrease of oxygen content and lowest nanoscale ordering of cations will result in the decrease of FM interactions and conductivity and the increase of antiferromagnetic interactions. Besides, the difference of the cationic electronegativities can lead to the different shrinkages of Co3+–O and Co4+–O bonds when strain is applied, especially under compressive strain, resulting in the changes in the cobalt spin states and a charge transfer between Co3+ and Co4+ ions, altering the physical properties of LBCO films. The combination and competition of the functions of the oxygen content, cationic ordering, and the cationic electronegativity lead to an abnormal change of the conductivity and magnetization for LBCO films with substrates. This provides a new way to understand the function of electronegativity in the strain-controlled physical properties of the films.