Ferromagnetism with strong magnetocrystalline anisotropy in A-site ordered perovskite YBaCo₂O₆ epitaxial thin films prepared: Via wet-chemical topotactic oxidation

A-site cation-ordered perovskite cobaltite, RBaCo₂O_x (R = rare earth element), exhibits fascinating physical properties, such as spin-state ordering and high oxygen conductivity, because of the large tetragonal distortion of the Co orbital. However, the distorted coordination geometry prefers oxygen vacancies, resulting in a difficulty in obtaining the stoichiometric phase (x = 6). For example, x in YBaCo₂O_x, which has largely distorted Co orbitals because of the small size of Y³⁺, has so far been limited to 5.52. To expand the available range of x, in this study, we performed a low-temperature topotactic oxidation of YBaCo₂O_5.3 epitaxial films using a strong oxidizing agent NaClO. The x value can be varied in a wide range of 5.3-6.0, maintaining the A-site cation-ordered perovskite structure, by changing the pH and temperature of NaClO. The single crystalline film with x = 6 exhibits large tetragonal distortion (c/a = 0.968) because of the small ionic radius of Y³⁺ and substrate-induced tensile strain. Unlike antiferromagnetic insulating YBaCo₂O_5.5, the fully oxidized film with x = 6 exhibits in-plane ferromagnetism and metallicity with a Curie temperature of 130 K possibly because of the double-exchange interaction between Co³⁺ and Co⁴⁺. Moreover, the YBaCo₂O₆ film exhibits huge magnetic anisotropy with a magnetic anisotropy constant of 1.5 × 10⁸ erg cm^-3, demonstrating that the A-site cation-ordered perovskite structure is promising for obtaining high magnetocrystalline anisotropic materials.