Half-Metallicity and Magnetic Anisotropy in Double-Perovskite GdBaCo₂O₆ Films Prepared via Topotactic Oxidation

Double-perovskite GdBaCo₂O_x is a promising material owing to its fascinating electronic and magnetic features, such as spin crossover, charge and spin-state ordering, magnetic-field or photo-induced antiferromagnetic-to-ferromagnetic phase transition, and large magnetoresistance (MR). These properties can be controlled by changing the oxygen content (x), and an increase of x is effective for stabilizing ferromagnetic order. However, a stoichiometric phase (x = 6) has not yet been obtained because the distorted coordination geometry prefers oxygen vacancies. In this study, we report the successful synthesis of GdBaCo₂O₆ epitaxial films via the topotactic oxidation of a film with x = 5.5. The transformation between x = 5.5 and 6 phases was reversible via low-temperature redox reactions. The x = 6 film exhibited ferromagnetic and metallic behavior below the Curie temperature (T_C) of 110 K and semiconducting behavior above T_C. The spontaneous magnetization of the x = 6 film was 4.8 μ_B/f.u., which is the largest in the double-perovskite cobaltite system owing to the ferromagnetic interaction between Co^3.5+ and Gd³⁺. It also exhibited strong magnetic anisotropy along the a axis (1.7 × 10⁷ erg/cm³) and anisotropic MR behavior associated with the Gd/Ba ordering along the c axis.