Nature of ferromagnetic ordered state in LaCoO3 epitaxial nano-thin film on LaAlO3 substrate

LaCoO3 (LCO) epitaxial nano-thin films with different thickness (∼30 nm, 60 nm, and 80 nm) were grown on (1 0 0), (1 1 0), and (1 1 1) oriented LaAlO3 (LAO) substrates, respectively, by a simple polymer assisted deposition method. X-ray diffraction analyses confirm that all the LCO films are epitaxially grown in accordance with the orientation of LAO substrates, with biaxial compressive strain in the ab plane. Due to the different strain directions, the Jahn-Teller (JT)-like tetragonal distortion of CoO6 octahedron is dominant in LCO film on (1 0 0) LAO, but the rotation of CoO6 octahedron is dominant in LCO film on (1 1 1) LAO, while it is intervenient between them in LCO film on (1 1 0) LAO. For the same oriented films, as a result of the lattice relaxation effect, the strain is slightly relaxed as the film thickness increasing. An obvious ferromagnetic (FM) transition at TC ∼ 85 K is observed in all the (1 0 0) oriented LCO films, but a very weak FM signal in (1 1 0) oriented LCO films and a nonmagnet-like characteristic in (1 1 1) oriented LCO films below ∼85 K. Combined with the structural analyses, it is found that the change of the FM signal is closely related to the stain-induced JT tetragonal distortion of CoO6 octahedron which can stabilize Co3+ ions in the intermediate spin (IS) state. The higher degree of the JT tetragonal distortion of CoO6 octahedron corresponds to the higher field-cooled (FC) magnetization. However, the rotation mode of CoO6 octahedron will weaken the degree of JT tetragonal distortion and reduce the population of IS Co3+.