Simple polymer assisted deposition and strain-induced ferromagnetism of LaCoO3 epitaxial thin films

LaCoO3 epitaxial films with different thicknesses (~ 20, 50 and 80 nm) were grown on (001) SrTiO3 substrates by a simple polymer assisted deposition method. X-ray diffraction analyses including θ/2θ symmetric scan, ω-scan and in-plane φ-scan indicate that single-phase (001) oriented LaCoO3 films with a pseudotetragonal structure were grown on (001) SrTiO3 substrates successfully, with a biaxial tensile strain from + 2.42% to + 2.60% and tetragonal distortion from 1.47% to 1.63%. Due to the lattice relaxation effect in epitaxial thin film, the biaxial tensile strain is slightly relaxed when the thickness of the LaCoO3 film increases, resulting in an increase of the c-axis constant in contrast to a decrease of the in-plane constants of the film. It is different from LaCoO3 bulk with a nonmagnetic ground state that all the epitaxial films exhibit a ferromagnetic transition at TC ~ 85 K. Combining with the structural and magnetic analyses, it is shown that the strain-induced ferromagnetism in LaCoO3 epitaxial films, corresponding to the higher spin states, origins from the decrease of the energy difference between eg and t2g levels, which is caused by an increase of the unit-cell volume and suppression of the CoO6 octahedral rotations. In addition, the change of FC curve with the thickness of the film reveals that the ferromagnetism is enhanced by the thickness decrease of LaCoO3 film due to the increase of the biaxial tensile strain.