Thickness dependent strain effect on ferromagnetic insulating properties of LaCoO3 thin films

Epitaxial tensile-strain is an important factor to determine the superior ferromagnetic insulating property of LaCoO3 (LCO) epitaxial films. In this paper, the whole development of LCO ferromagnetic insulating properties modulated by thickness-dependent strain effect were studied and we found that the film strain decreased and the ferromagnetic property weakened with increasing film thickness. The measurements of x-ray magnetic circular dichroism confirm that the spin moment of Co3+ ions is significantly larger than its orbital magnetic moment and it also reduces with increasing thickness. The x-ray absorption spectroscopy revealed that the increasing film thickness is unfavorable for the presence of eg electrons, as well as for the intermediate-spin and high-spin states. Overall, the enhancement of film thickness leads to the decrease of the Co-O bond length and the increase of crystal field splitting energy, unfavorable for the electron transfer from t2g to eg orbitals, and in turn for the stability of intermediate-spin and high-spin states. The electronic transport behavior of LCO film was proved to be a good insulating properties and the temperature dependence of the resistivity is consistent with the 3D-Mott's variable range hopping model. Our results not only demonstrated the feasibility of thickness-induced strain modulating the properties of LCO films, but also strengthen the understanding of the ferromagnetic insulating mechanism in LCO films.