Thin Film Fabrication and Novel Electronic Phases

In this chapter, we review the recent research on the spin-state ordering in epitaxial thin films of perovskite LaCoO3 from the viewpoint of optical spectroscopy and X-ray scattering. A remarkable feature is that a variety of spin/orbital orderings, which have not been identified in bulk samples, can be induced by tuning the epitaxial strain in thin film samples. In the moderately tensile-strained films, a spin-state/orbital ordering accompanying the ordering of high-spin-state site and low-spin-state site occurs around 130 K, and subsequently, a ferromagnetic/ferrimagnetic ordering emerges around 90 K. In the weakly strained films, another spin-state/orbital ordered phase with longer modulation period emerges around 40 K, followed by a ferromagnetic ordering transition around 20 K. By analyzing the Co-3d orbital state by the resonant soft X-ray scattering technique, we identified that two kinds of high-spin state with different site symmetries emerges in this phase. Moreover, by means of grazing-incidence resonant soft X-ray scattering technique, it is demonstrated that the transition temperature of spin-state ordering is higher in the surface-layer of film than inside the film due to the strain relaxation. These results demonstrate that the strain engineering of epitaxial thin films offers a fertile playground to study the spin-state/orbital ordering in the correlated spin-crossover material.