A-site ordering versus electronic inhomogeneity in colossally magnetoresistive manganite films

Epitaxial La3/4Ca1/4MnO3/MgO(100) (LCMO) thin film shows an unusual rhombohedral (R-3c) structure with a new perovskite superstructure at room temperature due to the CE-type ordering of La and Ca with modulation vector q=1/4[011]. A-site ordered film was found to be electronically homogeneous down to the 1 nm scale as revealed by scanning tunnelling microscopy/spectroscopy. In contrast, orthorhombic and A-site disordered LCMO demonstrate a mesoscopic phase separation far below the Curie temperature (T-C). Unique La/Ca ordering compensates the cation mismatch stress within one supercell, a(S)approximate to 1.55 nm, and enhances the electronic homogeneity. The phase separation does not seem to be a unique mechanism for the colossal magnetoresistance (CMR) as very large CMR approximate to 500% was also observed in A-site ordered films.