A-site Ordering versus Electronic Inhomogeneity in CMR-Manganite Films
V. Moshnyaga, L. Sudheendra, O. I. Lebedev, K. Gehrke, O. Shapoval, A. Belenchuk, S. A. Koester, B. Damaschke, G. van Tendeloo, K. Samwer
Abstract
Epitaxial La3/4Ca1/4MnO3/MgO(100) (LCMO) thin films show unusual rhombohedral (R-3c) structure with a new perovskite superstructure due to unique ordering of La and Ca at the A-site positions. Very sharp insulator-metal and para-ferromagnetic phase transitions at temperatures up to TMI ~ TC=295 K were observed. The ordered films were electronically homogeneous down to 1 nm scale as revealed by scanning tunnelling microscopy/spectroscopy. In contrast, orthorhombic and A-site disordered LCMO demonstrate broadened phase transitions as well as mesoscopic phase separation for T<<TC. The unique La/Ca ordering suppresses cation mismatch stress within one super-cell, a~1.55 nm, enhancing electronic homogeneity. Phase separation scenario seems not to be a unique mechanism for CMR as very large CMR=500 % was also observed in A-site ordered films.
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