Spontaneous atomic ordering and magnetism in epitaxially stabilized double-perovskites
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Spontaneous atomic ordering and magnetism in epitaxially stabilized double-perovskites Akira Ohtomo1,2, Suvankar Chakraverty3, Hisanori Mashiko1, Takayoshi Oshima1 and Masashi Kawasaki4,3,5 1 Department of Applied Chemistry, Tokyo Institute of Technology, Tokyo 152-8552, Japan. 2 ALCA, Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan. 3 Correlated Electron Research Group (CERG) and Cross-Correlated Materials Research Group (CMRG), RIKEN Advanced Science Institute, Wako 351-0198, Japan. 4 Quantum-Phase Electronics Center and Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan. 5 CREST, Japan Science and Technology Agency (JST), Tokyo 102-0075, Japan. ABSTRACT We report on the atomic ordering of B-site transition-metals and magnetic properties in double-perovskite oxides, La2CrFeO6 (LCFO) and La2VMnO6 (LVMO), which have never been reported to exist in ordered forms. These double-perovskite oxides are particularly interesting because of possible ferromagnetism (expected from the Kanamori-Goodenough rule for LCFO) and half-metallic antiferromagnetism (predicted for LVMO). Using pulsed-laser deposition technique with single solid-solution targets, we have prepared epitaxial films in ordered forms. Despite similar ionic characters of constituent transition-metals in each compound, the maximum B-site order attained was surprisingly high, ~90% for LCFO and ~80% for LVMO, suggesting a significant role of epitaxial stabilization in the spontaneous ordering process. Magnetization and valence state characterizations revealed that the magnetic ground state of both compounds was coincidently ferrimagnetic with saturation magnetization of ~2PB per formula unit, unlike those predicted theoretically. In addition, they were found to be insulating with optical band-gaps of 1.6 eV and 0.9 eV for LCFO and LVMO, respectively. Our results present a wide opportunity to explore novel magnetic properties of binary transition-metal perovskites upon epitaxial stabilization of the ordered phase. INTRODUCTION Double-perovskite oxides are an intriguing class of materials exhibiting a number of exotic properties, including the high-Curie temperature (TC) ferrimagnetism and the halfmetallicity [1]. They are generally expressed as A2B′B″O6, where A is an alkaline- or rare-earth element and B′ and B″ are different transition-metal elements. Figure 1(a) shows a schematic structure of the double-perovskite, where the transition-metals occupy the B-site alternately along the [111] direction to form a rock-salt-type sublattice. When 3d and 4d or 5d transitionmetals are combined, a large difference in the formal valences (FV) and ionic radii (rB) permits spontaneous ordering of transition-metal ions, thus facile to synthesize in a bulk form. Representative examples are Sr2Fe3+Mo5+O6 [2], Sr2Fe3+Re5+O6 [3,4], and Sr2Cr3+Re5+O6 [5]. While a few 3d-3d combinations are known to form the ordered phase, such as La2Mn4+B″O6 (B″ = Fe2+, Ni2+, Co2+) [6,7]. In any cases, the differences in FV and rB have to be enough large
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