Striping of orbital-order with charge-disorder in optimally doped manganites
Wei-Tin Chen1,2*, Chin-Wei Wang3, Ching-Chia Cheng1, Yu-Chun Chuang3, Arkadiy Simonov4, Nicholas C. Bristowe5, Mark S. Senn6
1Center for Condensed Matter Sciences and Center of Atomic Initiative for New Materials, National Taiwan University, Taipei, Taiwan
2Taiwan Consortium of Emergent Crystalline Materials, Ministry of Science and Technology, Taipei, Taiwan
3National Synchrotron Radiation Research Center, Hsinchu, Taiwan
4Materials Department, ETH Zürich, Zürich, Switzerland
5Centre for Materials Physics, Durham University, Durham, UK
6Department of Chemistry, University of Warwick, Coventry, UK
* Presenter:Wei-Tin Chen,
Novel functional materials design and discovery for magnetic, electronic, spintronic and energy technology applications stimulate much of modern chemistry, physics and materials sciences. Such functional materials are in particular interests due to their correlated electron systems ground states, and the sensitivity to changes in chemical composition and physical condition. These strongly-correlated materials tend to have dense and strongly-bonded structures, high pressure synthesis techniques therefore become one of the most important approaches in the novel materials exploration, and the high pressure condition may induce more interesting physical properties.

The phase diagrams of LaMnO3 perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions with specific doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic level understanding of the orbital ordered (OO) state at this doping level. Recently, another perovskite manganite compound CaMn7O12 was reported to be the Type-II multiferroic and exhibiting large magnetoelectric response. In order to have further understanding of the coupling between the crystal structure and physical properties, a series of A-site ordered quadruple perovskite AMn7O12 materials were prepared utilising high-pressure high-temperature synthesis techniques. Structural analysis with synchrotron x-ray and neutron diffraction techniques were performed and rich spin, charge and orbital couplings were observed in these materials. Distinct behaviours of the analogue AMn7O12 suggest that the phenomena of orbital and magnetic ordering and their intrinsic coupling are very sensitive to the A oxidation state and consequently B-cation valence. At specific doping ratio, the charge disordered Mn3.5+ with Mn3+ provides mechanism that long range orbital ordering can melt, resulting a conducting state. With detailed crystallographic analysis, the optimum doped CMR manganite mechanism can be well-addressed with 134 quadruple perovskite system.

Keywords: High pressure synthesis, orbital ordering, crystal structure, synchrotron x-ray powder diffraction, neutron powder diffraction