Striping of orbital-order with charge-disorder in optimally doped manganites

Wei-Tin Chen^1,2*, 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, email:weitinchen@ntu.edu.tw

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 Mn^3.5+ 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