Long range electronic phase separation in CaFe3O5

Ka H Hong,J Paul Attfield,Angel M Arevalo-Lopez,Clemens Ritter,James Cumby

doi:10.1038/s41467-018-05363-6

Ka H Hong, J Paul Attfield + Show 3 more

Open Access

https://doi.org/10.1038/s41467-018-05363-6

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Abstract

Incomplete transformations from ferromagnetic to charge ordered states in manganite perovskites lead to phase-separated microstructures showing colossal magnetoresistances. However, it is unclear whether electronic matter can show spontaneous separation into multiple phases distinct from the high temperature state. Here we show that paramagnetic CaFe3O5 undergoes separation into two phases with different electronic and spin orders below their joint magnetic transition at 302 K. One phase is charge, orbital and trimeron ordered similar to the ground state of magnetite, Fe3O4, while the other has Fe2+/Fe3+charge averaging. Lattice symmetry is unchanged but differing strains from the electronic orders probably drive the phase separation. Complex low symmetry materials like CaFe3O5 where charge can be redistributed between distinct cation sites offer possibilities for the generation and control of electronic phase separated nanostructures.

Highlights

Manganites such as La0.7Ca0.3MnO3 and magnetite, Fe3O4, share similar physics as both have a spinpolarised conducting state near ambient temperature due to double exchange between ferromagnetically aligned Mn3+ and Mn4+ or Fe2+ and Fe3+ spins, respectively
A single high temperature (HT) crystalline phase of CaFe3O5 is observed above TM = 302 K, but both powder synchrotron X-ray (Fig. 2a, b) and neutron diffraction data (Fig. 2c, d) reveal long range phase separation as diffraction peaks broaden or split into two components below the magnetic ordering transition as shown in Fig. 2a, c
Polycrystalline CaFe3O5 was prepared from stoichiometric quantities of CaFe2O4, Fe2O3, and Fe powders pressed into pellets, sealed in evacuated quartz tubes, and heated at 1100 °C for 12 h. (The CaFe2O4 was synthesised at ambient pressure using the ceramic technique outlined by Wan et al23. where CaCO3 and Fe2O3 powders were ground together in 1:1 ratio, pressed into pellets, heated at 850 °C for 4 h, reground and repelleted, and reheated at 1100 °C for 12 h.) Thermogravimetric analysis heating the sample in air at 10 °C min−1 to 900 °C, as shown in Supplementary Fig. 3, gave a mass increase of 2.789%, in agreement with the calculated value of 2.781% for oxidation of CaFe3O5

Summary

Introduction

Manganites such as La0.7Ca0.3MnO3 and magnetite, Fe3O4, share similar physics as both have a spinpolarised conducting state near ambient temperature due to double exchange between ferromagnetically aligned Mn3+ and Mn4+ or Fe2+ and Fe3+ spins, respectively. Both undergo charge ordering on cooling, which for magnetite is accompanied by a lattice distortion below the much-studied Verwey transition at 125 K1. This was recently found to result from a complex ordering of Fe2+/Fe3+ charge states, Fe2+ orbitals, and three-Fe trimeron groups[2].

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