Direct Observation of Oxygen Superstructures in Manganites

Abstract

We report the observation of superstructures associated with the oxygen 2p-states in two prototypical manganites using x-ray diffraction at the oxygen K-edge. We determine the nature of the orderings and discuss our picture with respect to novel theoretical models. In the stripe order system Bi0.31Ca0.69MnO3, hole-doped O states are found to be orbitally ordered, at the same propagation vector as the Mn orbital ordering, but no evidence is found to support a picture of oxygen charge stripes at this periodicity. In La 7 8 Sr 1 8 MnO3, we observe a 2p charge ordering described by alternating hole-poor and hole-rich MnO planes that is consistent with recent predictions.Fascinating macroscopic properties may emerge from the electronic and magnetic orderings that occur in 3d metal oxides doped with charge carriers. In doped cuprates, charge and spin stripes could be relevant to high-T C superconductivity [1], and the colossal magnetoresistance of doped manganites is fundamentally related to the stability, in a magnetic field, of an ordering of polarons together with an ordering of charge, orbital and magnetic moments on the Mn atoms [2]. The relationship between the orderings and the transport properties has been adressed both theoretically and experimentally concentrating on the 3d metal atoms, often neglecting the ligands' degrees of freedom. However, because the active states are not ionic 3d metal orbitals, but, rather hybridized metal 3d -oxygen 2p states, an ordering of the oxygens may be related to the ordering of the metals. In cuprates, the role of the oxygen atom is now always considered and, taking a recent example, the ordering of hole-doped oxygens was directly observed and discussed in the context of the stripes [3].In manganites, an electronic ordering on oxygens has not been detected yet although recent experiments suggested a crucial role of the O atoms in the polaronic, electronic, and magnetic orderings of the Mn atoms. A relation between conductivity and the O electronic configuration was identified [4]; spectroscopic studies indicated a small charge disproportionation on Mn atoms instead of an ionic charge ordering (Mn 4+ /Mn 3+ ) [5], and a crystallographic study proposed that the oxygen atom bridges an electron between two Mn atoms of nearly the same valence, coupling them magnetically [6]. At the heart of these results is the explicit role of the 2p − 3d hybridization in determining the magnetic exchange couplings and conductivity, the traditional picture being the one where the atomic 3d orbitals define exchange magnetic pathways between Mn atoms, the oxygen 2p states being filled.Meanwhile, new theoretical models beyond the Mncentered description also suggested that oxygen superstructures play an important role in determining the ground states. Ferrari et al. [7] explained the insulating behavior of the charge/orbitally orded half-doped system by an ordering of O holes -"oxygen stripes" of varying valences (incidentally, having the same periodicity as the Mn orbital ordering). Efremov et al...

Hard x-ray probe to study doping-dependent electron redistribution and strong covalency inLa1−xSr1+xMnOHerrero‐Martín1, Mirone2, Fernández-Rodríguez3 et al. 2010Phys. Rev. B284211View full textAdd to dashboardCite

“…Taken together the above discussion shows that the doping induced changes to the electronic structure involve both the oxygen 2p and the Mn 3d bands, which both play an active role, and contrast with the classical ionic model perspective as previously stated [6,8,30,31].…”

“…However, recent X-ray absorption spectroscopy (XAS) and soft resonant X-ray scattering (RXS) studies have remarked the prominent role played by O 2p states in these materials as well [6][7][8].…”

Anisotropy, orbital order, and colossal electroresistance in untwinnedLa0.9Sr0.1MnO3single crystals

“…But also homogeneous pictures of the low-temperature phases have been reported. 7,8 In summary, the different phases of the compounds in the range 0.1Յ x Յ 0175 are still under debate.…”

The effect of strain and strain symmetry on the charge-order transition in Bi0.4Ca0.6MnO3films

“…At x 4 0.5, X-ray diffraction at the oxygen K edge reveals orbital ordering of hole-doped, hybridized 2p-3d states [33], while phase separation into ferromagnetic and AF domains manifests itself as the presence of small polaron and charge-gap-like features observed in an optical reflectivity study [34]. Renner et al presented an intriguing visualization of phase separation and charge ordering in a lower-doped sample (x ¼ 0.24) in real space with atomic-scale resolution scanning probe microscopy data [35].…”