Charge Ordering In Half-doped Manganites Research Articles

Optical Study of Nanosize Effects on Charge Ordering in Half-Doped Manganites

Optical spectra are very sensitive to investigate strong charge correlations in transition-metal oxides. Recent studies on nanosized half-doped manganites frequently reported that the robust charge...

Ordered Phases and Magnetic Excitations in Doped Manganites

We present theoretical models concerning magnetic and charge ordering in half-doped manganites. The new approach to microscopic description of this group of compounds, taking into consideration the oxygen degrees of freedom, is discussed. The spin wave dispersion relations are shown both in the model of efiective spins on Mn ions and in the new approach when two Mn ions form a large efiective spin S = 7=2.

Charge ordering in half-doped manganites

Alternative descriptions for the ground state structure of the half-doped manganite ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ have been tested against high-resolution $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ x-ray and neutron powder diffraction data. The best fit is obtained using a $P{2}_{1}∕m$ symmetry model, which supports the striped charge and orbital ordering picture. The magnitude of the charge ordering is, however, only 25% of the ideal ${\mathrm{Mn}}^{3+}∕{\mathrm{Mn}}^{4+}$ separation. An alternative $Pnm{2}_{1}$ pseudosymmetry, used to describe Zener polaron ordering in ${\mathrm{Pr}}_{0.6}{\mathrm{Ca}}_{0.4}\mathrm{Mn}{\mathrm{O}}_{3}$ [Phys. Rev. Lett. 89, 097205 (2002)], gives a different, bistriped, charge and orbitally ordered model that does not fit the data as well.

Dimensional crossover and charge order in half-doped manganites and cobaltites.

We propose a generic model for understanding the effect of quenched disorder on charge-ordering in half-doped manganese and cobalt oxides with different crystal structures. Current experimental observations are explained in the light of the global phase diagram of the model.

A new interpretation of the CO state in half-doped manganites: new results from neutron diffraction and synchrotron radiation experiments

In the R 1− x D x MnO 3 ( x ∼0.5) manganites, the structural phase transition at T CO is commonly interpreted as a concomitant charge and orbital ordering (CO/OO) process driven by a co-operative Jahn–Teller effect and Coulomb repulsion forces. The low-temperature phase is supposed to contain well-separated and ordered Mn 3+ and Mn 4+ ionic species in an NaCl-like pattern. Structure refinement, from a neutron diffraction experiment below T CO on a Pr 0.6 Ca 0.4 MnO 3 single crystal, gives us a model for the displacement of atoms with respect to the high-temperature phase that invalidates the standard model based in the CO/OO picture. Our result is a non-centrosymmetric crystal structure with two non-equivalent MnO 6 octahedra, both being slightly elongated but displaying very similar average Mn–O distances (1.96 and 1.95 Å, respectively) and having off-centered Mn atoms. We argue that this is a proof of the absence of charge ordering in half-doped manganites in the sense of formation of separated Mn 3+ and Mn 4+ ionic species. A new qualitative interpretation of the CE-type spin ordering (SO) is proposed. The so-called CO transition is, in fact, a structural transition induced by the change in the mean free path of electrons that continue to be thermally activated below T CO by forming ferromagnetic Mn–Mn pairs stabilized by a local double-exchange process. The CE SO pattern results from the ordering of these pairs formed at T CO . High-resolution synchrotron powder diffraction shows a complex anisotropic/asymmetric strains appearing at the transition that can be phenomenologically fitted by additional phases. Complementary electron diffraction and microscopy have shown no trace of macroscopic phase separation.

Effective Hamiltonian for an extended Kondo-lattice model and a possible origin of charge ordering in half-doped manganites

An effective Hamiltonian is derived in the case of the strong Hund coupling and on-site Coulomb interaction by means of a projective perturbation approach. A physical mechanism for charge ordering in half-doped manganites ${(R}_{0.5}{X}_{0.5}{\mathrm{MnO}}_{3})$ is proposed. The virtual process of electron hopping results in antiferromagnetic superexchange and a repulsive interaction, which may drive electrons to form a Wigner lattice. The phase diagram of the ground state of the model is presented at half doping. In the case of formation of a Wigner lattice, we prove that spins of electrons are aligned ferromagnetically as well as that the localized spin background is antiferromagnetic. The influence of the on-site Coulomb interaction is also discussed.