Structure Of Manganites Research Articles

Effects of pressure on electron transport and atomic structure of manganites: Low to high pressure regimes

The pressure dependence of the resistivity and structure of La0.60Y0.07Ca0.33MnO3 has been explored in the pressure range from 1 atm to ~7 GPa. The metal to insulator transition temperature (TMI) was found to reach a maximum and the resistivity achieves a minimum at ~3.8 GPa. Beyond this pressure, TMI is reduced with a concomitant increase in the resistivity. Structural measurements at room temperature show that at low pressure (below 2 GPa) the Mn-O bond lengths are compressed. Between ~2 and ~4 GPa, a pressure induced enhancement of the Jahn-Teller (JT) distortion occurs in parallel with an increase in Mn-O1-Mn bond angle to ~180 (degree). Above ~4 GPa, the Mn-O1-Mn bond angle is reduced while the JT distortion appears to remain unchanged. The resistivity above TMI is well modeled by variable range hopping. The pressure dependence of the localization length follows the behavior of TMI.

Chemical H insertion into a low microtwinned EMD at temperatures close to 0 °C

Hydrogen has been inserted chemically by a new method at temperatures close to 0 °C into a low microtwinned EMD made by a suspension bath process. 33 Compounds covering the range 0 ≤ s ≤ 0.85 where s is the value in MnO1.97Hs were prepared. XRD patterns were line rich and showed that the insertion took place in three distinct stages; homogeneously between s = 0 and s = 0.28, then heterogeneously to s = 0.66 and finally homogeneously again for s ≥ 0.66. In the first homogeneous region the intergrowth structure was hinged ramsdellite–hinged pyrolusite: the inserted H was invisible to FTIR and must have been present as a sort of proton gas. The structural feature associated with this phenomenon was the near rectangular shape of the 2 × 1 tunnels of the preponderant hinged ramsdellite structure. The microtwinning was sufficient to restrict apical expansion of the ramsdellite octahedra to less than 2% and to prevent significant change of hinging angle during the first homogeneous region. In the second homogeneous region of H-insertion the intergrowth structure was groutite–hinged manganite. FTIR revealed that inserted H was present as O–H⋯O bonds in both parts of the intergrowth. The associated structural feature was the near parallelogram shape of the 2 × 1 tunnels of the preponderant groutite structure. In the central region, 0.28 ≤ s ≤ 0.66, H-insertion caused transformation of the end hinged ramsdellite–hinged pyrolusite structure (s = 0.28) into the initial groutite–hinged manganite structure (s = 0.66). Demicrotwinning must have accompanied the substantial expansion of apical distance and the major reduction in hinging angle required for this transformation. Throughout the heterogeneous region only the two phases represented by s = 0.28 and s = 0.66 were present.

Elastic interactions and superstructures in manganites

The role of elastic interactions between Jahn–Teller ions in the formation of various orbital- and charge-ordered structures in manganites and related compounds is analyzed. It is shown that such interactions alone are often sufficient to reproduce the structures observed in different regions of the phase diagram. A special attention is focused on stripe structures at high doping levels.

Observation of magnetic domain structure in phase-separated manganites by lorentz electron microscopy.

Magnetic domain structure in manganites was investigated by Lorentz electron microscopy, in order to understand some unusual physical properties, such as a colossal magnetoresistance (CMR) effect and a metal-to-insulator (MI) transition. In particular, we examined the spatial distribution of the charge/orbital ordered (CO/OO) insulator state and the ferromagnetic (FM) metallic state in phase-separated manganites, (La5/8-xPrx)Ca3/8MnO3 for x = 0.375, by obtaining both the dark-field and Lorentz images. We found an unusual coexistence of the CO/OO and FM metallic states with micrometer size below a MI transition temperature of 60 K. Our experimental findings provide direct evidence of the phase separation found in CMR manganites.

Superexchange interaction in insulating manganitesR1−xAxMnO3(x=0,0.5)

Manganites with general formulas $R{\mathrm{MnO}}_{3}$ and ${R}_{0.5}{A}_{0.5}{\mathrm{MnO}}_{3},$ where ${R}^{3+}$ is a rare-earth ion, and ${A}^{2+}$ is an alkaline-earth ion, are known as insulating pure or charge ordered compounds at low temperatures with some R and A. In the present work, it is shown how the charge ordering and crystal structure form the orbital structure, which strongly effects the superexchange interaction. An explicit space group and the magnitudes of the distortions of each crystal were used in the calculations. We assume an ideal insulating state of the crystals. The model presented in this work allows one to explain the $\mathrm{CE}$ and A magnetic structures in terms of charge and orbital structures using quantitative estimations of the exchange parameters. The model also allows one to find easy axis and noncollinear components of a multisublattice magnetic structure of manganites.

X-ray study of Nd0.5Sr0.5MnO3 manganite structure above and below the ferromagnetic metal–antiferromagnetic insulator spontaneous phase transition

The crystal structure of the manganite Nd0.5Sr0.5MnO3 is studied at temperature T=300 and 77.3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and P21/m, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.

Crystal and magnetic structure of Pr/sub 0.7/Sr/sub 0.3-x/MnO/sub 3/ (0≤x≤0.2) compounds

The electronic and magnetic properties of the ABO/sub 3/ type manganites (A=rare earth, B=Mn) depend sensitively on the ratio of divalent and trivalent atoms on the A sublattice. The substitution of rare-earth by Ca or Sr not only affects the total valence, but also the lattice spacing, which, in turn may affect the structure and magnetic ordering temperature. If these compounds are synthesized with less than stoichiometric amounts of the A site constituents, (relative to the Mn content), three possible outcomes may be considered: (a) single phase materials with vacancies on the A sublattice may be formed, (b) two phase materials with stoichiometric manganites and Mn/sub 3/O/sub 4/ may result, or (c) some combination of A and B may occur. We have carried out neutron diffraction (ND) studies on samples of Pr/sub 0.7/Sr/sub 0.3-x/MnO/sub 3/ with nominal x=0.0, 0.1 and 0.2, at 300 K and 10 K. The ND data clearly show the presence of Mn/sub 3/O/sub 4/ as a second phase in samples with x=0.1 and 0.2. Refinements of the manganite structure, based on the assumption that the ratio of Pr to Sr on the A site is equal to the nominal concentration, reveal that vacancies can be induced on the A site. In the x=0.2 sample, about 5.5% of the A sites are found to be vacant. The refined Mn moments in the z=0.0 and 0.1 samples at 10 K are found to be nearly equal (3.4 /spl mu//sub B/). The Pr moments are parallel to the Mn moments With magnitude of 0.27 /spl mu//sub B/ and 0.33 /spl mu//sub B/, respectively, in these two samples. The magnetic structure of x=0.2 sample is more complex due to the presence of an antiferromagnetic component.

Magnetic, charge and orbital orderings in manganites

We develop a theory to understand various types of electronic collective behaviors in manganites, such as magnetic, charge, and orbital orderings. Starting from an electronic model with strong correlations, we derive an effective Hamiltonian by utilizing the projection perturbation techniques, in which the orbital degeneracy of e g electrons in Mn ions are taken into account. Interplay of spin, charge and orbital degree of freedom leads to different magnetic and electronic structures in manganites.

Ferromagnetic ordering temperature and internal pressure in manganites La0.7Ca0.3−xSrxMnO3

Lanthanum manganites La0.7Ca0.3−xSrxMnO3 were synthesized and investigated. The jump of Tc at the magnetic ordering temperature dependence on concentration at x=0.15 was revealed. The structural phase transition from the orthorhombic phase (x<0.15) to the rhombohedral one (x≥0.15) corresponds to that concentration. Substitution of Ca2+ ion by Sr2+ leads to the increasing of the average ionic radius in A position (position of La, Ca, Sr) and plays the role of internal pressure. The revealed temperature Tc jump shows that the effect of internal pressure depends on the crystal structure of manganites.

Topological scenario for stripe formation in manganese oxides

The spin-charge-orbital complex structures of manganites are studied using topological concepts. The key quantity is the "winding number" w associated with the Berry-phase connection of an e(g) electron parallel transported through Jahn-Teller centers, along zigzag one-dimensional paths in an antiferromagnetic environment of t(2g) spins. From these concepts, it is shown that the "bi-stripe" and "Wigner-crystal" states observed experimentally have different w's. Predictions for the spin structure of the charge-ordered states for heavily doped manganites are discussed.

Raman phonons in La2?2xSr1+2xMn2O7 layered manganites

Phonons of polycrystalline La{sub 2-2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} (0.38 x 1.0) ceramic samples were studied at room temperature using Raman spectroscopy. The spectra are consistent with the group theory predictions for a body-centered tetragonal crystal structure (D{sup 17}{sub 4h}). Since no appreciable changes were observed in the Raman spectra for different extents of La doping, this suggests that the layered manganite structure is less sensitive to dopants than that of the related three-dimensional perovskites.

The relation between the electronic and magnetic structures of manganites in the tight-binding approximation

Analytic expressions for the dispersion curves E(k) of RMnO3 (R=La, Pr, Nd, Sm, and other RE elements) have been obtained in the tight-binding approximation for the main types of magnetic ordering on the Mn sublattice. The first calculation of E(k) taking into account the oxygen subsystem and mutual ordering of the manganese and RE sublattices is reported. The results obtained permit a qualitative interpretation of some features observed in the behavior of the rare-earth manganites.

Electronic structure in manganites

We examine the electronic structure in manganites and their anomalous properties by taking into account the Hund coupling, the degeneracy of e g orbitals in Mn ions and the electron-electron interaction. The effective Hamiltonian is derived, in which charge, spin and orbital degrees of freedom are included on an equal footing. The charge dynamics prefer the uniform orbital alignment, whereas the interaction between orbitals induces the alternate orbital ordering. As a result, the competing effects among those degrees of freedom cause unique electronic structure in manganites. We discuss the anisotropic antiferromagnetic order, anomalous magnetization process and dimensional cross-over under pressure observed in a variety of manganites.

Effective Hamiltonian in manganites:mStudy of the orbital and spin structures

In order to study nature of the electronic structures in manganites, the effective Hamiltonian is derived by taking the degeneracy of the ${\mathrm{e}}_{\mathrm{g}}$ orbitals and the strong electron correlation into account. The spin and orbital ordered phases in the insulating states are studied in the mean-field approximation applied to the effective Hamiltonian. It is shown that the A-type antiferromagnetic state is stabilized by the magnetic interaction, which strongly depends on the bond direction, originated from the orbital ordering and the electron-electron correlation. The spin and orbital excitations are also studied by utilizing the effective Hamiltonian. The spin excitations agree well with the recent results in the neutron-scattering experiments in ${\mathrm{LaMnO}}_{3}$. Although the numerical calculation is devoted to the insulating state, the effective Hamiltonian is relevant to the metallic state as well.

Effect of temperature on the structure of manganites

The effect of temperature on the structure of some “normal” manganites has been studied by high temperature X-ray diffraction and by differential thermal analysis. At rcom temperature they are isomorphous with hausmannite, with c/a > 1. On heating to elevated temperatures, the c,a, and c/a parameters are found to remain constant until just before a critical transformation temperature whereupon c/a falls abruptly to unity, coinciding with the structure transforming from tetragonal to cubic. This abrupt transformation is reminiscent of a first order process. The structure of the tetragonal forms at various temperatures, and the cubic form just after transition, have been determined by X-ray diffraction. The theoretical model for the variation of distortion with temperature has been discussed, and a correlation between the transition temperature and the cell distortion has been shown. An anomalous behaviour has been observed for magnesium manganite because of its tendency to change its cation distribution and alter, on heating, from a “normal” to a “random” structure. The site preference energy for the cations in MgMn 2O 4 has been calculated from the change in cation distribution with temperature. The D.T.A. data support the transition temperatures of various manganites as obtained by X-ray and also lead to the determination of the entropy of transformation.

The Symmetry and Crystal Structure of Manganite, Mn (OH)O

The Symmetry and Crystal Structure of Manganite, Mn (OH)O