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Manipulating magnetic domains is essential for many technological applications. Recent breakthroughs in Antiferromagnetic Spintronics brought up novel concepts for electronic device development. Imaging antiferromagnetic domains is of key importance to this field. Unfortunately, some of the basic domain types, such as antiphase domains, cannot be imaged by conventional techniques. Herein, we present a new domain projection imaging technique based on the localization of domain boundaries by resonant magnetic diffraction of coherent X rays. Contrast arises from reduction of the scattered intensity at the domain boundaries due to destructive interference effects. We demonstrate this approach by imaging antiphase domains in a collinear antiferromagnet Fe2Mo3O8, and observe evidence of domain wall interaction with a structural defect. This technique does not involve any numerical algorithms. It is fast, sensitive, produces large-scale images in a single-exposure measurement, and is applicable to a variety of magnetic domain types., Imaging the antiferromagnetic (AFM) domains facilitates the understanding and design of AFM spintronics but is still challenging. Here the authors show an imaging approach for antiphase domains in AFM Fe2Mo3O8 by resonantly scattered coherent soft X-rays, which is also applicable to collinear antiferromagnets.

We report the observation of the de Haas-van Alphen effect in IrTe2 measured using torque magnetometry at low temperatures down to 0.4 K and in high magnetic fields up to 33T. IrTe2 undergoes a major structural transition around 283 K due to the formation of planes of Ir and Te dimers that cut diagonally through the lattice planes, with its electronic structure predicted to change significantly from a layered system with predominantly three-dimensional character to a tilted quasi-two dimensional Fermi surface. Quantum oscillations provide direct confirmation of this unusual tilted Fermi surface and also reveal very light quasiparticle masses (less than 1 me), with no significant enhancement due to electronic correlations. We find good agreement between the angular dependence of the observed and calculated de Haas-van Alphen frequencies, taking into account the contribution of different structural domains that form while cooling IrTe2., Latex, 4 pages, 3 figures

Impurity-helium solids are porous gel-like materials held together by Van der Waals forces. They consist of impurity atoms, molecules or clusters of atoms and molecules, each surrounded by very thin layers of solid helium. Impurities studied include neon, krypton, and molecular and atomic nitrogen, hydrogen and deuterium. The pore sizes and cluster sizes in impurity-helium solids are determined by ugtrasound attenuation and X-ray diffraction. The ESR technique is employed to study atomic impurities. The tunnelling exchange chemical reactions D+H2→HD+D and D+HD→D2+H are observed. Large concentrations of atomic hydrogen (∼8⋅1017 per cm3) are produced in these reactions.

The crystal and magnetic structure of the ordered double perovskite Sr2MnReO6 was investigated by neutron diffraction. Monoclinic (space group P 21/n) distortion of the parent cubic double perovskite structure was revealed, and the refined Mn?O?Re bond angles deviate significantly from 180?. The monoclinic distortion produces antisymmetric exchange interactions, leading to a canted magnetic structure, for which a possible model is proposed and refined. No structural transitions were observed upon cooling or in an external magnetic field. Complementary x-ray synchrotron diffraction data support the neutron diffraction findings.

A series of states with different densities of stripes of Ir dimers is investigated using x-ray diffraction and density functional theory in layered nonmagnetic metal $\mathrm{IrT}{\mathrm{e}}_{2}$. With decreasing temperature, structures with and without inversion symmetry alternate. In noncentrosymmetric states, spin-orbit coupling splits the electronic energy bands into spin-polarized pairs. Factors affecting the stability of the observed dimerized states are established, and it is conjectured that an infinite series of alternating states with and without polarized bands is realized in $\mathrm{IrT}{\mathrm{e}}_{2}$. Switching dimerized states with different symmetries by changing temperature or strain enables control of band polarization, adding a new tool for spintronics and valleytronics research.

We review our recent x-ray scattering studies of charge and orbital order in doped manganites, with specific emphasis on the role of orbital correlations in Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable from the undoped structure with long range orbital order at low temperatures. For dopings 0.3, Comment: 22 pagegs, 7 figures

We study magnetic and multiferroic behavior in Ca${}_{3}$Co${}_{2\ensuremath{-}x}$Mn${}_{x}$O${}_{6}$ ($x\ensuremath{\sim}$ 0.97) by high-field measurements of magnetization ($M$), magnetostriction [$L$($H$)/$L$], electric polarization ($P$), and magnetocaloric effect. This study also gives insight into the zero- and low-magnetic-field magnetic structure and magnetoelectric coupling mechanisms. We measured $M$ and $\ensuremath{\Delta}$$L$/$L$ up to pulsed magnetic fields of 92 T, and determined the saturation moment and field. On the controversial topic of the spin states of Co${}^{2+}$ and Mn${}^{4+}$ ions, we find evidence for $S=\frac{3}{2}$ spins for both ions with no magnetic-field-induced spin-state crossovers. Our data also indicate that Mn${}^{4+}$ spins are quasi-isotropic and develop components in the $ab$ plane in applied magnetic fields of 10 T. These spins cant until saturation at 85 T, whereas the Ising Co${}^{2+}$ spins saturate by 25 T. Furthermore, our results imply that the mechanism for suppression of electric polarization with magnetic fields near 10 T is flopping of the Mn${}^{4+}$ spins into the $ab$ plane, indicating that appropriate models must include the coexistence of Ising and quasi-isotropic spins.

Using inelastic neutron scattering technique, we measured the spin wave dispersion over the entire Brillouin zone of room temperature multiferroic BiFeO3 single crystals with magnetic excitations extending to as high as 72.5 meV. The full spin waves can be explained by a simple Heisenberg Hamiltonian with a nearest neighbor exchange interaction (J=4.38 meV), a next nearest neighbor exchange interaction (J'=0.15 meV), and a Dzyaloshinskii-Moriya-like term (D=0.107 meV). This simple Hamiltonian determined, for the first time, for BiFeO3 provides a fundamental ingredient for understanding of the novel magnetic properties of BiFeO3., Comment: 10 pages, 3 figures

Polarized small-angle neutron scattering studies of single-crystalline multiferroic ${\mathrm{BiFeO}}_{3}$ reveal a long-wavelength spin density wave generated by $\ensuremath{\sim}1\ifmmode^\circ\else\textdegree\fi{}$ spin canting of the spins out of the rotation plane of the antiferromagnetic cycloidal order. This signifies weak ferromagnetism within mesoscopic regions of dimension 0.03 microns along $[1\overline{1}0]$, to several microns along [111], confirming a long-standing theoretical prediction. The average local magnetization is $0.06\text{ }\text{ }{\ensuremath{\mu}}_{B}/\mathrm{Fe}$. Our results provide an indication of the intrinsic macroscopic magnetization to be expected in ferroelectric ${\mathrm{BiFeO}}_{3}$ thin films under strain, where the magnetic cycloid is suppressed.

Inorganic compounds with the AB2X4 spinel structure have been studied for many years, because of their unusual physical properties. The spinel crystallographic structure, first solved by Bragg in 1915, has cations occupying both tetrahedral (A) and octahedral (B) sites. Interesting physics arises when the B-site cations become mixed in valence. Magnetite (Fe3O4) is a classic and still unresolved example, where the tendency to form ordered arrays of Fe2+ and Fe3+ ions competes with the topological frustration of the B-site network. The CuIr2S4 thiospinel is another example, well known for the presence of a metal-insulator transition at 230 K with an abrupt decrease of the electrical conductivity on cooling accompanied by the loss of localized magnetic moments. Here, we report the determination of the crystallographic structure of CuIr2S4 below the metal-insulator transition. Our results indicate that CuIr2S4 undergoes a simultaneous charge-ordering and spin-dimerization transition-a rare phenomenon in three-dimensional compounds. Remarkably, the charge-ordering pattern consists of isomorphic octamers of Ir83+S24 and Ir84+S24 (as isovalent bi-capped hexagonal rings). This extraordinary arrangement leads to an elegant description of the spinel structure, but represents an increase in complexity with respect to all the known charge-ordered structures, which are typically based on stripes, slabs or chequerboard patterns.

Ferroelectric Ising chain magnet Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ ($x\simeq$0.96) was studied in magnetic fields up to 33 T. Magnetization and neutron scattering measurements reveal successive metamagnetic transitions from the zero-field $\uparrow \uparrow \downarrow \downarrow$ spin configuration to the $\uparrow \uparrow \uparrow \downarrow$ state with a broad magnetization plateau, and then to the $\uparrow \uparrow \uparrow \uparrow$ state. The absence of hysteresis in these plateaus reveals an intriguing coupling between the intra-chain state and the three-dimensional geometrically frustrated magnetic system. Inversion symmetry, broken in the $\uparrow \uparrow \downarrow \downarrow$ state, is restored in the $\uparrow \uparrow \uparrow \downarrow$ state, leading to the complete suppression of the electric polarization driven by symmetric superexchange., accepted for publication as a Brief Report in Physical Review B

We report discovery of collinear-magnetism-driven ferroelectricity in the Ising chain magnet ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{6}$ ($x\ensuremath{\approx}0.96$). Neutron diffraction shows that ${\mathrm{Co}}^{2+}$ and ${\mathrm{Mn}}^{4+}$ ions alternating along the chains exhibit an up-up-down-down ($\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$) magnetic order. The ferroelectricity results from the inversion symmetry breaking in the $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$ spin chain with an alternating charge order. Unlike in spiral magnetoelectrics where antisymmetric exchange coupling is active, the symmetry breaking in ${\mathrm{Ca}}_{3}(\mathrm{Co},\mathrm{Mn}{)}_{2}{\mathrm{O}}_{6}$ occurs through exchange striction associated with symmetric superexchange.

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 Bi0.31 Ca0.69 MnO3, 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., 5 pages

Recent state-of-the-art crystallographic investigations of transition metal spinel compounds have revealed that the d- orbital charge carriers undergo ordering transitions with the formation of local "molecular bonding" units such as dimers in MgTi2O4, octomers in CuIr2S4, and heptamers in AlV2O4. Herein, we provide a unifying scheme involving one- dimensional orbital wave ordering applicable to all of these spinels. The relative phase of the orbitals in the chains is shown to be crucial to the formation of different local units, and thus both the amplitude and phase of the orbital wave play important roles. Examination of Horibe et al.'s [1] structure for AlV2O4 serves as the vehicle for developing the general behavior for such orbital wave ordering. Ordered AlV2O4 will be seen to organize into three equivalent chains in 2D Kagome planes coupled so as to form units of three dimer bonds. Three additional equivalent chains manifest a more complex tetramerization with three different charge states and two different bonding schemes. The orbital wave ordering scheme developed is extended to other spinel and related compounds with local triangular transition metal coordination and partial filling of the t2g-d orbitals.

We report neutron-diffraction measurements of the magnetic phase diagram of the colossal magnetoelectric $\mathrm{Dy}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$. Our measurements reveal the magnetic origin of the complex dielectric anomalies in this material. Frustrated magnetic interactions result in a large density of low-energy magnetic states which can be selected by the application of relatively small magnetic fields. The unusual combination of magnetic frustration and strong magnetoelastic coupling is responsible for the remarkable changes of the dielectric properties of $\mathrm{Dy}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ in small applied magnetic fields.

We report a study of electronic excitations in manganites exhibiting a range of ground states, using resonant inelastic x-ray scattering (RIXS) at the Mn K edge. Excitations with temperature dependent changes correlated with the magnetism were observed as high as 10 eV. By calculating Wannier functions, and finite-q response functions, we associate this dependence with intersite d-d excitations. The calculated dynamical structure factor is found to be similar to the RIXS spectra.

Neutron scattering measurements on a magnetoresistive manganite La$_{0.75}$(Ca$_{0.45}$Sr$_{0.55}$)$_{0.25}$MnO$_3$ show that uncorrelated dynamic polaronic lattice distortions are present in both the orthorhombic (O) and rhombohedral (R) paramagnetic phases. The uncorrelated distortions do not exhibit any significant anomaly at the O-to-R transition. Thus, both the paramagnetic phases are inhomogeneous on the nanometer scale, as confirmed further by strong damping of the acoustic phonons and by the anomalous Debye-Waller factors in these phases. In contrast, recent x-ray measurements and our neutron data show that polaronic correlations are present only in the O phase. In optimally doped manganites, the R phase is metallic, while the O paramagnetic state is insulating (or semiconducting). These measurements therefore strongly suggest that the {\it correlated} lattice distortions are primarily responsible for the insulating character of the paramagnetic state in magnetoresistive manganites., 10 pages, 8 figures embedded

We report a resonant x-ray-diffraction study of the magnetoresistant perovskite Pr 0 . 6 Ca 0 . 4 MnO 3 . We discuss the spectra measured above and below the semiconductor-insulator transition temperature with the aid of a detailed formal analysis of the energy and polarization dependences of the structure factors and ab initio calculations of the spectra. In the low-temperature insulating phase, we find that inequivalent Mn atoms order in a CE-type pattern and that the crystallographic structure of La 0 . 5 Ca 0 . 5 MnO 3 [Radaelli et al., Phys. Rev. B 55, 3015 (1997)] can also describe this system in detail. Instead, the alternative structure proposed for the so-called Zener-polaron model [Daoud-Aladine et al., Phys. Rev. Lett. 89, 097205 (2002)] is ruled out by crystallographic and spectroscopic evidence. Our analysis supports a model involving orbital ordering. However, we confirm that there is no direct evidence of charge disproportionation in the Mn K-edge resonant spectra. Therefore, we consider a CE-type model in which there are two Mn sublattices, each with partial e g occupancy. One sublattice consists of Mn atoms with the 3x 2 -r 2 or 3y 2 -r 2 orbitals partially occupied in an alternating pattern, the other sublattice with the x 2 -y 2 orbital partially occupied.

S K and Ir ${L}_{3}$ x-ray absorption measurements across the temperature-induced metal (M) to insulator (I) transition in ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ are presented. Dramatic S K-edge changes reflect the Ir d-electronic state redistribution across this transition. These changes, along with a detailed consideration of the I-phase structure, motivate a model in which the I-phase stabilization involves an interplay of charge and d-orbital orientation ordering along Ir chains, a quadrupling of the Ir-chain repeat unit, and correlated dimer spin-singlet formation.

We report x-ray scattering studies of nanoscale structural correlations in the paramagnetic phases of the perovskite manganites La$_{0.75}$(Ca$_{0.45}$Sr$_{0.55}$)$_{0.25}$MnO$_3$, La$_{0.625}$Sr$_{0.375}$MnO$_3$, and Nd$_{0.45}$Sr$_{0.55}$MnO$_3$. We find that these correlations are present in the orthorhombic $O$ phase in La$_{0.75}$(Ca$_{0.45}$Sr$_{0.55}$)$_{0.25}$MnO$_3$, but they disappear abruptly at the orthorhombic-to-rhombohedral transition in this compound. The orthorhombic phase exhibits increased electrical resistivity and reduced ferromagnetic coupling, in agreement with the association of the nanoscale correlations with insulating regions. In contrast, the correlations were not detected in the two other compounds, which exhibit rhombohedral and tetragonal phases. Based on these results, as well as on previously published work, we propose that the local structure of the paramagnetic phase correlates strongly with the average lattice symmetry, and that the nanoscale correlations are an important factor distinguishing the insulating and the metallic phases in these compounds., Comment: a note on recent experimental work, and a new reference added

We present results of a systematic x-ray scattering study of the effects of Mg doping on the high-field incommensurate phase of ${\mathrm{CuGeO}}_{3}.$ Lorentzian-squared line shapes, the changing of the first-order transition to second order, and the destruction of long-range order with infinitesimal doping are observed, consistent with random-field effects in a three-dimensional $\mathrm{XY}$ system. Values for the soliton width in pure and lightly doped ${\mathrm{CuGeO}}_{3}$ are deduced. We find that even a very small doping has a drastic effect on the shape of the lattice modulation.

At low temperatures, spinel ${\mathrm{CuIr}}_{2}{\mathrm{S}}_{4}$ is a charge-ordered spin-dimerized insulator with triclinic lattice symmetry. We find that x rays induce a structural transition in which the local triclinic structure is preserved, but the average lattice symmetry becomes tetragonal. These structural changes are accompanied by a thousandfold reduction in the electrical resistivity. The transition is persistent, but the original state can be restored by thermal annealing. We argue that x-ray irradiation disorders the lattice dimerization pattern, producing a state in which the orientation of the dimers is preserved, but the translational long-range order is destroyed.

Inorganic compounds with the AB2X4 spinel structure have been studied for many years, because of their unusual physical properties. The spinel crystallographic structure, first solved by Bragg in 1915, has cations occupying both tetrahedral (A) and octahedral (B) sites. Interesting physics arises when the B-site cations become mixed in valence. Magnetite (Fe3O4) is a classic and still unresolved example, where the tendency to form ordered arrays of Fe2+ and Fe3+ ions competes with the topological frustration of the B-site network. The CuIr2S4 thiospinel is another example, well known for the presence of a metal-insulator transition at 230 K with an abrupt decrease of the electrical conductivity on cooling accompanied by the loss of localized magnetic moments. Here, we report the determination of the crystallographic structure of CuIr2S4 below the metal-insulator transition. Our results indicate that CuIr2S4 undergoes a simultaneous charge-ordering and spin-dimerization transition-a rare phenomenon in three-dimensional compounds. Remarkably, the charge-ordering pattern consists of isomorphic octamers of Ir83+S24 and Ir84+S24 (as isovalent bi-capped hexagonal rings). This extraordinary arrangement leads to an elegant description of the spinel structure, but represents an increase in complexity with respect to all the known charge-ordered structures, which are typically based on stripes, slabs or chequerboard patterns.

We report synchrotron x-ray scattering studies of charge/orbitally ordered (COO) nanoclusters in Nd$_{0.7}$Sr$_{0.3}$MnO$_3$. We find that the COO nanoclusters are strongly suppressed in an applied magnetic field, and that their decreasing concentration follows the field-induced decrease of the sample electrical resistivity. The COO nanoclusters, however, do not completely disappear in the conducting state, suggesting that this state is inhomogeneous and contains an admixture of an insulating phase. Similar results were also obtained for the zero-field insulator-metal transition that occurs as temperature is reduced. These observations suggest that these correlated lattice distortions play a key role in the Colossal Magnetoresistance effect in this prototypical manganite.

We report x-ray scattering studies of broad peaks located at a (0.5 0 0)/(0 0.5 0)-type wavevector in the paramagnetic insulating phases of La_{0.7}Ca_{0.3}MnO_{3} and Pr_{0.7}Ca_{0.3}MnO_{3}. We interpret the scattering in terms of correlated polarons and measure isotropic correlation lengths of 1-2 lattice constants in both samples. Based on the wavevector and correlation lengths, the correlated polarons are found to be consistent with CE-type bipolarons. Differences in behavior between the samples arise as they are cooled through their respective transition temperatures and become ferromagnetic metallic (La_{0.7}Ca_{0.3}MnO_{3}) or charge and orbitally ordered insulating (Pr_{0.7}Ca_{0.3}MnO_{3}). Since the primary difference between the two samples is the trivalent cation size, these results illustrate the robust nature of the correlated polarons to variations in the relative strength of the electron-phonon coupling, and the sensitivity of the low-temperature ground state to such variations., Comment: 13 pages, 6 figures

In this paper, we report polarized optical microscopy and electrical transport studies of manganese oxides that reveal that the charge ordering transition in these compounds exhibits typical signatures of a martensitic transformation. We demonstrate that specific electronic properties of charge-ordered manganites stem from a combination of martensitic accommodation strain and effects of strong electron correlations. This intrinsic strain is strongly affected by the grain boundaries in ceramic samples. Consistently, our studies show a remarkable enhancement of low field magnetoresistance and the grain size effect on the resistivity in polycrystalline samples and suggest that the transport properties of this class of manganites are governed by the charge-disordered insulating phase stabilized at low temperature by virtue of martensitic accommodation strain. High sensitivity of this phase to strains and magnetic field leads to a variety of striking phenomena, such as unusually high magnetoresistance (10^10 %) in low magnetic fields., Comment: Short paper, 4 figures, to appear in Rapid Communication

We report x-ray scattering studies of short-range structural correlations and diffuse scattering in ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}.$ On cooling, this material undergoes a series of transitions, first from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) phase, and then to a charge-ordered (CO) insulating state. Highly anisotropic structural correlations were found in both the PI and FM states of this compound. The correlations increase with decreasing temperature, reaching a maximum at the CO transition temperature. Below this temperature, they abruptly collapsed. Single-polaron diffuse scattering was also observed in both the PI and FM states suggesting that substantial local lattice distortions are present in these phases. We argue that our measurements indicate that nanoscale regions exhibiting layered orbital order exist in the paramagnetic and ferromagnetic phases of ${\mathrm{Nd}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3}.$

We report x-ray scattering studies of diffuse peaks in the paramagnetic insulating phase of La0.7Ca0.3MnO3. The peaks occur at a (0.5 0 0)/(0 0.5 0)-type wavevector, and are found to disappear abruptly upon cooling through the transition into the ferromagnetic metallic phase. Based on the wavevector and measured correlation length of 1–2 lattice constants, the peaks are found to be consistent with the formation of CE-type bipolarons, and their collapse at Tc is attributed to the energy gain associated with itineracy in the ferromagnetic state.

The insulator-metal transition in single crystal ${\mathrm{La}}_{5/8\ensuremath{-}y}{\mathrm{Pr}}_{y}{\mathrm{Ca}}_{3/8}{\mathrm{MnO}}_{3}$ with $y\ensuremath{\approx}0.35$ was studied using synchrotron x-ray diffraction, electric resistivity, magnetic susceptibility, and specific heat measurements. Despite the dramatic drop in the resistivity at the insulator-metal transition temperature ${T}_{\mathrm{MI}},$ the charge-ordering (CO) peaks exhibit no anomaly at this temperature and continue to grow below ${T}_{\mathrm{MI}}.$ Our data suggest then, that in addition to the CO phase, another insulating phase is present below ${T}_{\mathrm{CO}}.$ In this picture, the insulator-metal transition is due to the changes that occur within this latter phase. The CO phase does not appear to play a major role in this transition. We propose that a percolationlike insulator-metal transition occurs via the growth of ferromagnetic metallic domains within the parts of the sample that do not exhibit charge ordering. Finally, we find that the low-temperature phase-separated state is unstable against x-ray irradiation, which destroys the CO phase at low temperatures.

In this work we report a study of the photoinduced insulator-to-metal transition in manganese oxide perovskites of the formula Pr1-xCaxMnO3. The transition is closely related to the magnetic field induced insulator-to-metal transition (CMR effect) observed in these materials. It is accompanied by a dramatic change in the magnetic properties and lattice structure: the material changes from an insulating charge-ordered canted antiferromagnet to a ferromagnetic metal. We present an investigation of the transport and structural properties of these materials over the course of the transition (which usually takes about an hour to complete). The current-voltage characteristics exhibited by the material during the transition are highly nonlinear, indicating a large inhomogeneity of the transitional state. Possible practical applications of this novel type of transition are briefly discussed. We also report a high-resolution x-ray diffraction study of the charge-ordering in these materials. The temperature dependent charge ordering structure observed in these compounds is more complex than previously reported.

We report a synchrotron x-ray scattering study of the magnetic-field-induced incommensurate phase of the spin-Peierls compound CuGeO{sub 3}, in magnetic fields up to 13 T. By measuring first and third harmonics of the incommensurate Bragg reflections as a function of field we show that the lattice modulation has the form of a soliton lattice. The soliton half-width, 13.6 lattice spacings, is surprisingly large. Dilution of the CuO{sub 2} spin-1/2 chains with spin-0 (Zn) and spin-1 (Ni) impurities results in a short-range-ordered incommensurate state with an anisotropic correlation length comparable to the average impurity separation. {copyright} {ital 1996 The American Physical Society.}

The colossal magnetoresistance (CMR) effect in perovskite manganites is a magnetic-field-induced transition from a paramagnetic insulating (PI) to a ferromagnetic metallic state. The large electrical resistivity of the PI state lies at the heart of the CMR effect. This enhanced resistivity stems, in part, from strong electron–lattice coupling and the associated local lattice distortions. Both uncorrelated local distortions (Jahn–Teller polarons) and correlated distortions are present in the PI state. The latter are believed to signal the presence of nanoscale orbital correlations. In this paper, we describe recent x-ray and neutron-scattering studies of the orbital correlations in pseudocubic manganites Ln1−xBxMnO3. Possible microscopic structures giving rise to these correlations are discussed. It is found that the correlations are ubiquitous in the orthorhombic PI phase of hole-doped manganites, and that their properties are defined by a single parameter—the doping level x. The correlations, however, are absent in the other paramagnetic phase exhibited by the manganites—the rhombohedral phase. The latter phase is metallic in the doping range in which the CMR effect is observed. Since the uncorrelated lattice distortions are present in both these phases, the insulating character of the PI state in CMR manganites results from the presence of the correlated lattice distortions. The orbital correlations, therefore, play the key role in the CMR effect.