Abstract: Single crystalline ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ ( $0.74\ensuremath{\le}x\ensuremath{\le}0.82$) were studied with neutron scattering, electron diffraction, and bulk magnetic measurement. We discovered dynamic ferromagnetic spin correlations at high temperatures, which are replaced by antiferromagnetic spin fluctuations at a concomitant charge ordering and structural transition. Our results indicate that thermal-activated hopping of the Jahn-Teller active ${e}_{g}$ electrons in these insulating materials, nevertheless, induces ferromagnetic interaction through the double-exchange mechanism. It is the ordering of these charges which competes with the double-exchange ferromagnetic metallic state.

Abstract: The metal-insulator (M-I) phase transitions relevant to charge ordering (CO) have been investigated for perovskite-type (Nd{sub 1{minus}y}Sm{sub y}){sub 1/2}Sr{sub 1/2}MnO{sub 3} (0{le}y{le}1) crystals, in which the one-electron bandwidth (W) is systematically controlled by varying the averaged ionic radius of the A site and by application of quasihydrostatic pressure (P). Competition between the ferromagnetic double exchange and the antiferromagnetic CO interactions give rise to complex M-I phase diagrams with temperature (T) and W (y and/or P) as the parameters. The M-I phase boundaries are associated with critically W- and T-dependent hystereses, which result in unique appearance of the metastable state. We have demonstrated the pressure-induced phase transition from the metastable ferromagnetic metal to the thermodynamically stable charge-ordered insulator for the y=0.875 crystal locating near the critical M-I phase boundary. With decrease of W, the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature (T{sub c}) and enhances the electron-lattice coupling. Consequently, the lattice-coupled first-order I-M transition is observed at T{sub c} in the small-W region of y{ge}0.5. It was found that application of magnetic field also induces the phase transition from the insulator with antiferromagnetic spin correlations to the ferromagnetic metal, which is accompanied by lattice-structuralmore » change. {copyright} {ital 1997} {ital The American Physical Society}« less

Abstract: The charge ordering process of a mixed valence LuFe 2 O 4 system has been studied, focusing attention on the frustration effect associated with the charge configuration. It is revealed that the system undergoes successive phase transitions following the sequence of disorder→2D CDW→3D CDW. In the 2D-CDW state, the charge density wave defined by the wave vector (1/3, 1/3), is formed in the hexagonal layer. On the nodal lines of the CDW, the charge order is still undetermined due to the charge frustration. From a dielectric viewpoint, this phase is viewed as random stacking of 2D ferroelectric layers. In the 3D-CDW state, antiphase stacking of the 2D ordered layer develops. At the same time, the in-plane charge order exhibits a doubly modulated long-period structure. From a dielectric viewpoint, this phase is characterized by an incommensurate antiferroelectric phase. Relevance of the charge ordering mechanism to the observed anomalously large dielectric constant in LuFe 2 O 4 is discussed.

Abstract: A. K. Cheetham Several compounds are known to exhibit magnetoresistance, a change in electrical resistivity in the presence of magnetic field. Such materials are of interest both for technology and basic science. In their Perspective, Rao and Cheetham discuss recent results on magnetoresistance in rare-earth manganate materials, which exhibit especially interesting fundamental physical phenomena.

Abstract: Giant magnetoresistance, and spin-, charge-, and orbital-ordering are some of the properties displayed by manganates that make these materials of interest in magnetic recording, sensor, and actuator technology. New and significant results on the giant magnetoresistance found in films as well as polycrystalline and single-crystal samples of rare earth manganates are reviewed along with related aspects. The unique features of these systems and the as-yet unsolved problems are highlighted. Charge-ordering as opposed to spin-ordering is also discussed and suggestions for future directions are given.

Abstract: Resistivity and ac-susceptibility measurements demonstrate that the cation size mismatch at the $A$ site in perovskite manganites influences greatly the electrical and magnetic properties which are associated with colossal magnetoresistance. It has been reported that the Pr${}_{1\ensuremath{-}x}$Ca${}_{x}$MnO${}_{3}$ system does not exhibit an insulator-metal transition and remains insulating at all temperatures because of charge ordering at high temperatures. We show that the cation size mismatch at the $A$ site, introduced by replacing the Pr${}^{3+}$ ions by larger La${}^{3+}$ and smaller Y${}^{3+}$ ions with constant average $A$-site ionic radius $〈{r}_{A}〉$=1.18 \AA{}, suppresses charge ordering and the samples with $x$=0.25 and 0.3 undergo insulator-metal transitions at 130 and 60 K, respectively, without applying a magnetic field. For 0.30 $l~xl~$ 0.5, they all show a spin-glass behavior.

Abstract: A polarized neutron diffraction experiment gives direct evidence for a charge order in the extremely low-carrier system Yb4As3. Inelastic neutron scattering experiments on Yb4As3 revealed the existence of low-energy spin excitations that are characteristic of a one-dimensional antiferromagnetic coupling at low temperatures. The results indicate that the one-dimensional arrangement of Yb3+ ions caused by the charge order plays a main role for the heavy-electron behavior in Yb4As3.

Abstract: The crystal structures of Sr2HoMn2O7 and Sr2YMn2O7 have been determined at 290 and 1.7 K from neutron and X-ray powder diffraction data. Both are distorted Ruddlesden−Popper structures formed by the intergrowth of rock-salt-like layers and perovskite-like blocks of tilted MnO6 octahedra (space group P42/mnm; for Ho at 290 K a = 5.40388(5), c = 19.9050(2) A). The majority of the lanthanide cations are located in the rock-salt layers. Neither structure changes significantly on cooling to 1.7 K. There is evidence for neither charge ordering of Mn3+ and Mn4+ cations nor for long-range magnetic ordering. The magnetic susceptibility maximum observed previously is thus confirmed as signifying a transition to a spin-glass phase. The behavior of these compounds is contrasted with that of other Mn oxides which show long-range magnetic order and colossal magnetoresistance.

Abstract: We have used tunnelling spectroscopy (TS) to probe the density of states (DOS) of the rare earth manganate , which shows a transition to a charge ordered (CO) antiferromagnetic insulating state from a ferromagnetic metallic state. We find that a gap opens up in the DOS around the Fermi level, , as the solid is cooled below the charge ordering temperature . The charge ordering gap was found to be strongly temperature dependent for and for it reaches a limiting value of nearly 0.27 eV. Above the temperature dependence of the DOS at was found to be similar to that of the solids such as which show colossal magnetoresistance but no charge ordering.

Abstract: The fascinating properties like giant magnetoresistance (GMR) effect, metal-insulator transition, charge ordering phenomenon etc. have made the divalent ion doped RMnO3 (R = rare-earth elements) an attractive system for investigation. Resistivity of these compounds shows a peak near the ferromagnetic transition temperature (Tc). The application of magnetic field inhibits the spin-disorder scattering and the resistivity decreases drastically. Keeping electrondoped superconductor Nd2−xCexCuO4 in mind we have doped RMnO3 (R = La, Pr, Nd) with tetravalent Ce ion. These compounds are very susceptible to the annealing treatment and belong to the orthorhombic perovskite phase. They show a very high value of resistivity at the peak and under the magnetic field the GMR effect is observed. For La0.7Ce0.3MnO3 and Pr0.7Ce0.3MnO3 the magnetoresistance ratio reaches about 54% and 82.5% respectively at 7.7 T. With the increase of the temperature the magnetic state changes from ferromagnetic to paramagnetic regime. This magnetic transition is not very sharp and the resistivity peak appears at a temperature higher than Tc.

Abstract: The systematic investigation of the magnetic susceptibility of the CMR manganites Pr1−xCaxMnO3 versus temperature has been performed for 0.25 ≤ x ≤ 0.50. Due to the similar size of calcium and praseodymium, these results show the important role of the mixed valence of manganese upon the complex magnetic behaviour of these compounds. They demonstrate that the appearance of antiferromagnetism coincides with charge ordering, TN = TCO varying with x, from 250K for x = 0.50 to 225K for x = 0.35. A strong competition between ferromagnetism and antiferromagnetism is observed for 0.35 ≤ x 170 K) and going through canted antiferromagnetic or weak ferromagnetic states for intermediate temperatures (70 K < T < 170 K). For 0.25 ≤ x ≤ 0.30, a strong ferromagnetic state is observed for 95 K ≤ T ≤ 150 K, with a transition to a spin glass like state below 95–110 K.

Abstract: In Yb pnictides with anti-Th 3 p 4 structure, the valence of Yb decreases from about 2.25 in Yb 4 As 3 to about 2 in Yb 4 Bi 3 with increasing weight of pnictogen. In Yb 4 As 3 , charge ordering around 300 K is observed and in the charge ordered phase heavy fermion behavior is seen in spite of extremely low carrier concentration. On the other hand, Yb 4 Sb 3 is a valence fluctuating system. So, to make systematic studies from Yb 4 As 3 to Yb 4 Sb 3 we grew single crystals of Yb 4 (As 1− x Sb x ) 3 . With increasing Sb content, the absolute value of the resistivity decreases due to the increase of the number of carriers and the charge ordering temperature decreases reflecting the increase of the screening effect, and at 29% charge ordering seems to disappear. The γ-value, however, is still large even in Sb 29% sample. The large γ-value of Yb 4 (As 1− x Sb x ) 3 is considered to be independent ofthe carrier concentration.

Abstract: Photoemission spectra and the TB-LMTO-ASA band structure of the mixed valent europium compound EuPdP (ZrBeSi type structure) were studied and compared with the results obtained on isostructural SrPdP and EuAuAs and the ThCr2Si2-type compound EuNi2P2. EuNi2P2 is a homogeneous mixed valent compound, whereas the situation in EuPdP is not clear. The Mossbauer spectroscopy suggests static mixed valency in EuPdP corresponding to partially localized 4 f electrons, but from the photoemission measurement we know that the situation in EuPdP is different from the static mixed valent Eu3O4. Band structure calculations as well as photoemission spectra indicate a 4 f density of states in the vicinity of the Fermi level in EuPdP, which belongs to a van Hove Singularity (vHS). In EuPdP we therefore expect homogeneous mixed valence, similar to EuNi2P2 with itinerant 4 f electrons. The absence of homogeneous mixed valence is possibly associated with the layered structure of EuPdP, enabling charge ordering through a Martensitic phase transition. A high density of states associated with a singularity at the Fermi energy seems to be responsible for the structural instability in inhomogeneous mixed valent europium compounds.

Abstract: Charge ordering of a mixed valence system LuFe2O4 has been studied by neutron scattering technique, focusing attention on the charge frustration effect on triangular lattice. It is revealed that the system undergoes successive phase transitions following the sequence of disorder → two-dimensional charge density wave state (2D-CDW) → three-dimensional charged density wave state (3D-CDW). In 2D-CDW state, the charge density wave defined by the wave vector ( 1 3 1 3 ) is formed in the hexagonal layer.

Abstract: Mixed valence compound Yb4As3 which shows the charge ordering of Yb3+ and Yb2+ around 300 K has been considered to be a heavy fermion system with extremely low carrier concentration. So the main interest in this system is to know whether the heavy fermion behavior surely comes from such low carrier concentration. To make this situation clear, single crystals of Yb4(As1-xPx)3 were grown and their transport properties, magnetic susceptibility and specific heat were measured. With increasing concentration of phosphor the system becomes semiconductive, however, the specific heat and magnetic susceptibility are almost the same as those of pure Yb4As3. This result shows that the large γ-value of Yb4As3 is not due to the mass enhancement as seen in usual heavy fermion systems but arises from another magnetic correlation.

Abstract: We have observed the diffuse scattering distributions associated with the charge ordering of Fe2+ and Fe3+ above the Verwey transition temperature (T V\cong123 K) in magnetite, Fe3O4, by synchrotron X-rays. At a specific wavelength of λ= 1.7421 or 1.7425 A close to the Fe K absorption edge, the anomalous scattering effect makes it possible to distinguish Fe2+ ions from Fe3+ in the X-ray diffuse intensities. X-shaped diffuse patterns appeared at 130 and 150 K on the a1* - a2* plane around the 440-type reciprocal lattice point, which were not observed at 296 K. Even for the measurement at 130 K, the X-shaped pattern disappeared when the X-rays had a lower energy resolution. Cross-shaped diffuse patterns were also observed around the 400 and 800 reciprocal lattice points. The theoretical calculations suggest that these diffuse streaks are not due to thermal diffuse scattering. Our analysis of Huang scattering revealed that the observed diffuse patterns were plausibly caused by the local displacement with Fe2+ and Fe3+ pairing among the B sites.

Abstract: Manganese perovskites have recently been the subject of renewed attention, due to the “colossal” magnetoresistance (CMR) and the coupled metal-insulator and magnetic transitions displayed at the spin-ordering temperature TC by some of these compounds. The charge localization mechanism above Tc appears at present to be the single most important issue in manganite research. Neutron and synchrotron X-ray diffraction studies have revealed, for appropriate doping ranges, a strong competition between the metallic state, which is ferromagnetic and shows small lattice distortions, and a charge-localized/ordered state that becomes antiferromagnetic at low temperatures and shows large lattice distortions of the Jahn-Teller type. In the case of La0.5Ca0.5MnO3 and Pr0.7Ca0.3MnO3, these two states can coexist giving rise to transient inhomogeneities. The possibility that a similar situation may be reproduced in the paramagnetic phase will be discussed, in the light of recent neutron-diffuse-scattering data suggestive of extended ferromagnetic fluctuations above TC.

Abstract: We report low frequency dielectric dispersions of ErFe2O4 in which equal numbers of Fe2+ and Fe3+ ions occupy the sites on a triangular lattice. The first dispersion observed between 200 K and 300 K is explained qualitatively on a model that spontaneous polarization is present in the crystal below the Verwey transition. The second dispersion at higher temperatures is characterized by a few glass-like features. It was interpreted by successive freezing of the local dipole moments accompanied by short-range ordering of Fe2+ and Fe3+ ions.

Abstract: The discovery of giant magnetoresistance (GMR) in rare earth manganates of the general formula Ln 1−x A x MnO 3 (Ln=rare earth, A=divalent cation) has aroused much interest not only because of its technological implications, but also due to the fascinating features and mechanism of the phenomemon in these oxides. GMR is observed in these manganates when they become ferromagnetic and transform from an insulating state to a metallic state close to the Curie temperature. The essential features of magnetoresistance in the manganates can be understood on the basis of the double-exchange mechanism, but this is too simplistic to account for all the observed data. The most curious property of the manganates relates to the high resistivity exhibited in the so-called metallic state. Charge ordering competes with the double-exchange interaction responsible for ferromagnetism and GMR in these materials. The charge-ordered (charge-crystal) insulating state in the rare earth manganates can be melted into a metallic and ferromagnetic charge-liquid state by applying a magnetic field, thus providing a unique case of charge and spin separation in solids. The observation of GMR in Tl 2 Mn 2 O 7 shows that there can be causes other than double-exchange for the phenomenon.

Abstract: We report the Sr-concentration dependence of the spin and charge ordering in La2−xSrxNiO4 with 0.27 ⩽ x ⩽ 0.5. In this range of the Sr concentration x, the characteristic wave vector ϵ for the magnetic and charge order varies approximately linear in x, i.e. ϵ ∼ x. The transition temperature of spin order is always lower than that of stripe order. Two transition temperatures increase for x x c = 1 3 , and then tend to decrease as x further increases.

Abstract: Thin epitaxial films of (La1−xPrx)0.7Ca0.3MnO3 (x=0,0.25,0.5,0.75,l) were grown on LaAlO3, SrTiO3 and ZrO2(Y2O3) by aerosol MOCVD at 750°C Variation of x greatly influences the electrical properties of material by changing of the tolerance factor t. The tensile strain in the films on perovskite substrates produces the effect on the maximum resistivity temperature Tp comparable with the effect of chemical pressure nearby critical value of t ∼ 0.91. By variation of x the colossal magnetoresistance in very low magnetic fields was achieved in the thin films: an applied field of 0.3 T was enough for 32 times decrease of the resistivity of (La0.35Pr0.35Ca0.3MnO3 film on LaAlO3 at 155 K. The charge ordering starting in (La1\t-xPrx)0.7Ca0.3MnO3 at 160-210 K leads to the switching from Arrhenius law (lgρ∼T1) to Mott law (lgρ∼ T1/4) of resistivity. Large-angle boundaries dominating the structure of the films on ZrO2(Y2O3) increase the resistivity both above and below Tp.

Abstract: Magnetoelastic effects measured previously on Ce x La 1- x B 6 were reanalyzed based on the dynamical Jahn-Teller effect as a single site effect and the paired distortion as an interaction effect developed in recent papers for RB 6 in general. In the dilute systems, we observe an anomalous Curie-Weiss term caused from competition between the single site effect and the pair effect, while in the dense systems anomalies due to pair formation are observed even in the paramagnetic region. Strong frustration inherent to the pair ordering plays an important role. Anomalous properties in PrB 6 and NdB 6 were also interpreted based on the above common model in good agreement with the experimental results, in particular with anomalous neutron scattering measurement. Magnetoelastic effects were measured also on PrB 6 and NdB 6 . In PrB 6 the effect is 100 time stronger than that in CeB 6 due to induced mixing through distortions between the nearly degenerated ground levels, Γ 1 and Γ 5 . The charge dipolar ordering ...

Abstract: The authors found dynamic ferromagnetic correlations at the room temperature in perovskite manganites which have an antiferromagnetic insulating ground state. They are replaced by antiferromagnetic correlations when the electrons order. The results indicate that the double exchange mechanism is turned off by electron localization at low temperatures.

Abstract: In the context of problem of possible connection between high-Tc superconductivity and lattice instability of ferro- or antiferroelectric type the model considering the electron Hubbard correlations as well as the local anharmonic vibrations of crystal lattice (described by pseudospins) is investigated. The strong on-site coupling between electrons and pseudospins leads to the formation of mixed vibronic like states and the spectrum of low-frequency excitations contains three pseudospin-type branches. This splitting is caused by difference between internal fields acting on the pseudospin at various occupations of electron states on the site. The transverse dielectric susceptibility xϰ achieves high values when the frequency of one of these excitations reaches its minimum. The essential feature of the model is the appearance of either dielectric or charge ordering type instabilities in the temperature dependencies of dynamical susceptibility xϰ(q, ω). The generalized random phase approximation tak...

Abstract: In this work the authors report a study of the photoinduced insulator-to-metal transition in manganese oxide perovskites of the formula Pr{sub 1{minus}x}Ca{sub x}MnO{sub 3}. 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. The authors 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. They 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.