Charge-orbital Ordering Research Articles

Orbital ordering of layered manganites from resonant soft X-ray scattering

We present measurements of Mn L -edge resonant soft X-ray scattering on single-layered manganites with different sizes of cations. Orbital ordering of Pr 0.5 Ca 1.5 MnO 4 exhibits a stronger three-dimensional character and a dramatically enhanced transition temperature, as compared with those of La 0.5 Sr 1.5 MnO 4 . The c -axis correlation length of orbital ordering in Pr 0.5 Ca 1.5 MnO 4 is about half of the in-plane correlation length. Our results indicate that reduction in one-electron bandwidth and quenched disorder strongly enhances the stabilization of charge–orbital ordering.

Asymmetric behavior of charge-orbital order in electron/hole-dopedA-site ordered manganitesEu(Ba1−yLay)Mn2O6and(Eu1−xCax)BaMn2O6

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Effect of Quenched Disorder on Charge–Orbital–Spin Ordering in Single-Layer Manganites

Structural and magnetic properties have been investigated for half-doped single-layer manganites RE 0.5 Sr 1.5 MnO 4 [ RE = La, (La, Pr), Pr, Nd, Sm, and Eu]. Analyses of electron diffraction and ac susceptibility measurements have revealed that the long-range charge–orbital ordering (CO–OO) state as observed in La 0.5 Sr 1.5 MnO 4 is suppressed for the other materials: the CO–OO transition temperature, as well as the correlation length decreases with a decrease in the cation size of RE . Such a short-range CO–OO state shows a spin-glass behavior at low temperatures. A new electronic phase diagram is established with quenched disorder as the control parameter.

Charge-orbital ordering of magnetite studied by soft resonant X-ray scattering

We report direct experimental evidence for the existence of charge and orbital ordering in magnetite. In particular, measurements of oxygen K-edge resonant X-ray scattering on magnetite reveal that the oxygen 2p states in the vicinity of the Fermi level exhibit a charge-orbital ordering with a spatial periodicity of the doubled lattice parameter of the undistorted cubic phase.

Charge-Orbital Ordering and Verwey Transition in Magnetite Measured by Resonant Soft X-Ray Scattering

We report experimental evidence for the charge-orbital ordering in magnetite below the Verwey transition temperature T(V). Measurements of O K-edge resonant x-ray scattering on magnetite reveal that the O 2p states in the vicinity of the Fermi level exhibit a charge-orbital ordering along the c axis with a spatial periodicity of the doubled lattice parameter of the undistorted cubic phase. Such a charge-orbital ordering vanishes abruptly above T(V) and exhibits a thermal hysteresis, correlating closely with the Verwey transition in magnetite.

Correlation between A-site cation randomness and colossal magnetoresistance effect in perovskite manganite

We have achieved over 1000% colossal magnetoresistance (CMR) at room temperature by introducing well-controlled disorder to the A-site ordered manganites, Sm 1 - x La x + y Ba 1 - y Mn 2 O 6 . The La-doping to the A-sites (Sm and Ba) of mother compound, SmBaMn 2 O 6 , has mainly three different effects: (1) ionic-size randomness (IR) by Sm(La), (2) charge randomness (CR) by Ba(La) and (3) carrier doping by La/Ba. Any CMR associated with the melting of charge/orbital order (CO) phase by magnetic field is not caused by only IR, because the CO state is very stable in the homogeneous electrostatic potential at the A-site. However, a slight CR destabilizes the CO state and realizes phase separation state between CO and ferromagnetic domains, resulting in an insulator–metal transition under magnetic field, CMR effect. Therefore, the electrostatic potential disorder (CR) at the A-site is an essential role for CMR effect.

Charge and complex orbital ordering in [formula omitted

To discuss charge and orbital ordering in magnetite ( Fe 3 O 4 ), a spinless three-band Hubbard model was investigated by the exact diagonalization of finite size clusters and the Hartree–Fock approximation. A state with noncollinear orbital moments is obtained with both the methods. The orbital moments are not induced by the ordered spin moment via the spin–orbit interaction as usual magnetic materials but by spontaneous breaking of the time reversal symmetry within the orbital degrees of freedom. The result is consistent with a large orbital moment found in the measurements of the magnetic circular dichroism of Fe 2p X-ray absorption spectra.

Electronically smecticlike liquid-crystal phase in a nearly half-doped manganite

We use neutron scattering to study the spin and charge-orbital ordering (CO-OO) in the nearly half-doped perovskite manganite ${\mathrm{Pr}}_{0.55}{({\mathrm{Ca}}_{0.8}{\mathrm{Sr}}_{0.2})}_{0.45}\mathrm{Mn}{\mathrm{O}}_{3}$ (PCSMO). On cooling from room temperature, PCSMO first enters into a CO-OO state below ${T}_{\mathit{CO}}$ and then becomes a CE-type long-range ordered antiferromagnet below ${T}_{N}$. At temperatures above ${T}_{N}$ but below ${T}_{\mathit{CO}}$ $({T}_{N}&lt;T&lt;{T}_{\mathit{CO}})$, the spins in PCSMO form highly anisotropic smectic liquid-crystal-like texture with ferromagnetic (FM) quasi-long-range ordered one-dimensional zigzag chains weakly coupled antiferromagnetically. Such a magnetic smecticlike phase results directly from the spin-orbit interaction and demonstrates the presence of textured ``electronic soft'' phases in doped Mott insulators.

Pseudomorphic strain effect on the charge-orbital ordering pattern in Pr0.5Sr0.5MnO3 epitaxial thin films

We accomplished coherent epitaxial growth of a Pr0.5Sr0.5MnO3 thin film on (LaAlO3)0.3–(SrAl0.5Ta0.5O3)0.7 (110) substrate, which demonstrates a robust charge-orbital ordering (COO) phase transition. In addition to the isotropic transport properties, a drastic recovery of high-resistivity state (&amp;gt;20Ωcm) from a field-cooled metallic state (∼0.5mΩcm) was observed at 5K with a release of a magnetic field. These results indicate a possible modification of COO pattern into CE-type COO state [d(3x2−r2∕3y2−r2)] in the film from A-type stripe COO state [d(x2−y2)] observed in bulk samples by a substrate-induced strain.

Critical phase control of the charge-orbital ordered manganite thin films

Utilizing the tetragonal symmetry breaking on (110) substrates, we realized a first-order phase transition arising from the charge-orbital ordering (COO) in epitaxial thin films for two types of representative COO materials, (Pr,Ca)MnO 3 and (Nd,Sr)MnO 3. In addition, we demonstrated a dramatic substrate-dependent crossover from charge-ordered insulator to metal for (Nd,Pr) 0.5Sr 0.5MnO 3 as proof of the control of the one-electron bandwidth by an anisotropic substrate-induced strain. The critical phase control of spin, charge, and orbital ordering in the perovskite manganite epitaxial thin films demonstrated here constitutes a breakthrough toward strongly correlated electron devices in oxide electronics.

Charge-Orbital Ordering and Verwey Transition in Magnetite

Local density approximation + Hubbard U (LDA + U) band structure calculations reveal that magnetite (Fe3O4) forms an insulating charge-orbital-ordered state below the Verwey transition temperature. The calculated charge ordering is in good agreement with that inferred from recent experiments. We found an associated t(2g) orbital ordering on the octahedral Fe2+ sublattice. Such an orbital ordering results primarily from the on-site Coulomb interaction. This finding unravels such fundamental issues about the Verwey transition as the mechanism for the charge ordering and for the formation of the insulating gap, as well as the nonobedience of the Anderson's criterion for the charge ordering.

Scientific Review: A New Epithermal Neutron Diffractometer at KENS (EXCED)

Neutron scattering experiments on highly absorbing materials like boron (10B), cadmium (113Cd), rare earths (Sm, Eu, Gd, Dy), etc., are extremely difficult to perform and have so far discouraged research activities on relevant compounds. However, recent topical phenomena such as charge ordering and orbital ordering have highlighted the importance of studying 4f-electron systems and raised a large demand for neutron scattering studies. Unfortunately, the preparation of isotope enriched (weakly absorbing) samples are costly, and very often the large amount of sample required for experiments can not be produced. An alternative way consists of reducing the absorption cross-section _a by utilizing epithermal neutrons of the order of several eV.

Charge order, orbital order, and electron localization in the Magnéli phase Ti 4O 7

The metal–insulator transition of the Magnéli phase Ti 4O 7 is studied by means of augmented spherical wave (ASW) electronic structure calculations as based on density functional theory and the local density approximation. The results show that the metal–insulator transition arises from a complex interplay of charge order, orbital order, and singlet formation of those Ti 3d states which mediate metal–metal bonding inside the four-atom chains characteristic of the material. Ti 4O 7 thus combines important aspects of Fe 3O 4 and VO 2. While the charge ordering closely resembles that observed at the Verwey transition, the orbital order and singlet formation appear to be identical to the mechanisms driving the metal–insulator transition of vanadium dioxide.

Novel structures and electromagnetic properties of the A-site-ordered/disordered manganitesRBaMn2O6/R0.5Ba0.5MnO3 (R = Y and rare earth elements)

The A-site-ordered/disordered manganese perovskites,RBaMn2O6/ R0.5Ba0.5MnO3 (R = Y and rare earthelements), are reviewed. RBaMn2O6 displays remarkable features: (1) the charge/orbital order (CO) transitionat relatively high temperatures far above 300 K, (2) a new stackingvariation of the CE-type CO with a fourfold periodicity along thec-axis, (3) the presence of structural transition possibly accompanied bydx2−y2 orbital order and (4) electronic phase segregation. These novel structural andelectromagnetic properties are discussed in terms of the structural characteristic that theMnO2 square sublattice is sandwiched by two types of rock-salt layer,R O and BaO, with different lattice sizes. Such structure introduces a strong frustration to theMnO2 sublattice and gives a new perturbation to the competition of multipledegrees of freedom among charge, orbital, spin and lattice. InR0.5Ba0.5MnO3 with a primitive cubic perovskite cell, on the other hand, the magnetic glassystates are dominant as the ground state. A peculiar behaviour, steplikeultrasharp magnetization and resistivity changes, has been observed inPr0.5Ba0.5MnO3.

導電性セラミック材料 Ａサイト秩序型／無秩序型ペロブスカイトＭｎ酸化物ＲＢａＭｎ２Ｏ６／Ｒ０．５Ｂａ０．５ＭｎＯ３の構造と物性

The structures and electromagnetic properties of A-site ordered/disordered perovskite manganites, RBaMn2O6/R0.5Ba0.5MnO3(R=Y and rare earth elements), have been investigated. RBaMn2O6 exhibits remarkable features; (1) the charge/orbital order (CO) transition at relatively higher temperatures far above 300K, (2) a new stacking variation of the CE-type CO with a 4-fold periodicity along the c-axis, (3) the presence of structural transition possibly accompanied by dx2-y2 orbital order, (4) electronic phase separation. These novel structural and electromagnetic properties are discussed in terms of the structural characteristic that the MnO2 square sub-lattice is sandwiched by two types of rock-salt layers, RO and BaO with different lattice sizes, which introduces a strong frustration to the MnO2 sub-lattice. In R0.5Ba0.5MnO3 with a primitive cubic perovskite cell, on the other hand, the magnetic glassy states are dominant as the ground state. A peculiar behavior, the steplike ultrasharp magnetization and resistivity changes, has been observed in Pr0.5Ba0.5MnO3.

Random potential effect near the bicritical region in perovskite manganites as revealed by comparison with the ordered perovskite analogs.

The orbital-charge-spin ordering phase diagram for half-doped perovskites Ln(1/2)Ba(1/2)MnO3 (Ln = rare earth) with ordered Ln/Ba cations has been investigated comparatively with that of the Ln/Ba solid-solution analogs. A large modification of the phase diagram is observed upon the A-site disordering near the original bicritical point between the charge-orbital ordering and ferromagnetic metallic phases. The random potential by quenched disorder inherent in the A-site solid solution is found to suppress the respective long-range orders and gives rise to the colossal magnetoresistive state.

Structure, magnetic properties and phase separation of [formula omitted

Structure and magnetic properties of Nd 0.5Ca 0.5Mn 1− x Ga x O 3 (0⩽ x⩽0.1) were investigated systematically through X-ray diffraction, magnetization and electron spin resonance (ESR) measurements. It was found that substituting Mn with Ga reduces the charge ordering temperature ( T CO) and induces an additional peak in magnetization–temperature curve at low temperature. Both the temperature and magnetic field dependence of the magnetization in zero-field-cooling and field-cooling modes suggest that, in the Ga-doped samples, spin-glass-like phase induced by Ga-doping and antiferromagnetic charge-orbital-ordered (COO) phase coexist at low temperature. Furthermore, the ESR spectra indicate that, when the temperature is between the Neel temperature T N and the T CO, the paramagnetic phase and antiferromagnetic COO phase coexist and the volume fraction of the former increases with temperature at the expense of the latter. In addition, magnetization evidences for the interaction between Nd and Mn ions are present.

New Type of Charge/Orbital Ordering above Room Temperature in the Perovskite Bi2/3Sr1/3MnO3

A new type of charge ordering stable up to T 1 500 K has been evidenced by electron microscopy in the Mn 3+ -rich perovskite Bi 2/3 Sr 1/3 MnO 3 . This new superstructure, based on a Imma-type subcell is characterized by a double modulation with a = 2a p √2, b = 4a p , c = a p √2. X-ray powder diffraction evidences a second structural transition at T 2 580 K and suggests a displacive modulation of the oxygen atoms below T 2 . These results are interpreted as the correlated coexistence of charge-orbital ordering and modulated displacive ordering of oxygen atoms, in the form of double Mn stripes, below 500 K.

Bicritical features of the metal-insulator transition in bandwidth-controlled manganites: Single crystals ofPr1−x(Ca1−ySry)xMnO3

The phase change from a charge- and orbital-ordered insulator to a ferromagnetic metal has been investigated for single crystals of ${\mathrm{Pr}}_{0.55}({\mathrm{Ca}}_{1\ensuremath{-}y}{\mathrm{Sr}}_{y}{)}_{0.45}{\mathrm{MnO}}_{3}$ with controlled one-electron bandwidth. At $0&lt;~y&lt;~0.2,$ the ground state is the charge- and orbital-ordered insulator, while for $y&gt;~0.25$ it changes to a ferromagnetic metal. At around $y=0.25,$ the critical temperatures for the charge-orbital ordering and the ferromagnetic transition coincide with each other (\ensuremath{\approx}200 K), forming the bicritical point in the electronic phase diagram. Around this insulator-metal phase boundary, prototypical colossal magnetoresistance emerges in the metallic regime, while the charge-orbital ordering is melted by a relatively low magnetic field of a few tesla in the charge- and orbital-ordered insulating regime. The phase diagrams of ${\mathrm{Pr}}_{1\ensuremath{-}x}({\mathrm{Ca}}_{1\ensuremath{-}y}{\mathrm{Sr}}_{y}{)}_{x}{\mathrm{MnO}}_{3}$ $(0&lt;~y&lt;~1)$ with various doping levels of $x=0.35,$ 0.45, and 0.5 indicate that such a competition between the charge-orbital ordering and the ferromagnetic metal is critically dependent on the doping level. In particular, the commensurate doping level $(x=0.5)$ tends to stabilize the charge- and orbital-ordered state even beyond the bicritical point in the lower-temperature side of the ferromagnetic phase.

Field and pressure effects on correlations associated with charge-orbital ordering in manganites

Field and pressure effects on correlations associated with charge-orbital ordering in manganites