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We here report on the study of the crystallographic and magnetic properties of layered perovskites Ca3Mn2-xRuxO7 (x≤0.9). We observe a solid solution between Mn and Ru atoms in the whole series and all samples present the same orthorhombic structure independently of the Ru content. Different magnetic structures, depending on the Ru content in the sample, have been determined using neutron powder diffraction. For low Ru doping (x≤0.1), there is a dominant G-type antiferromagnetic ordering in the perovskite bilayers but, differently from undoped Ca3Mn2O7, the magnetic moments are located on the ab plane. For higher Ru concentration (x≥0.3), the predominant G-type ordering is preserved along the y axis while an A-type component is developed along the x axis and its intensity increases as Ru content does. This component is characterized by a ferromagnetic ordering in the a direction of one of the Mn(Ru)O6 layers, coupled antiferromagnetically with the neighbor Mn(Ru)O6 layer within the same bilayer. The study of the macroscopic magnetic properties shows that ferromagneticlike correlations are enhanced with increasing Ru content as deduced from the shift to higher temperature of the onset of the magnetic transition temperature. The magnetic transitions take place in two steps. At higher temperatures (140-200 K), short-range magnetic correlations are established. Tiny spontaneous magnetization is observed in the hysteresis loops with small coercive field. At TN≈115-125K, long-range antiferromagnetic ordering is developed. The ferromagnetic component remains with a strong increase of coercivity. We discuss in the paper the possible origins of this ferromagnetic contribution., The authors would like to acknowledge the use of Servicio General de Apoyo a la Investigacion from Universidad de Zaragoza. For financial support, we thank the Spanish Ministerio de Economia y Competitividad [Projects No. RTI2018098537-B-C22 and No. RTI2018-098537-B-C21, cofunded by ERDF from EU, the "Severo Ochoa" Programme (FUNFUTURE (CEX2019-000917) and Diputacion General de Aragon (Project No. E12-17R, RASMIA). We thank the ILL for the beamtime allocation under Experiment No. 5-31-2788., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

The Pr0.50Sr0.50CoO3 perovskite exhibits unique magnetostructural properties among the rest of ferromagnetic (FM)/metallic Ln0.50Sr0.50CoO3 compounds. The sudden orthorhombic-tetragonal (Imma→I4/mcm) structural transition produces an unusual magnetic behavior vs temperature and external magnetic fields. The symmetry change is responsible for a spontaneous spin rotation in this metallic oxide. We have studied half-doped Ln0.50(Sr1−xAx)0.50CoO3 cobaltites varying the ionic radius rA of A-site cations (divalent cations and lanthanides) to complete the T-rA phase diagram. The influence of the structural distortion and the A-cation size for the occurrence of a spontaneous spin reorientation in the metallic state has been investigated. As the magnetization reorientation is driven by the temperature-induced collapse of the orthorhombic distortion, a careful investigation of the structural symmetry is presented, varying the structural distortion of the Sr-rich half-doped cobaltites by means of both compositional and temperature changes. The region in the phase diagram of these FM/metallic cobaltites where Fm′m′m magnetic symmetry replaces Im′m′a was determined. In that region, the magnetization direction has rotated 45 ° within the a−b plane from the former to the latter., We acknowledge financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MINCIU), through Projects No. RTI2018-098537-B-C21 and No. RTI2018-098537-B-C22, cofunded by ERDF from the EU, and “Severo Ochoa” Programme for Centres of Excellence in R&D [FUNFUTURE (CEX2019-000917-S)]. X.Z. was financially supported by China Scholarship Council (CSC) with No. 201706080017. A.R. and X.Z.’s work was done as a part of the Ph.D. program in Materials Science at Universitat Autònoma de Barcelona. We also acknowledge ALBA (2014071047, 2015091512), ILL, and D1B-CRG (MINCIU) for provision of beam time (CRG-D1B-14-293, CRG-2149-142, 5-31-1963).

We report a comprehensive study on the magnetic ground state of La1.5Ca0.5CoO4 combining single crystal neutron diffraction and resonant magnetic x-ray scattering at the CoL2,3 edges. Three single-crystal samples obtained from the same boule were investigated exhibiting magnetic phase transitions from a high-temperature paramagnetic phase to an antiferromagnetic phase at TN≈52K. Single crystal neutron diffraction reveals that the crystal structure at room temperature shows an orthorhombic A-centered lattice but with a and b axes almost equal in length. The structural phase transition (charge-ordering-like) from the parent tetragonal cell takes place above 523 K into the space group A2mm where two nonequivalent compressed and expanded CoO6 octahedra are ordered showing a checkerboard pattern in the ab plane. The charge segregation between the nonequivalent Co sites is about 0.4(1) electrons. Resonant magnetic x-ray reflections indexed as (1/4,1/4,0)t, (1/4,1/4,1)t, and (1/4,1/4,1/2)t in the parent tetragonal cell were observed at low temperature at the CoL2,3-edge energy range. The resonant spectral shape, with a noticeable absence of any resonant enhancement at the CoL2 edge, indicates that only Co2+-like ions participate in the magnetic ordering. The polarization analysis discloses that the orientation of Co magnetic moments is the same for the three magnetic orders and they are long-range ordered along the diagonal in the ab plane of the parent tetragonal cell with a slight tilt in the c axis. Despite the onset temperatures for the three resonant magnetic reflections being the same, ≈55K, different thermal behavior is observed between (1/4,1/4,1/2)t and (1/4,1/4,L)t (L=integer) reflections whose intensities maximize at different temperatures, suggesting the coexistence of two magnetic arrangements. Moreover, the intensity of the (1/4,1/4,1/2)t magnetic reflection is at least ten times larger than that of the (1/4,1/4,L) t (L=integer) ones. On the other hand\, neutron diffraction measurements only detect a single type of antiferromagnetic ordering following the propagation vector k=(1/4,1/4,1/2)t that involves half of the Co atoms in the unit cell. We conclude that the bulk magnetic order in La1.5Ca0.5CoO4 corresponds then to this propagation vector k=(1/4,1/4,1/2)t while (1/4,1/4,0)t and (1/4,1/4,1)t magnetic reflections correspond to a minority magnetic phase that must be due to changes in the oxygen stoichiometry near the surface., For financial support, we thank the Spanish Ministerio de Ciencia, Innovación y Universidades (Project No. RTI2018-098537-B-C22 co-funded by ERDF from EU) and Diputación General de Aragón (Project No. E12-20R).

We present a comprehensive structural study of the charge-orbital ordering and magnetic phase transitions observed in the A-site ordered SmBa Mn2 O6 perovskite combining synchrotron radiation x-ray powder diffraction and symmetry-adapted modes analysis. In SmBa Mn2 O6, successive phase transitions in charge, spin, and lattice degrees of freedom take place with decreasing temperature at TCO1≈380K, TCO2≈190K, and TN≈250K. The main difference between the two charge-ordered phases concerns the stacking sequence along the c axis, which is double for the high temperature charge-ordered phase and has led to controversy in the literature. We show that both charge-ordered phases are pseudosymmetric with respect to the ideal undistorted tetragonal structure of A-site ordered RBa Mn2O6 perovskites and lead to two nonequivalent Mn sites. However, the charge segregation stabilizes at about 0.35e− in the low temperature charge-ordered phase, clearly below the nominal separation of one charge unit between Mn3+ and Mn4+ and undergoes a prominent increase in the high temperature charge-ordered phase when warming above ≈250K. The two Mn sites are anisotropic in both charge-ordered phases but the analysis of the active modes discloses that only the low temperature charge-ordered phase displays a Jahn-Teller-like distortion for one of the Mn sites. In addition, this low temperature charge-ordered phase has polar symmetry compatible with ferroelectricity along the a axis., For financial support, we thank the Spanish Ministerio de Ciencia, Innovación y Universidades (Projects No. RTI2018-098537-B-C22 and No. RTI2018-098537-B-C21 cofunded by ERDF from EU, and Severo Ochoa FUNFUTURE, CEX2019-000917-S) and Diputación General de Aragón (Project No. E12-20R).

We report here a structural study of RBaMn2O6 (R = La, Pr, and Nd) compounds by means of synchrotron radiation x-ray powder diffraction and Raman spectroscopy. The three compounds are A-site ordered perovskites adopting the prototypical tetragonal structure at high temperature. A ferromagnetic transition is observed in the LaBaMn2O6 sample and the lattice parameters undergo anisotropic changes at TC related to the orientation of the magnetic moments. Both PrBaMn2O6 and NdBaMn2O6 have a structural transition coupled to an electronic localization and an antiferromagnetic transition. In both cases, the x-ray diffraction patterns reveal that the lowtemperature phase is orthorhombic with lattice parameters a + b, b − a, and c with respect to the tetragonal phase. Two possible solutions belonging to the space groups Pmam and P21am can yield accurate refinements of the x-ray patterns. However, the active modes in the low-temperature phase disclosed by the Raman spectroscopy clearly point to the noncentrosymmetric space group, P21am. The symmetry analysis of this transition unveils that the primary modes belong to the irreducible representations M5− and GM5− and the main distortions correspond to rotations of the MnO6 octahedra and an asymmetric combination of stretching and scissoring modes of the basal oxygens in these octahedra. We conclude that the low-temperature phase is polar and the main contribution comes from the displacement of oxygen atoms from their centrosymmetric positions. However, negligible contribution from the asymmetric stretching associated with a Jahn-Teller distortion is found in this structural transition, suggesting the lack of ferroic orbital ordering of eg (3dx2−y2 ) orbitals in the orthorhombic ab plane. There is only one inequivalent site for the Mn atom in the low-temperature polar phase so charge ordering cannot account for the electronic localization having a structural origin., The authors would like to acknowledge the Servicio General de Apoyo a la Investigación from Universidad de Zaragoza. Granted beam time at ALBA synchrotron is appreciated (Proposal No. 2018093038). For financial support, we thank the Spanish Ministerio de Ciencia, Innovación y Universidades (Projects No. RTI2018-098537-B-C22 and No. RTI2018-098537-B-C21 cofunded by ERDF from EU, and Severo Ochoa FUNFUTURE, CEX2019-000917-S) and Diputación General de Aragón (Project No. E12-20R).

The Pr-based cobaltate Pr0.5Ca0.5CoO3 (PCCO) presents in bulk form a singular simultaneous valence and spin-state transition that turns the metallic state into insulator, and displays a large and ultrafast photoresponse in the insulating phase. Epitaxial thin films of PCCO have been grown by deposition on LaAlO3(001) (LAO) substrate, chosen to minimize the mismatch with the film. The films grow epitaxially with two times the substrate periodicity (2a0×2a0×2a0) and the long perovskite axis perpendicular to the surface. We report a reduction of the structural symmetry from Pnma (bulk) to P212121 (film). The P212121 symmetry revealed by synchrotron x ray remains at low temperatures. These PCCO films are metallic, and ferromagnetic below TC=170K, confirming the stabilization of excited Co3+ spin states and the suppression of the concurrent Co spin-state, valence, and metal-insulator transitions. Z-contrast imaging and electron-energy-loss spectroscopy elemental maps reveal long-range ordered oxygen vacancy planes unexpectedly stacking parallel to the interface, in spite of the tensile character of the PCCO/LAO heterostructure. In contrast to the general tendency reported for strained La0.5Sr0.5CoO3−δ (LSCO) films, we show that a nominal tensile strain can be also compatible with the presence of alternating O vacancy planes parallel to the interface, instead of perpendicular to it. That occurs thanks to the double cell of the film and the formation of the (1/2, 1/2, 1/2) superstructure studied in this work. These results expand the possibilities of controlling interfacial physical properties via engineering of ordered oxygen-defect structures in strongly correlated oxides., We acknowledge financial support from the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN), through Projects No. MAT2015-686760-02-2-P, No. RTI2018-098537-B-C21, and No. RTI2018-098537-BC22, and “Severo Ochoa” Programme for Centres of Excellence in R&D (Grant No. SEV-2015-0496). The former are cofunded by ERDF of European Union, as Project No. EFA194/16 TNSI (POCTEFA/UE-FEDER). We thank also the Spanish MICINN and the Consejo Superior de Investigaciones Científicas for financial support and for provision of synchrotron radiation facilities (Grant No. CRG-BM25B, ESRF). J.G. acknowledges the Ramon y Cajal program (Grant No. RYC-2012-11709). Research at ORNL (STEM-EELS) was supported by the US Department of Energy (DOE), Basic Energy Sciences (BES), Materials Sciences and Engineering Division (SJP and MV).

We propose a model for the Co charge disproportionation in La2−xCaxCoO4±δ (0.4≤x≤0.7) and La1.5Sr0.5CoO4 layered cobaltites based on the photon energy, azimuthal angle, and polarization dependence of the scattered intensity of selected reflections in a resonant x-ray scattering experiment. Tetragonal superlattice (h/2,h/2,l)-type reflections were detected at room temperature for all the samples independently from the Ca (or Sr) doping rate in agreement with a checkerboard ordering of two different Co sites. The corresponding average charge disproportion is accounted for by a semiempirical model from which the imaginary part of the resonant atomic scattering factors of each of the two Co atoms is obtained resulting in about 0.5±0.1 electron. Moreover, no forbidden (h/4,h/4,l) -type reflections were observed at room temperature indicating the lack of any local anisotropy ordering in contrast with the behavior found in the related layered manganites. Finally, we found the pattern of small distortions (∼0.05Å) around the two Co sites compatible with the resonant x-ray scattering results. The symmetry of this displacement pattern is consistent with the A2mm (or Ammm) orthorhombic structure of the ordered phase and the structural transition is accounted for by the condensation of two soft modes—X1+(B2u) and X1+(A1)—acting on the oxygen atoms., For financial support, we thank the Spanish Ministerio de Economía y Competitividad Project No. MAT2015-68760-C2-1-P and Diputación General de Aragón Project No. E69.

We present a detailed study on the magnetic structure and magnetoelectric properties of several double perovskites R2CoMnO6(R=Ho, Tm, Yb, and Lu). All of these samples show an almost perfect (∼94%) ordering of Co2+ and Mn4+ cations in the unit cell. Our research reveals that the magnetic ground state strongly depends on the R size. For samples with larger R (Ho and Tm), the ground state is formed by a ferromagnetic order (F type) of Co2+ and Mn4+ moments, while R either remains mainly disordered (Ho) or is coupled antiferromagnetically (Tm) to the Co/Mn sublattice. For samples with smaller R (Yb or Lu), competitive interactions lead to the formation of an E-type arrangement for the Co2+ and Mn4+ moments with a large amount of extended defects such as stacking faults. The Yb3+ is partly ordered at very low temperature. The latter samples undergo a metamagnetic transition from the E into the F type, which is coupled to a negative magnetodielectric effect. Actually, the real part of dielectric permittivity shows an anomaly at the magnetic transition for the samples exhibiting an E-type order. This anomaly is absent in samples with F-type order, and, accordingly, it vanishes coupled to the metamagnetic transition for R=Yb or Lu samples. At room temperature, the huge values of the dielectric constant reveal the presence of Maxwell-Wagner depletion layers. Pyroelectric measurements reveal a high polarization at low temperature, but the onset of pyroelectric current is neither correlated to the kind of magnetic ordering nor to the magnetic transition. Our study identified the pyroelectric current as thermally stimulated depolarization current and electric-field polarization curves show a linear behavior at low temperature. Therefore, no clear ferroelectric transition occurs in these compounds., We acknowledge ILL and D1B-CRG Ministerio de Economía y Competitividad (MINECO) for beam time allocation (experiment codes CRG-2419, CRG-2154, and 5-31-2233) and the use of Servicio General de Apoyo a la Investigación (SAI), Universidad de Zaragoza. We are thankful for financial support from Spanish MINECO Projects No. MAT2015-68760-C2-1 and No. MAT2015- 68760-C2-2, cofunded by European Regional Development Fund (ERDF) from the European Union, the Severo Ochoa Programme (Grant No. SEV-2015-0946), and DGA (Grant No. E69-CAMRADS).

In half-doped Pr0.50A0.50CoO3 metallic perovskites, the spin-lattice coupling brings about distinct magnetostructural transitions for A=Ca and A=Sr at temperatures close to ∼100 K. However, the ground magnetic properties of Pr0.50Sr0.50CoO3 (PSCO) strongly differ from Pr0.50Ca0.50CoO3 ones, where a partial Pr3+ to Pr4+ valence shift and Co spin transition makes the system insulating below the transition. This paper investigates and describes the relationship between the Imma→I4/mcm symmetry change [Padilla-Pantoja, García-Muñoz, Bozzo, Jirák, and Herrero-Martín, Inorg. Chem. 53, 12297 (2014)] and the original magnetic behavior of PSCO versus temperature and external magnetic fields. The FM1 and FM2 ferromagnetic phases, above and below the magnetostructural transition (TS1∼120K) have been investigated. The FM2 phase of PSCO is composed of [100] FM domains, with magnetic symmetry Im′m′a (mx≠0, mz=0). The magnetic space group of the FM1 phase is Fm′m′m (with mx=my). Neutron data analyses in combination with magnetometry and earlier reports results agrees with a reorientation of the magnetization axis by 45 within the ab plane across the transition, in which the system retains its metallic character. The presence below TS1 of conjugated magnetic domains, both of Fm′m′m symmetry but having perpendicular spin orientations along the diagonals in the xy plane of the tetragonal unit cell, is at the origin of the anomalies observed in the macroscopic magnetization. A relatively small field μ0H[z]30mT is able to reorient the magnetization within the ab plane, whereas a higher field (μ0H] 1.2T at 2 K) is necessary to align the Co moments perpendicular to the ab plane. Such a spin reorientation, in which the orbital and spin components of the Co moment rotate joined by 45, was not observed previously in analogous cobaltites without praseodymium., We thank the spanish Ministerio de Economía y Competitividad (MINECO) for financial support under Projects No. MAT2012-38213-C02-02 and -01 and No. MAT2015-68760-2-2-P and -1-P (cofunded by the European Regional Development Fund [ERDF] from the European Union) and No. CSD2007-00041 (NANOSELECT). J.P.-P. and J.A.R.V. thank CSIC for JAE-Pre and JAE-Doc contracts, respectively.

We have carried out an investigation on the magnetoelectric properties of the presumed multiferroic Y2CoMnO6 with different degrees of Co/Mn atomic ordering. The magnetic ground state was studied by neutron diffraction, showing a collinear ferromagnetic (FM) ordering of Co and Mn moments with a small antiferromagnetic canting. No superstructure peaks from an E-type magnetic structure were detected in our measurements. Magnetic measurements reveal FM transitions with pinned magnetic domains. The degree of Co/Mn ordering affects the Curie temperature only a little, but has strong effects on the magnetic hysteresis loops, and the FM moment signal at high field increases with increasing such order. The loops display steps at critical fields whose number and extent depends on each specimen. The most ordered sample exhibits the greatest steps ascribed to the alignment of magnetic domains separated by antiphase boundaries. All samples are insulators exhibiting low dielectric loss and dielectric constants at low temperature. On warming, they show a step increase in the real dielectric permittivity accompanied by peaks in the dielectric loss typical of thermally activated hopping processes. At room temperature, the huge values of the dielectric constant reveal the presence of Maxwell-Wagner depletion layers. Pyroelectric measurements reveal a high polarization at low temperature for these compounds that increases with increasing the Co/Mn ordering. There is no correlation between the magnetic transition and the onset of pyroelectric current. No significant changes are observed in the pyroelectric effect measured under an external magnetic field, so magnetoelectric coupling is negligible. This paper identifies the pyroelectric current as thermally stimulated depolarization current ascribed to the reorientation of defect dipoles with activation energy of about 0.05 eV. Therefore, no ferroelectric transition occurs in these compounds, discarding the existence of intrinsic magnetoelectric multiferroicity., For financial support we thank the Spanish Ministerio de Economía y Competitividad (MINECO) (Projects No. MAT2012-38213-C02-01 and -02 and No. MAT2015-68760-C1-1 and -2-P, cofunded by the European Regional Development Fund [ERDF] from the European Union) and Diputación General de Aragón (DGA, project E-69). J.A. Rodríguez-Velamazan acknowledges CSIC for the JAEdoc contract.

We have investigated the role of Mn3+ Jahn-Teller distortion in driving ferromagnetism in the LaMn1-xScxO3 series. The replacement of Mn by Sc in LaMnO3 decreases the orthorhombic distortion of the Pbnm cell, but the unit cell remains distorted even in the LaScO3 sample. The analysis of the x-ray diffraction patterns indicates a continuous evolution from the typical Jahn-Teller distorted octahedron in LaMnO3 into a nearly regular one in LaScO3. Surprisingly, x-ray absorption spectroscopy measurements at the Mn and Sc K edges reveal the stability of both Jahn-Teller distorted MnO6 octahedron and nearly regular ScO6 octahedron along the whole Sc-substituted series. Moreover, the structure is described as a random distribution of Jahn-Teller distorted MnO6 octahedra spatially ordered as in LaMnO3 and nearly regular ScO6 octahedra. This result contrasts with the pseudocubic phase and the appearance of regular MnO6 octahedra in LaMn1-xGaxO3 for x>0.5. Thus the occurrence of Jahn-Teller distortion strongly depends on the distorted orthorhombic crystallographic structure of the matrix in which the Mn3+ atom is allocated. Besides, a ferromagnetic ground state is observed for x>0.5 in both series independently of the presence (or not) of Jahn-Teller distortions around Mn3+, which discards either the spin flipping or the vibronic superexchange models proposed for the ferromagnetism in LaMn1-xBxO3 (B=Sc or Ga)., The authors thank ESRF, ALBA, and Elettra Synchrotrons for granting beam time. Financial support from the Spanish MINECO (Projects No. MAT2012-38213-C02-01 and No. MAT2011-23791) and the Diputacion General de Aragón (CAMRADS) are acknowledged.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We report a complete characterization of the crystal structure between 400 and 80 K for RFe2O4 (R = Rm, Yb, and Lu) compounds using high resolution x-ray synchrotron powder diffraction. The three samples have a hexagonal structure (space group R3¯m) characterized by a sequence of double layers of mixed valence iron and oxygen atoms forming two-dimensional triangular layers separated by a single R-O layer along the c axis. This structure is stable down to 80 K for TmFe2O4 and YbFe2O4 though a sudden expansion in the c axis is observed at around 300 K coupled to a variation in the electrical properties. However, LuFe2O4 exhibits two structural transitions upon cooling. The splitting of some reflections and the occurrence of superstructure peaks below 320 K reveal a structural phase transition. The unit cell is monoclinic (space group C2/m), and there are four nonequivalent Fe sites with a maximum charge disproportionation of 0.5 e. The hexagonal to monoclinic transition is characterized by a sudden expansion of the c axis on cooling, and it seems to be driven by the condensation of Y2 modes. At lower temperatures (~170 K) additional splitting of several peaks indicate that the unit cell is no longer monoclinic but triclinic (space group P1¯). This transition is characterized by a contraction of the monoclinic ab plane, while the c axis remains almost unchanged. There are six nonequivalent Fe sites in the triclinic cell, and the charge disproportionation magnitude is little affected., Financial support from the Spanish Ministerio de Economía y Competitividad (Project No.MAT2012-38213-C02-01), and Diputación General de Aragón (DGA) to Caracterización de Materiales con Radiación de Sincrotrón (CAMRADS) group is acknowledged. S. Lafuerza thanks DGA for the research grant.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We here report an investigation of the electronic states in the RFe2O4(R=Lu,Yb,Tm,Y) mixed-valence ferrites by means of soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements together with ab initio theoretical calculations. The presence of Fe+2 and Fe+3 pure ionic species is discarded in the XAS spectra at the OK edge in both experimental data and simulations based on the multiple scattering theory. Similarly, no trace of Fe+2/Fe+3 contributions is detected in the XMCD spectra at the FeK edge. On the other hand, the XAS and XMCD spectra at the FeL2,3 edges can be well described in terms of Fe+2/Fe+3 contributions, and are also supported by multiplet calculations. This finding can be interpreted as the existence of a mixture of 3d5/3d6 configurations at the Fe atoms. Alternative ferrimagnetic spin orderings based on a trimodal Fe valence distribution are also proposed and discussed. Finally, a possible explanation for the strong dependence of the FeL2,3 edges XMCD signal magnitude on both the sample surface preparation and detection method is presented., Financial support from the Spanish MINECO (Project No. MAT2012-38213-C02-01) and Diputación General de Aragón (DGA-CAMRADS) is acknowledged.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We present here an x-ray absorption study of the RFe2O4 series (LuFe2O4, YFe2O4, YbFe2O4, and LuCoFeO4) at the Fe K-edge. Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectra were measured at temperatures ranging from 100 K to 390 K crossing the charge ordering (CO) transition temperatures on both isotropic and oriented powder samples. Unpolarized and polarized x-ray absorption spectra with the x-ray polarization parallel and perpendicular to the hexagonal c axis were obtained separately. The XANES spectra show almost no dependence with either polarization or temperature. This indicates that, in contrast to its average crystallographic structure, the local electronic and geometrical state of the Fe atom is barely anisotropic, and it remains the same above and below the proposed CO. Moreover, the linear combination of two spectra corresponding to the pure ionic states Fe2+ and Fe3+ does not fit to the experimental XANES of either LuFe2O4, YFe2O4, or YbFe2O4, discarding total ionic segregation in favor of an intermediate mixed valence state. The maximum charge disproportionation compatible with the XANES spectra is 0.5 ± 0.1 electrons in the whole temperature range. The polarized EXAFS spectra have allowed us to analyze the local structure separating the different contributions mixed up in the polycrystal. Best-fit results show that the five oxygen atoms of the first coordination shell are at nearly the same distance but with a large Debye-Waller factor. Neither the interatomic distances nor the Debye-Waller factors of the first oxygen coordination shell change with temperature, indicating that the dynamical structural distortion of the high-temperature symmetric hexagonal phase freezes upon cooling down. Thus, the local structure instability of the mixed valence is in the origin of the structural transitions. © 2014 American Physical Society., Financial support from the Spanish MINECO (Project No. MAT2012-38213-C02-01) and Diputación General de Aragón (DGA-CAMRADS) are gratefully acknowledged.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., We have investigated the low-temperature phases of LuFe2O4 by resonant x-ray scattering (RXS) at the Fe K edge to determine both the ordering sequence and magnitude of charge segregation. Two successive charge ordering (CO) phases have been detected. Resonant superlattice (1/3,1/3,l/2) reflections appear below the so-called CO phase at TCO ¿ 320 K. Additionally, resonant superlattice (1/3,1/3,l) reflections are observed below 240 K concurrent with the onset of the magnetic ordering. The ¿-¿¿ polarization dependence for all the measured superlattice reflections indicates the absence of local anisotropy of the electronic density at the Fe atom. The energy dependence of the resonant intensity for these reflections has been quantitatively analyzed following the monoclinic C2/m structure in the CO phase between 320 and 240 K and the triclinic P1¯ structure below 240 K. We find a four-modal charge segregation among the Fe atoms in the C2/m phase with formal valences Fe2.77+,Fe2.63+,Fe2.36+, and Fe2.22+ whereas the simplest charge distribution that explains successfully all the RXS data in the P1¯ phase is the trimodal Fe2.8+,Fe2.5+, and Fe2.2+. Both ordering models imply the lack of charge segregation along the c axis discarding a polar configuration and thus the occurrence of ferroelectricity., Financial support from the Spanish MINECO (Project No. MAT2012-38213-C02-01) and DGA (CAMRADS) is acknowledged.

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY)., Room temperature Fe and Co K-edge x-ray magnetic circular dichroism, synchrotron x-ray powder diffraction, and magnetization measurements were carried out to investigate the stability of the ferrimagnetic ground state in CoxFe3-xO4 (x=0, 1, 1.5, and 2) ferrites under pressure up to about 30 GPa using diamond anvil cells. The x-ray magnetic circular dichroism at the Fe K-edge is observed to decrease continuously up to the highest reached pressure by ∼50% in Fe3O4 (at 25 GPa) and by ∼70% in Co1.5Fe1.5O4 (at 16 GPa) and Co2FeO4 (at 21 GPa). In CoFe2O 4, the suppression by ∼80% of the x-ray magnetic circular dichroic signal occurs simultaneously at both Fe and Co K-edges between 24 and 27 GPa. However, a continuous decrease of the dichroic signal with pressure is only observed at the Co K-edge, whereas the Fe K-edge dichroic intensity drops drastically. The synchrotron x-ray powder diffraction measurements indicate the occurrence of structural phase transitions at critical pressures in coincidence with the suppression of ferrimagnetism. In CoFe2O4, bulk magnetization measurements up to 26 GPa confirm the disappearance of ferrimagnetism and indicate an almost linear dependence of the magnetization with the magnetic field in the high pressure phase. We thus conclude that high-pressure CoFe2O4 is either paramagnetic or antiferromagnetic. © 2013 American Physical Society., This research was financed by the Spanish MICINN (Project No. MAT2012-38213-C02-01) and DGA (CAMRADS).

We have studied the incommensurate-ordered phase in overdoped La 0.4Sr 1.6MnO 4 by resonant x-ray diffraction at the Mn K edge. Weak resonant superlattice (h±0.2h±0.20) and (h±0.4h±0.40) reflections of the tetragonal structure were found below ∼240K. The energy, azimuth angle, and polarization dependencies of the resonant scattering have revealed sinusoidal modulations of the oxygen motions that are transverse and longitudinal to the tetragonal [110] direction. This result discards (Mn3 +,Mn4 +)-like stripe-type order but point to a charge-density-modulation picture. © 2012 American Physical Society., Financial support from the Spanish MICINN (project FIS08-03951 and MAT2007-61621) and DGA (CAMRADS) is acknowledged. J. H.-M. thanks CSIC for the JAEdoc contract, which is cofinanced by the European Social Fund.

13 páginas, 10 figuras, 4 tablas.-- Trabajo presentado como póster al 21st ESRF Users' Meeting celebrado en Grenoble del 7 al 10 de Febrero del 2011., The semiconductor-insulator phase transition of the single-layer manganite La0.5Sr1.5MnO4 has been studied by means of high-resolution synchrotron x-ray powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude that a concomitant structural transition from tetragonal I4/mmm to orthorhombic Cmcm phases drives this electronic transition. A detailed symmetry-mode analysis reveals that condensation of three soft modes― Δ2(B2u), X1 +(B2u) and X1 +(A)―acting on the oxygen atoms accounts for the structural transformation. The Δ2 mode leads to a pseudo Jahn-Teller distortion (in the orthorhombic bc plane only) on one Mn site (Mn1), whereas the two X1 + modes produce an overall contraction of the other Mn site (Mn2) and expansion of the Mn1 one. The X1 + modes are responsible for the tetragonal superlattice (1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two different Mn sites. A strong enhancement of the scattered intensity has been observed for these superlattice reflections close to the Mn K edge, which could be ascribed to some degree of charge disproportion between the two Mn sites of about 0.15 electrons. We also found that the local geometrical anisotropy of the Mn1 atoms and its ordering originated by the condensed Δ2 mode alone perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type reflections without invoking any orbital ordering., This work has been financially supported by the Spanish MICINN (project FIS08-03951) and D.G.A. (CAMRADS project).

6 páginas, 4 figuras.-- PACS number(s): 71.30.+h, 78.70.Dm, 75.30.Wx, 75.25.Dk.-- et al., X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 were performed at the Pr M4,5, Pr L3, and Ca L2,3 absorption edges as a function of temperature below 300 K. Ca spectra show no changes down to 10 K while a noticeable thermally dependent evolution takes place at the Pr edges across the metal-insulator transition. Spectral changes are analyzed by different methods, including multiple scattering simulations, which provide quantitative details on an electron loss at Pr 4f orbitals. We conclude that in the insulating phase a fraction [15(±5)%] of Pr3+ undergoes a further oxidation to adopt a hybridized configuration composed of an admixture of atomiclike 4f1 states (Pr4+) and f-symmetry states on the O 2p valence band (Pr3+L̲ states) indicative of a strong 4f-2p interaction., We acknowledge financial support from MICINN (Spanish government) under Projects No. MAT2009-09308 and Nanoselect Project No. CSD2007-00041. We acknowledge ILL (and the CRG-D1B), and HZB for the provision of beam time. The ALICE diffractometer is funded through the BMBF under Contract No. 05KS7PC1. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 226716. J.H.M. thanks CSIC for JAEdoc contract.

6 páginas, 5 figuras, 1 tabla.-- PACS number(s): 71.30.+h, 75.25.Dk, 75.30.Wx, 78.70.Dm.-- et al., X-ray absorption spectroscopy measurements in Pr0.5Ca0.5CoO3 and (Pr,Y)0.55Ca0.45CoO3 compositions reveal that the valence of praseodymium ions is stable and essentially +3 (Pr [4f2]) in the metallic state, but abruptly changes when carriers localize approaching the oxidation state +4 (Pr [4f1]). This mechanism appears to be the driving force of the metal-insulator transition. The ground insulating state of Pr0.5Ca0.5CoO3 is a homogeneous Co3.5-δ state stabilized by a charge transfer from Pr to Co sites: 1/2Pr3++Co3.5→1/2Pr3+2δ+Co3.5-δ, with 2δ ≈ 0.26e-., Financial support from MICINN (Spanish government) under Projects No. MAT2006-11080-C02-02 and No. MAT2009-09308, and NANOSELECT under Project No. CSD2007-00041 is acknowledged. The ALICE diffractometer is funded through the BMBF under Contract No. 05KS7PC1. The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 226716

8 páginas, 7 figuras.-- PACS number(s): 71.28.+d, 78.70.En, 71.10.-w, 75.47.Lx.-- et al., The doping dependence of the occupied 3d states at the Mn sites of La1−xSr1+xMnO4 (x=0, 0.3, and 0.5) was studied by means of nonresonant hard x-ray emission spectroscopy. For the single-crystalline samples, we observe a linear dichroism of the Mn Kβ main lines (3p to 1s transitions) that is strongest for x=0 and decreases with increasing hole doping x. At the same time, the Mn Kβ main lines of polycrystalline samples, i.e., the angular-integrated spectra, remain almost unchanged upon hole doping. The linear dichroism in the single crystals and the invariance of the Kβ lines in polycrystals lead to the conclusion that hole doping causes a spatial redistribution of the 3d electrons without reducing the total charge and spin density at the manganese sites. This implies that the doped holes in La1−xSr1+xMnO4 must have a strong oxygen 2p character. Many-body cluster calculations are presented that support these findings., We acknowledge the financial support from Spanish MICINN, Project no. FIS2008-03951. J.G. acknowledges the German DFG through the Emmy-Noether Program, J. F.-R. the Gobierno del Principado de Asturias for the financial support from Plan de Ciencia, Tecnología e Innovación PCTI de Asturias 2006– 2009.

6 páginas, 4 figuras, 1 tabla.-- PACS number(s): 71.30.+h, 61.05.fm, 75.30.Kz, 75.47.De, Remarkable structural changes take place during the metal to insulator first-order transition in Pr0.50Ca0.50CoO3: a contraction of the unit-cell volume, the bending of the Co-O-Co bond angle, and a decrease in (Pr,Ca)-O bond distance. However, the Co-O bond length and the volume of the CoO6 octahedra remain almost unaltered during the transition. In view of these data and the absence of transition in (Nd2/3La1/3)0.5Ca0.5CoO3, we review the current spin-state transition picture, emphasizing that an active role is played by the A-site Pr cations that could be related with the effect of Pr in 123 superconducting cuprates., Financial support from MICINN (Spanish government) under Projects No. MAT2006-11080-C02-02 and No. NANOSELECT CSD2007-00041, Generalitat de Catalunya (Grant No. 2005-GRQ-00509) and FAME European Network of Excellence is acknowledged.

The crystal and local structures of La1−xSrxFeO3−δ (0⩽x⩽1) samples have been studied by x-ray diffraction and x-ray absorption spectroscopy techniques. The Fe-O bond length decreases with increasing x. Accordingly, the x-ray absorption near edge spectroscopy (XANES) spectra reveal a chemical shift of the iron K edge to higher energies. Both results agree with an Fe valence increase as La is substituted with Sr. Extended x-ray absorption fine structure spectroscopy and XANES show that the chemical state of Fe atoms in intermediate compositions can be described either by a bimodal distribution of formal Fe3+ and Fe4+ ions or by an Fe3.x+ intermediate valence. The large value of the Debye-Waller factors obtained for intermediate compositions indicates that hole doping produces local disorder around the Fe ions. These factors show unusually large values below the metal-insulator (MI) transition for x=2∕3 or 3∕4. We show that a significant charge disproportionation of the type 2Fe4+→Fe3++Fe5+ cannot account for the local structure observed below the MI transition temperature of these samples. We suggest that an electronic localization arises from an order-disorder transition between dynamic and static distortions, resulting in the opening of a gap at the Fermi level., The authors acknowledge the financial support from CICyT (Project No. MAT05-04562) and DGA (CAMRADS and PIP018/2005).

12 pages, 12 figures, 2 tables.-- PACS number(s): 75.30.Kz, 75.47.Gk, 71.30.+h, We have applied three complementary neutron techniques (neutron diffraction, neutron depolarization, and small-angle neutron scattering) to investigate the crossover from long-range to short-range magnetic ordering in (Nd1−xTbx)0.55Sr0.45MnO3 manganite compounds. The obtained results indicate that the ground state at T=1.5 K is a long-range C-type canted ferromagnetic state for 0x(Tb)0.5 with ferromagnetic domains larger than several hundreds of nm that decrease as the Tb content increases. In the concentration range 0.5x(Tb)0.6, the ferromagnetic domain size decreases below ~100 nm and these large ferromagnetic clusters coexist in this concentration range with smaller short-range ferromagnetic clusters (around one nanometer size). For x(Tb)0.6, only short-range ferromagnetic and A-type and C-type antiferromagnetic clusters are present and their size is below ~5 nm. The obtained results are discussed in the context of the global phase diagram recently discussed for RE0.55Sr0.45MnO3 and in the context of the quantum critical point features recently suggested in manganites., Financial support by the Spanish Ministry of Science (through projects SAB2003-0255, MAT2002-04657, and MAT2005-05565-C02-02 including FEDER funding), by the European Project SCOOTMO and by the Aragon Regional Government (through project E26) is acknowledged.

5 pages, 4 figures, 1 table.-- PACS number(s): 61.10.Ht, 75.50.Dd, 75.50.Gg, We have investigated the local structure around the Mn atom on highly diluted manganites with formal Mn3+ and Mn3.5+ ions by x-ray absorption fluorescence spectra. Both x-ray absorption near-edge structure and extended x-ray absorption fine structure were recorded for LaMn1–xGaxO3 (x=0.8 and 0.9) and La0.9Ca0.1Mn0.2Sc0.8O3 compounds at room temperature. The local structure around the Mn atom in the gallium samples consist of a regular octahedron of oxygen atoms contrasting with the tetragonal Jahn-Teller distortion expected for a formal Mn3+ ion. This result suggests that the ordered tetragonal distortions in LaMnO3 are not originated by a single ion mechanism like the Jahn-Teller one., Supported by the Spanish CICyT Project No. MAT2005- 04562 and DGA.