Browsing by Author "Conder, K"

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    Chemical pressure effects on crystal and magnetic structures of bilayer manganites PrA2Mn2O7 (A = Sr or Ca)
    (AIP Publishing, 2016-06-03) Deng, G; Sheptyakov, D; Pomjakushin, V; Medarde, M; Pomjakushina, E; Conder, K; Kenzelmann, M; Studer, AJ; Gardner, JS; McIntyre, GJ
    The crystal and magnetic structures of the bilayer manganites PrSr2Mn2O7 (PSMO) and PrCa2Mn2O7 (PCMO) have been studied by neutron powder diffraction. It was found that PSMO crystallizes in space group I4/mmm, while PCMO adopts space group Cmc21 at room temperature. The difference in the structure arises from chemical pressure induced by the Ca substitution for Sr on the A sites, which causes different Jahn-Teller distortions. In PSMO, the MnO6 octahedra suffer a small elongated distortion, while those in PCMO adopt strong compressed distortion along the axial direction. In addition, the octahedra in PCMO show a+b0c0 rotation and a0b+c+ tilting in the Glazer notation in comparison to PSMO. As a result, these two compounds adopt very different magnetic structures: The magnetic structure of PSMO is an A-type magnetic structure (Im'm'm) with propagation vector k = (0, 0, 1) and magnetic moments in the ab plane. In contrast, a C-type antiferromagnetic magnetic structure (Cm'c2′1) with the multiple propagation vectors (k = (0, 12, 12) and (0, 12, 0)) and magnetic moments mainly along the b axis is found in PCMO. The critical exponent of the magnetic phase transition is around 0.345 for PSMO and 0.235 for PCMO, indicating 3D and 2D XY transitions, respectively. The strong Jahn-Teller distortion induced by the chemical pressure is believed to suppress the double exchange and favour super-exchange in PCMO, leading to the dramatic difference in the magnetic structure. © 2016 Author(s). Published by AIP Publishing.
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    Coexistence of long-range magnetic ordering and singlet ground state in the spin-ladder superconductor SrCa13Cu24O41
    (American Physical Society, 2013-11-26) Deng, GC; Kenzelmann, M; Danilkin, SA; Studer, AJ; Pomjakushin, V; Imperia, P; Pomjakushina, E; Conder, K
    A long-range magnetic order was discovered in the quasi-one-dimensional spin-ladder compound SrCa13Cu24O41 by susceptibility, specific heat, and neutron diffraction experiments. The temperature dependence of the magnetic Bragg peak intensity could be well fitted to the power law with a transition temperature TN = 4.23 K and a critical exponent β = 0.28, indicating a three-dimensional phase transition for a low-dimensional magnet. A computer program was coded and found two possible magnetic structure models fitting best with all the observed magnetic peaks. These models suggest the spin-ladder sublattice is magnetically ordered with Cu moments aligning along the a axis. The spin interactions are primarily antiferromagnetic along rungs and legs, while there are ferromagnetic clusters along legs. Surprisingly, the singlet-triplet spin-gap excitation is observable above and below TN, indicating a coexistence of the spin-singlet ground state and long-range magnetic ordering state in this compound. © 2013 American Physical Society.
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    Determination of hole distribution in Sr14-xCaxCu24O41 using soft x-ray absorption spectroscopy at the Cu L3 edge.
    (American Physical Society, 2013-07-22) Huang, MJ; Deng, G; Chin, YY; Hu, ZW; Cheng, JG; Chou, FC; Conder, K; Zhou, JS; Pi, TW; Goodenough, JB; Lin, HJ; Chen, CT
    The physical properties of Sr14-xCaxCu24O41 are determined by the hole distribution between the edge-shared CuO2 chain and the corner-shared Cu2O3 two-leg ladder, but inconsistent results on the hole distribution were obtained in various experimental works in recent decades. In this work we reinvestigate the hole distribution by soft x-ray absorption spectroscopy at the Cu L-3 edge. By comparing with the pure ladder and the pure chain systems, we can unambiguously distinguish between the hole distributions in the chain and the ladder. We have found that there are 5.3 holes in the edge-shared chain and 0.7 holes in the corner-shared ladder on average for Sr14Cu24O41. Upon Ca substitution, the holes gradually transfer from the edge-shared chain to the corner-shared ladder, and there are up to 1.21 holes in the corner-shared ladder for Sr1.8Ca12.2Cu24O41. © 2013, American Physical Society.
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    Direct observation of charge order and an orbital glass state in multiferroic LuFe2O4
    (American Physical Society, 2009-08-14) Mulders, AM; Lawrence, SM; Staub, U; García-Fernández, M; Scagnoli, V; Mazzoli, C; Pomjakushina, E; Conder, K; Wang, YD
    Geometrical frustration of the Fe ions in LuFe2O4 leads to intricate charge and magnetic order and a strong magnetoelectric coupling. Using resonant x-ray diffraction at the Fe K edge, the anomalous scattering factors of both Fe sites are deduced from the (h/3 k/3 l/2) reflections. The chemical shift between the two types of Fe ions equals 4.0(1) eV corresponding to full charge separation into Fe2+ and Fe3+. The polarization and azimuthal angle dependence of the superlattice reflections demonstrate the absence of differences in anisotropic scattering revealing random orientations of the Fe2+ orbitals characteristic of an orbital glass state. © 2009, American Physical Society
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    Effects of 18O isotope substitution in multiferroic RMnO3 (R=Tb, Dy)
    (Australian Institute of Physics, 2015-02-02) Graham, PJ; Narayanan, N; Reynolds, NM; Li, F; Rovillain, P; Bartkowiak, M; Hester, JR; Kimpton, JA; Yethiraj, M; Pomjakushina, E; Conder, K; Kenzelmann, M; McIntyre, GJ; Hutchison, WD; Ulrich, C
    Multiferroic materials demonstrate desirable attributes for next-generation multifunctional devices as they exhibit coexisting ferroelectric and magnetic orders. In type-II multiferroics, coupling exists that allows ferroelectricity to be manipulated via magnetic order and vice versa, offering potential in high-density information storage and sensor applications. Despite extensive investigations into the subject, questions of the physics of magnetoelectric coupling in multiferroics remain, and competing theories propose different mechanisms. The aim of this investigation was to study changes in the statics and dynamics of structural, ferroelectric and magnetic orders with oxygen-18 isotope substitution to shine light into the coupling mechanism in multiferroic RMnO3 (R=Tb, Dy) systems. We have performed Raman spectroscopy on 16O and 18O-substituted TbMnO3 single crystals. Oxygen-18 isotope substitution reduces all phonon frequencies significantly. However, specific heat measurements determine no changes in Mn3+ (28 and 41 K) magnetic phase transition temperatures. Pronounced anomalies in peak position and linewidth at the magnetic and ferroelectric phase transitions are seen. While the anomalies at the sinusoidal magnetic phase transition (41 K) are in accordance to the theory of spin-phonon coupling, further deviations develop upon entering the ferroelectric phase (28 K). Furthermore, neutron diffraction measurements on 16O and 18O-substituted DyMnO3 powders show structural deviations at the ferroelectric phase transition (17 K) in the order of 100 fm. These results indicate that the structure is actively involved in the emergence of ferroelectricity in these materials.
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    Evolution of charge order through the magnetic phase transition of LuFe2O4
    (American Physical Society, 2012-07-13) Bartkowiak, M; Mulders, AM; Scagnoli, V; Staub, U; Pomjakushina, E; Conder, K
    The charge order in multiferroic LuFe(2)O(4) has been investigated with resonant x-ray diffraction at the Fe K edge in the combined charge ordered and magnetic phase. The energy dependence of the charge order reflection (1/3 1/3 7/2) has been analyzed in detail to investigate the charge disproportionation between the iron sites as a function of temperature. It is found that the charge disproportionation is constant within 0.02e across the Neel temperature T(N). The charge order reflection exhibits a decrease in intensity with increasing temperature which is attributed to an increase in the probability of electron hopping. We confirm the increase in polarization at T(N) is not of static origin but rather dynamic. Our observations are consistent with antiferromagnetically aligned magnetic moments inhibiting the double exchange mechanism and reducing the probability of electrons hopping between Fe(2+) and Fe(3+) in the magnetic phase. © 2012, American Physical Society.
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    Ferroelectric charge order stabilized by antiferromagnetism in multiferroic LuFe(2)O(4)
    (American Physical Society, 2011-10-03) Mulders, AM; Bartkowiak, M; Hester, JR; Pomjakushina, E; Conder, K
    Neutron diffraction measurements on multiferroic LuFe(2)O(4) show changes in the antiferromagnetic (AFM) structure characterized by wave vector q = (1/3 1/3 1/2) as a function of electric field cooling procedures. The increase of intensity from all magnetic domains and the decrease in the two-dimensional (2D) magnetic order observed below the Neel temperature are indicative of increased ferroelectric charge order (CO). The AFM order changes the dynamics of the CO state, and stabilizes it. It is determined that the increase in electric polarization observed at the magnetic ordering temperature is due to a transition from paramagnetic 2D charge order to AFM three-dimensional charge order.© 2011, American Physical Society
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    Floating zone crystal growth and magnetic properties of bilayer manganites Pr(Sr1−xCax)(2)Mn2O7
    (Elsevier Science BV, 2012-08-15) Deng, G; Thiyagarajan, R; Radheep, DM; Pomjakushina, E; Medarde, M; Krzton-Maziopa, A; Wang, S; Arumugam, S; Conder, K
    Single crystals of a bilayer manganite series Pr(Sr1-xCax)(2)Mn2O7 (x=0, 0.4, 0.9, 1) have been grown using traveling solvent floating zone technique under various oxygen partial pressure. It was found that the crystal quality is very sensitive to oxygen partial pressure applied during growth and highly Ca doped crystals could only be obtained at elevated oxygen pressure. Whereas a secondary phase was observed for PrCa2Mn2O7 grown at oxygen partial pressure below 1 bar, a pure phase can be achieved at 8 bar of oxygen. This was evidenced studying composition mapping by X-ray fluorescence spectroscopy. X-ray Laue diffraction and X-ray powder diffraction have been carried out on the samples for the quality check of the grown crystals. Susceptibility measurements indicate that all samples are antiferromagnetic at low temperature. They also reveal the existence of two anomalies at similar to 320 K and 370 K, which coincide with the charge ordering/orbital ordering transitions described in the literature. © 2012, Elsevier Ltd.
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    High oxygen pressure single crystal growth of highly Ca-doped spin ladder compound Sr(14-x)Ca(x)Cu(24)O(41) (x > 12)
    (Elsevier, 2011-07-15) Deng, G; Radheep, DM; Thiyagarajan, R; Pomjakushina, E; Wang, S; Nikseresht, N; Arumugam, S; Conder, K
    Large size high-quality Ca-doped Sr14−xCaxCu24O41 (x=12.2, 12.6 and 13) spin ladder superconducting single crystals were grown using a modified mirror floating zone furnace with oxygen pressure up to 35 bar. The qualities of the as-grown single crystals were confirmed by polarized optical microscopy, neutron diffraction and X-ray diffraction methods. The sample compositions and homogeneity were measured using micro-X-ray fluorescence spectroscopy. The lattice parameters of each sample have been refined using the Rietveld method. The oxygen stoichiometry measurements which were performed by hydrogen reduction/thermogravimetry method show that all the three samples are slightly oxygen deficient even though high oxygen pressure was applied during growth. The high oxygen pressure is indispensible for growing highly Ca-doped Sr14−xCaxCu24O41 single crystals. The susceptibility was measured along c-axis for all three compositions, fitting well with the one dimensional dimer model, consistent with the reported values of undoped Sr14Cu24O41 parent compounds.(C) 2011 Elsevier B.V.
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    Magnetic and electronic co states in the layered cobaltate GdBaCo2O5.5-x
    (American Physical Society, 2008-08) García-Fernández, M; Scagnoli, V; Staub, U; Mulders, AM; Janousch, M; Bodenthin, Y; Meister, D; Patterson, BD; Mirone, A; Tanaka, Y; Nakamura, T; Grenier, S; Huang, YJ; Conder, K
    We have performed nonresonant x-ray diffraction, resonant soft and hard x-ray magnetic diffraction, soft x-ray absorption, and x-ray magnetic circular dichroism measurements to clarify the electronic and magnetic high-spin (HS) state at the states of the Co3+, ions in GdBaCo2O5.5. Our data are consistent with a Co-Py(3+) pyramidal sites and a Co-Oc(3+), low-spin (LS) state at the octahedral sites. The structural distortion with a doubling of the a axis (2a(p)X2a(p)X2a(p) cell) shows alternating elongations and contractions of the pyramids, and indicates that the metal-insulator transition is associated with orbital order in the t(2g) orbitals of the Co-Py(3+) HS state. This distortion corresponds to an alternating ordering of xz and yz orbitals along the a and c axes for the Co-Py(3+). The orbital ordering and pyramidal distortion lead to deformation of the octahedra but the Co-Oc(3+) LS state does not allow an orbital order to occur for the Co-Oc(3+), ions. The soft x-ray magnetic diffraction results indicate that the magnetic moments are aligned in the ab plane but are not parallel to the crystallographic a or b axes. The orbital order and the doubling of the magnetic unit cell along the c axis support a noncollinear magnetic structure. The x-ray magnetic circular dichroism data indicate that there is a large orbital magnetic contribution to the total ordered Co moment. © 2008, American Physical Society
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    Magnetoelectric coupling in TbMnO3 explored via Raman spectroscopy
    (Australian Institute of Physics, 2013-02-06) Graham, PJ; Bartkowiak, M; Rovillain, P; Mulders, AM; Yethiraj, M; Pomjakushina, E; Conder, K; Kenzelmann, M; Ulrich, C
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    Resonant soft x-ray powder diffraction study to determine the orbital ordering in a-site-ordered SmBaMn2O6
    (American Physical Society, 2008-02) García-Fernández, M; Staub, U; Bodenthin, Y; Lawrence, SM; Mulders, AM; Buckley, CE; Weyeneth, S; Pomjakushina, E; Conder, K
    Soft x-ray resonant powder diffraction has been performed at the Mn L-2,L-3 edges of A-site-ordered SmBaMn2O6. The energy and polarization dependences of the (1/4 1/4 0) reflection provide direct evidence for a (x(2) - z(2))/(y(2) - z(2))-type orbital ordering in contrast to single-layer manganite. The temperature dependence of the reflection indicates an orbital reorientation transition at approximate to 210 K, below which the charge- and orbital-ordered MnO2 sheets show AAAA-type of stacking. The concurring reduction of the ferromagnetic superexchange correlations leads to further charge localization. © 2008, American Physical Society
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    Spin dynamics of edge-sharing spin chains in SrCa13Cu24O41
    (American Physical Society, 2018-11-12) Deng, GC; Yu, DH; Mole, RA; Pomjakushina, E; Conder, K; Kenzelmann, M; Yano, SI; Wang, CW; Rule, KC; Gardner, JS; Luo, HQ; Li, S; Ulrich, C; Imperia, P; Ren, W; Cao, SX; McIntyre, GJ
    The low-energy magnetic excitation from the highly Ca-doped quasi-one-dimensional magnet SrCa13Cu24O41 was studied in the magnetic ordered state by using inelastic neutron scattering. We observed the gapless spin-wave excitation, dispersive along the a and c axes but nondispersive along the b axis. Such excitations are attributed to the spin wave from the spin-chain sublattice. Model fitting to the experimental data gives the nearest-neighbor interaction Jc as 5.4 meV and the interchain interaction Ja=4.4meV. Jc is antiferromagnetic and its value is close to the nearest-neighbor interactions of the similar edge-sharing spin-chain systems such as CuGeO3. Comparing with the hole-doped spin chains in Sr14Cu24O41, which shows a spin gap due to spin dimers formed around Zhang-Rice singlets, the chains in SrCa13Cu24 O41 show a gapless excitation in this paper. We ascribe such a change from gapped to gapless excitations to holes transferring away from the chain sublattice into the ladder sublattice upon Ca doping. ©2018 American Physical Society
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    Spin dynamics of quasi-one-dimensional spin-ladder system SrCa13Cu24O41 in the long-range magnetic ordering state
    (Australian Institute of Physics, 2018-01-31) Deng, GC; Yu, DH; Mole, RA; Yano, SI; Wang, CW; Rule, KC; Gardner, JS; Luo, H; Li, S; Ulrich, C; Imperia, P; Ren, W; Cao, SX; Pomjakushina, E; Conder, K; Kenzelmann, M; McIntyre, GJ
    Sr14-xCaxCu24O41 is a quasi-one-dimensional magnet, which consists of two sublattices: spin ladder and spin chain, forming an incommensurate crystal structure along the c axis, namely, the ladder leg or chain direction. The highly Ca-doped compounds undergo a superconducting phase transition under hydrostatic pressure ~ 3GPa, which is really intriguing since the hole-doped even-leg spin-ladder system was theoretically predicted as superconductors by charge-pairing mechanism through antiferromagnetic interaction on the rungs of spin ladders. In the previous study, we discovered that all compounds with different Ca content have a singlet ground state with a spin-gap ~ 32meV. In the highly Ca-doped sample SrCa13Cu24O41, a long-range magnetic ordering takes place at ~ 4.2K. Interestingly, the singlet spin-liquid state and the long-range magnetic ordering coexist in this compound. In this study, we further investigated its spin dynamics in the ordered phase by using inelastic neutron scattering on PELICAN and SIKA at OPAL. We observed the gapless spin-wave excitation, dispersive along the a and c axes but nondispersive along the b axis, indicating the nature of a 2D magnet. A dynamic model has been proposed to fit the experimental data, indicating three major exchange interactions along rungs (JR), legs (JL) and between neighbor ladders (JInter). This study helps us to understand the origin of the spin liquid ground state in this low-dimensional magnet, in which hole-doping should be attributed to induce the long-range magnetic ordering due to the disorder-induced order effect.
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    Spin-gap evolution upon Ca doping in the spin-ladder series Sr(14−x)CaxCu24O41 studied by inelastic neutron scattering
    (Americal Physical Society, 2013-07-03) Deng, G; Tsyrulin, N; Bourges, P; Lamago, D; Rønnow, HM; Kenzelmann, M; Danilkin, SA; Pomjakushina, E; Conder, K
    The spin-gap evolution upon Ca doping in Sr14-xCaxCu24O41 was systematically investigated using inelastic neutron scattering. We discover that the singlet-triplet spin-gap excitation survives in this series with x up to 13, indicating the singlet dimer ground state in these compounds. This observation corrects the previous speculation that the spin gap collapses at x similar to 13 by the NMR technique. The strong intensity modulation along Q(H) in x = 0 gradually evolves into a Q-independent feature in x > 11. This could be attributed to the localized Cu moment magnetism developing into an itinerant magnetism with increasing x. It is a surprise that the spin gap persists in the normal state of this spin-ladder system with metallic behavior, which evidences the possibility of magnetically mediated carrier pairing mechanism in a two-leg spin-ladder lattice. © 2013, American Physical Society.
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    Structural evolution of one-dimensional spin-ladder compounds Sr14−xCaxCu24O41 with Ca doping and related evidence of hole redistribution
    (American Physical Society, 2011-10-17) Deng, G; Pomjakushin, V; Petříček, V; Pomjakushina, E; Kenzelmann, M; Conder, K
    Incommensurate crystal structures of spin-ladder series Sr(14-x)Ca(x)Cu(24)O(41) (x = 3, 7, 11, and 12.2) were characterized by powder neutron scattering method and refined using the superspace group Xmmm(00 gamma)ss0 [ equivalent to superspace group Fmmm(0, 0, 1 + gamma)ss0; X stands for nonstandard centering (0, 0, 0, 0), (0, 1/2, 1/2, 1/2), (1/2, 1/2, 0, 0), (1/2, 0, 1/2, 1/2)] with a modulated structure model. The Ca doping effects on the lattice parameters, atomic displacement, Cu-O distances, Cu-O bond angles, and Cu bond valence sum were characterized. The refined results show that the CuO(4) planar units in both chain and ladder sublattices become closer to square shape with an increase of Ca doping. The Cu bond valence sum calculation provided new evidence for the charge transfer from the chains to ladders (approximately 0.16 holes per Cu from x = 0 to 12.2). The charge transfer was attributed to two different mechanisms: (a) the Cu-O bond distance shrinkage on the ladder and (b) increase of the interaction between two sublattices, resulting in Cu-O bonding between the chains and ladders. The low-temperature structural refinement resulted in the similar conclusion with a slight charge backflow to the chains.© 2011, American Physical Society