Showing 1–12 of 12 results for author: Gibbs, D

Distinct magnetic phase transition at the surface of an antiferromagnet

Authors: Sean Langridge, Gavin M. Watson, Doon Gibbs, Joseph J. Betouras, Nikitas I. Gidopoulos, Frank Pollmann, Martin W. Long, Christian Vettier, Gerry H. Lander

Abstract: In the majority of magnetic systems the surface is required to order at the same temperature as the bulk. In the present study, we report a distinct and unexpected surface magnetic phase transition, uniquely at a lower temperature than the Néel temperature. Employing grazing incidence X-ray resonant magnetic scattering we have observed the near surface behavior of uranium dioxide. UO$_2$ is a non-… ▽ More In the majority of magnetic systems the surface is required to order at the same temperature as the bulk. In the present study, we report a distinct and unexpected surface magnetic phase transition, uniquely at a lower temperature than the Néel temperature. Employing grazing incidence X-ray resonant magnetic scattering we have observed the near surface behavior of uranium dioxide. UO$_2$ is a non-collinear, triple-q, antiferromagnet with the U ions on an face-centered-cubic lattice. Theoretical investigations establish that at the surface the energy increase, due to the lost bonds, reduces when the spins near the surface rotate, gradually losing their normal to the surface component. At the surface the lowest-energy spin configuration has a double-q (planar) structure. With increasing temperature, thermal fluctuations saturate the in-plane crystal field anisotropy at the surface, leading to soft excitations that have ferromagnetic $XY$ character and are decoupled from the bulk. The structure factor of a finite two-dimensional $XY$ model, fits the experimental data well for several orders of magnitude of the scattered intensity. Our results support a distinct magnetic transition at the surface in the Kosterlitz-Thouless universality class. △ Less

Submitted 22 April, 2014; v1 submitted 11 December, 2013; originally announced December 2013.

Comments: 7 pages including Supplemental material, 4 figures

Journal ref: Phys. Rev. Lett. 112, 167201 (2014)

Resonant x-ray diffraction study of the magnetoresistant perovskite Pr0.6Ca0.4MnO3

Authors: S. Grenier, J. P. Hill, Doon Gibbs, K. J. Thomas, M. v. Zimmermann, C. S. Nelson, V. Kiryukhin, Y. Tokura, D. Casa, T. Gog, C. Venkataraman

Abstract: We report a x-ray resonant diffraction study of the perovskite Pr0.6Ca0.4MnO3. At the Mn K-edge, this technique is sensitive to details of the electronic structure of the Mn atoms. We discuss the resonant x-ray spectra measured above and below the charge and orbital ordering phase transition temperature (TCOO = 232 K), and present a detailed analysis of the energy and polarization dependence of… ▽ More We report a x-ray resonant diffraction study of the perovskite Pr0.6Ca0.4MnO3. At the Mn K-edge, this technique is sensitive to details of the electronic structure of the Mn atoms. We discuss the resonant x-ray spectra measured above and below the charge and orbital ordering phase transition temperature (TCOO = 232 K), and present a detailed analysis of the energy and polarization dependence of the resonant scattering. The analysis confirms that the structural transition is a transition to an orbitally ordered phase in which inequivalent Mn atoms are ordered in a CE-type pattern. The Mn atoms differ mostly by their 3d orbital occupation. We find that the charge disproportionation is incomplete, 3d^{3.5-δ} and 3d^{3.5+δ} with δ\ll0.5 . A revised CE-type model is considered in which there are two Mn sublattices, each with partial e_{g} occupancy. One sublattice consists of Mn atoms with the 3x^{2}-r^{2} or 3y^{2}-r^{2} orbitals partially occupied, the other sublattice with the x^{2}-y^{2} orbital partially occupied. △ Less

Submitted 19 January, 2004; v1 submitted 9 May, 2003; originally announced May 2003.

Comments: 15 pages, 15 figures

Substrate-induced strain effects on Pr_{0.6}Ca_{0.4}MnO_{3} films

Authors: C. S. Nelson, J. P. Hill, Doon Gibbs, M. Rajeswari, A. Biswas, S. Shinde, R. L. Greene, T. Venkatesan, A. J. Millis, F. Yokaichiya, C. Giles, D. Casa, C. T. Venkataraman, T. Gog

Abstract: We report the characterization of the crystal structure, low-temperature charge and orbital ordering, transport, and magnetization of Pr_{0.6}Ca_{0.4}MnO_{3} films grown on LaAlO_{3}, NdGaO_{3}, and SrTiO_{3} substrates, which provide compressive (LaAlO_{3}) and tensile (NdGaO_{3} and SrTiO_{3}) strain. The films are observed to exhibit different crystallographic symmetries than the bulk materia… ▽ More We report the characterization of the crystal structure, low-temperature charge and orbital ordering, transport, and magnetization of Pr_{0.6}Ca_{0.4}MnO_{3} films grown on LaAlO_{3}, NdGaO_{3}, and SrTiO_{3} substrates, which provide compressive (LaAlO_{3}) and tensile (NdGaO_{3} and SrTiO_{3}) strain. The films are observed to exhibit different crystallographic symmetries than the bulk material, and the low-temperature ordering is found to be more robust under compressive-- as opposed to tensile-- strain. In fact, bulk-like charge and orbital ordering is not observed in the film grown on NdGaO_{3}, which is the substrate that provides the least amount of nominal and measured, but tensile, strain. This result suggests the importance of the role played by the Mn--O--Mn bond angles in the formation of charge and orbital ordering at low temperatures. Finally, in the film grown on LaAlO_{3}, a connection between the lattice distortion associated with orbital ordering and the onset of antiferromagnetism is reported. △ Less

Submitted 12 March, 2003; originally announced March 2003.

Comments: 12 pages, 7 figures

Coherent X-ray Scattering from Manganite Charge and Orbital Domains

Authors: C. S. Nelson, J. P. Hill, Doon Gibbs, F. Yakhou, F. Livet, Y. Tomioka, T. Kimura, Y. Tokura

Abstract: We report coherent x-ray scattering studies of charge and orbital domains in manganite systems. The experiments were carried out on LaMnO_3 and Pr_{0.6}Ca_{0.4}MnO_3, with the incident photon energy tuned near the Mn K edge. At room temperature, the orbital speckle pattern of LaMnO_3 was observed to be constant over a timescale of at least minutes, which is indicative of static orbital domains o… ▽ More We report coherent x-ray scattering studies of charge and orbital domains in manganite systems. The experiments were carried out on LaMnO_3 and Pr_{0.6}Ca_{0.4}MnO_3, with the incident photon energy tuned near the Mn K edge. At room temperature, the orbital speckle pattern of LaMnO_3 was observed to be constant over a timescale of at least minutes, which is indicative of static orbital domains on this timescale. For Pr_{0.6}Ca_{0.4}MnO_3, both charge and orbital speckle patterns were observed. The observation of the latter rules out the presence of fast orbital fluctuations, while long time series data-- on the order of several minutes-- were suggestive of slow dynamic behavior. In contrast, the charge order speckle patterns were static. △ Less

Submitted 19 June, 2002; originally announced June 2002.

Comments: 6 pages, 4 figures

Common features of nanoscale structural correlations in magnetoresistive manganites with ferromagnetic low-temperature state

Authors: V. Kiryukhin, T. Y. Koo, A. Borissov, Y. J. Kim, C. S. Nelson, J. P. Hill, D. Gibbs, S-W. Cheong

Abstract: We report x-ray scattering studies of nanoscale structural correlations in Nd$_{1-x}$Sr$_x$MnO$_3$ and La$_{1-x}$(Ca,Sr)$_x$MnO$_3$, $x$=0.2--0.5. We find that the correlated regions possess a temperature-independent correlation length of 2-3 lattice constants which is the same in all samples. The period of the lattice modulation of the correlated regions is proportional to the Ca/Sr doping conc… ▽ More We report x-ray scattering studies of nanoscale structural correlations in Nd$_{1-x}$Sr$_x$MnO$_3$ and La$_{1-x}$(Ca,Sr)$_x$MnO$_3$, $x$=0.2--0.5. We find that the correlated regions possess a temperature-independent correlation length of 2-3 lattice constants which is the same in all samples. The period of the lattice modulation of the correlated regions is proportional to the Ca/Sr doping concentration $x$. Remarkably, the lattice modulation periods of these and several other manganites with a ferromagnetic ground state fall on the same curve when plotted as a function of $x$. Thus, the structure of the correlated regions in these materials appears to be determined by a single parameter, $x$. We argue that these observations provide important clues for understanding the Colossal Magnetoresistance phenomenon in manganites. △ Less

Submitted 5 December, 2001; originally announced December 2001.

Comments: 6 pages, 5 embedded eps figures

Orbital Correlations in Doped Manganites

Authors: J. P. Hill, C. S. Nelson, M. v. Zimmermann, Y. -J. Kim, D. Gibbs, D. Casa, B. Keimer, Y. Murakami, C. Venkataraman, T. Gog, Y. Tomioka, Y. Tokura, V. Kiryukhin, T. Y. Koo, S. -W. Cheong

Abstract: We review our recent x-ray scattering studies of charge and orbital order in doped manganites, with specific emphasis on the role of orbital correlations in Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable from the undoped structure with long range orbital order at low temperatures. For dopings 0.3<x<0.5, we find scattering consistent with a charge and orbitally ordere… ▽ More We review our recent x-ray scattering studies of charge and orbital order in doped manganites, with specific emphasis on the role of orbital correlations in Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable from the undoped structure with long range orbital order at low temperatures. For dopings 0.3<x<0.5, we find scattering consistent with a charge and orbitally ordered CE-type structure. While in each case the charge order peaks are resolution limited, the orbital order exhibits only short range correlations. We report the doping dependence of the correlation length and discuss the connection between the orbital correlations and the finite magnetic correlation length observed on the Mn^3+ sublattice with neutron scattering techniques. The physical origin of these domains, which appear to be isotropic, remains unclear. We find that weak orbital correlations persist well above the phase transitions, with a correlation length of 1-2 lattice constants at high temperatures. Significantly, we observe similar correlations at high temperatures in La_0.7Ca_0.3MnO_3, which does not have an orbitally ordered ground state, and we conclude that such correlations are robust to variations in the relative strength of the electron-phonon coupling. △ Less

Submitted 3 May, 2001; originally announced May 2001.

Comments: 22 pagegs, 7 figures

A Resonant X-ray Scattering Study of Octahedral Tilt Ordering in LaMnO$_3$ and Pr$_{1-x}$Ca$_x$MnO$_3$

Authors: M. v. Zimmermann, C. S. Nelson, Y. -J. Kim, J. P. Hill, Doon Gibbs, H. Nakao, Y. Wakabayashi, Y. Murakami, Y. Tomioka, Y. Tokura, T. Arima, C. -C. Kao, D. Casa, C. Venkataraman, Th. Gog

Abstract: We report an x-ray scattering study of octahedral tilt ordering in the manganite series Pr$_{1-x}$Ca$_x$MnO$_3$ with x=0.4 and 0.25 and in LaMnO$_3$. The sensitivity to tilt ordering is achieved by tuning the incident x-ray energy to the L$_I$, L$_{II}$ and L$_{III}$ absorption edges of Pr and La, respectively. The resulting energy-dependent profiles are characterized by a dipole-resonant peak a… ▽ More We report an x-ray scattering study of octahedral tilt ordering in the manganite series Pr$_{1-x}$Ca$_x$MnO$_3$ with x=0.4 and 0.25 and in LaMnO$_3$. The sensitivity to tilt ordering is achieved by tuning the incident x-ray energy to the L$_I$, L$_{II}$ and L$_{III}$ absorption edges of Pr and La, respectively. The resulting energy-dependent profiles are characterized by a dipole-resonant peak and higher energy fine structure. The polarization dependence is predominantly $σ$-to-$π$ and the azimuthal dependence follows a sin-squared behavior. These results are similar to those obtained in recent x-ray scattering studies of orbital ordering carried out in these same materials at the Mn K edge. They lead to a description of the cross-section in terms of Templeton scattering in which the tilt ordering breaks the symmetry at the rare earth site. The most interesting result of the present work is our observation that octahedral tilt ordering persists above the orbital ordering transition temperatures in all three samples. Indeed, we identify separate structural transitions which may be associated with the onset of orbital and tilt ordering, respectively, and characterize the loss of tilt ordering versus temperature in LaMnO$_3$. △ Less

Submitted 19 March, 2001; originally announced March 2001.

Comments: 24 pages, 8 figures

Correlated Polarons in Dissimilar Perovskite Manganites

Authors: C. S. Nelson, M. v. Zimmermann, J. P. Hill, Doon Gibbs, V. Kiryukhin, T. Y. Koo, S-W. Cheong, D. Casa, B. Keimer, Y. Tomioka, Y. Tokura, T. Gog, C. T. Venkataraman

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

Submitted 29 November, 2000; originally announced November 2000.

Comments: 13 pages, 6 figures

X-ray Resonant Scattering Studies of Orbital and Charge Ordering in Pr$_{1-x}$Ca$_x$MnO$_3$

Authors: M. v. Zimmermann, C. S. Nelson, J. P. Hill, Doon Gibbs, M. Blume, D. Casa, B. Keimer, Y. Murakami, C. -C. Kao, C. Venkataraman, T. Gog, Y. Tomioka, Y. Tokura

Abstract: We present the results of a systematic x-ray scattering study of the charge and orbital ordering in the manganite series Pr$_{1-x}$Ca$_x$MnO$_3$ with $x$=0.25, 0.4 and 0.5. The temperature dependence of the scattering at the charge and orbital wavevectors, and of the lattice constants, was characterized throughout the ordered phase of each sample. It was found that the charge and orbital order w… ▽ More We present the results of a systematic x-ray scattering study of the charge and orbital ordering in the manganite series Pr$_{1-x}$Ca$_x$MnO$_3$ with $x$=0.25, 0.4 and 0.5. The temperature dependence of the scattering at the charge and orbital wavevectors, and of the lattice constants, was characterized throughout the ordered phase of each sample. It was found that the charge and orbital order wavevectors are commensurate with the lattice, in striking contrast to the results of earlier electron diffraction studies of samples with $x$=0.5. High momentum-transfer resolution studies of the x=0.4 and 0.5 samples further revealed that while long-range charge order is present, long-range orbital order is never established. Above the charge/orbital ordering temperature T$_o$, the charge order fluctuations are more highly correlated than the orbital fluctuations. This suggests that charge order drives orbital order in these samples. In addition, a longitudinal modulation of the lattice with the same periodicity as the charge and orbital ordering was discovered in the x=0.4 and 0.5 samples. For x=0.25, only long-range orbital order was observed with no indication of charge ordering, nor of an additional lattice modulation. We also report the results of a preliminary investigation of the loss of charge and orbital ordering in the x=0.4 sample by application of a magnetic field. Finally, the polarization and azimuthal dependence of the charge and orbital ordering in these compounds is characterized both in the resonant and nonresonant limits, and compared with the predictions of current theories. The results are qualitatively consistent with both cluster and LDA+U calculations of the electronic structure. △ Less

Submitted 13 July, 2000; originally announced July 2000.

Comments: 37 pages, 22 figures

X-ray Anomalous Scattering Study of a Charge-Ordered State in NaV2O5

Authors: H. Nakao, K. Ohwada, N. Takesue, Y. Fujii, M. Isobe, Y. Ueda, M. v. Zimmermann, J. P. Hill, D. Gibbs, J. C. Woicik, I. Koyama, Y. Murakami

Abstract: Charge ordering of V4+ and V5+ in NaV2O5 has been studied by an X-ray diffraction technique using anomalous scattering near a vanadium K-absorption edge to critically enhance a contrast between the two ions. A dramatic energy dependence of the superlattice intensities is observed below Tc=35 K. Consequently, the charge ordering pattern is the zigzag-type ladders with the unit cell 2a*2b*4c, but… ▽ More Charge ordering of V4+ and V5+ in NaV2O5 has been studied by an X-ray diffraction technique using anomalous scattering near a vanadium K-absorption edge to critically enhance a contrast between the two ions. A dramatic energy dependence of the superlattice intensities is observed below Tc=35 K. Consequently, the charge ordering pattern is the zigzag-type ladders with the unit cell 2a*2b*4c, but not the chain-type originally proposed for the spin-Peierls state. Charge disproportionation suggested in our model as the average valence V^{4.5+-delta_c/2} is observed below T_C, showing continuous variation of delta_c as a function of temperature. △ Less

Submitted 19 September, 2000; v1 submitted 8 March, 2000; originally announced March 2000.

Comments: 4 pages, 4 figures

Spin Dynamics and Orbital State in LaTiO_3

Authors: B. Keimer, D. Casa, A. Ivanov, J. W. Lynn, M. v. Zimmermann, J. P. Hill, D. Gibbs, Y. Taguchi, Y. Tokura

Abstract: A neutron scattering study of the Mott-Hubbard insulator LaTiO$_{3}$ (T$_{\rm N}=132$ K) reveals a spin wave spectrum that is well described by a nearest-neighbor superexchange constant $J=15.5$ meV and a small Dzyaloshinskii-Moriya interaction ($D=1.1$ meV). The nearly isotropic spin wave spectrum is surprising in view of the absence of a static Jahn-Teller distortion that could quench the orbi… ▽ More A neutron scattering study of the Mott-Hubbard insulator LaTiO$_{3}$ (T$_{\rm N}=132$ K) reveals a spin wave spectrum that is well described by a nearest-neighbor superexchange constant $J=15.5$ meV and a small Dzyaloshinskii-Moriya interaction ($D=1.1$ meV). The nearly isotropic spin wave spectrum is surprising in view of the absence of a static Jahn-Teller distortion that could quench the orbital angular momentum, and it may indicate strong orbital fluctuations. A resonant x-ray scattering study has uncovered no evidence of orbital order in LaTiO$_{3}$. △ Less

Submitted 18 October, 2000; v1 submitted 1 February, 2000; originally announced February 2000.

Comments: final version, Phys. Rev. Lett. 85, 3946 (2000)

Journal ref: Phys. Rev. Lett. 85, 3946 (2000)

Interplay between charge, orbital and magnetic order in Pr_{1-x}Ca_xMnO_3

Authors: M. v. Zimmermann, J. P. Hill, Doon Gibbs, M. Blume, D. Casa, B. Keimer, Y. Murakami, Y. Tomioka, Y. Tokura

Abstract: We report resonant x-ray scattering studies of charge and orbital order in Pr_{1-x}Ca_{x}MnO_3 with x=0.4 and 0.5. Below the ordering temperature, T_O=245 K, the charge and orbital order intensities follow the same temperature dependence, including an increase at the antiferromagnetic ordering temperature, T_N. High resolution measurements reveal, however, that long range orbital order is never… ▽ More We report resonant x-ray scattering studies of charge and orbital order in Pr_{1-x}Ca_{x}MnO_3 with x=0.4 and 0.5. Below the ordering temperature, T_O=245 K, the charge and orbital order intensities follow the same temperature dependence, including an increase at the antiferromagnetic ordering temperature, T_N. High resolution measurements reveal, however, that long range orbital order is never achieved. Rather, an orbital domain state is formed. Above T_O, the charge order fluctuations are more highly correlated than the orbital fluctuations. Similar phenomenology is observed in a magnetic field. We conclude that the charge order drives the orbital order at the transition. △ Less

Submitted 19 May, 1999; originally announced May 1999.

Comments: 4 pages, 5 figures