B. Roessli

Paul Scherrer Institut, Aargau, Switzerland

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Publications (203)480.38 Total impact

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    ABSTRACT: Frustrated systems exhibit remarkable properties due to the high degeneracy of their ground states. Stabilised by competing interactions, a rich diversity of typically nanometre-sized phase structures appear in polymer and colloidal systems, while the surface of ice pre-melts due to geometrically frustrated interactions. Atomic spin systems where magnetic interactions are frustrated by lattice geometry provide a fruitful source of emergent phenomena, such as fractionalised excitations analogous to magnetic monopoles. The degeneracy inherent in frustrated systems may prevail all the way down to absolute zero temperature, or it may be lifted by small perturbations or entropic effects. In the geometrically frustrated Ising--like magnet Ca3Co2O6, we follow the temporal and spatial evolution of nanoscale magnetic fluctuations firmly embedded inside the spin--density--wave magnetic structure. These fluctuations are a signature of a competing ferrimagnetic phase with an incommensurability that is different from, but determined by the host. As the temperature is lowered, the fluctuations slow down into a super-paramagnetic regime of stable spatiotemporal nano-structures.
    04/2014;
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    ABSTRACT: We report a neutron scattering study of a ferroelectric phase transition in Sr0.61Ba0.39Nb2O6 (SBN-61). The ferroelectric polarization is along the crystallographic c-axis but the transverse acoustic branch propagating along the 〈1, 1, 0〉 direction does not show any anomaly associated with this transition. We find no evidence for a soft transverse optic phonon. We do, however, observe elastic diffuse scattering. The intensity of this scattering increases as the sample is cooled from a temperature well above the phase transition. The susceptibility associated with this diffuse scattering follows the anomaly of the dielectric permittivity of SBN-61 well. Below Tc the shape of this scattering is consistent with the scattering expected from ferroelectric domain walls. Our results suggest that despite apparent chemical disorder SBN-61 behaves as a classic order-disorder uniaxial ferroelectric with critical fluctuations in the range <10(-11) s.
    Journal of Physics Condensed Matter 04/2014; 26(18):185901. · 2.22 Impact Factor
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    ABSTRACT: At low temperatures, Tb_{2}Ti_{2}O_{7} enters a spin liquid state, despite expectations of magnetic order and/or a structural distortion. Using neutron scattering, we have discovered that in this spin liquid state an excited crystal field level is coupled to a transverse acoustic phonon, forming a hybrid excitation. Magnetic and phononlike branches with identical dispersion relations can be identified, and the hybridization vanishes in the paramagnetic state. We suggest that Tb_{2}Ti_{2}O_{7} is aptly named a "magnetoelastic spin liquid" and that the hybridization of the excitations suppresses both magnetic ordering and the structural distortion. The spin liquid phase of Tb_{2}Ti_{2}O_{7} can now be regarded as a Coulomb phase with propagating bosonic spin excitations.
    Physical Review Letters 01/2014; 112(1):017203. · 7.73 Impact Factor
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    ABSTRACT: The magnetic structure of the metallic antiferromagnet Ag2NiO2 with the Néel temperature TN = 56 K has been investigated by means of a neutron diffraction technique using a powder sample in the temperature range between 5 and 65 K. The antiferromagnetic (AF) diffraction peaks are clearly observed below TN and can be indexed with the propagation vector [Formula: see text]. Based on the results of both a representational analysis and a Rietveld refinement of the magnetic peaks, the AF spin structure is determined as an A-type AF structure with ml = m0cos(2πk ⋅l), where ml is the moment at the lth Ni(3+) site and m0 = (0.31,0,0.65) μB at 5 K.
    Journal of Physics Condensed Matter 06/2013; 25(28):286005. · 2.22 Impact Factor
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    ABSTRACT: The magnetic structure of the metallic antiferromagnet Ag 2 NiO 2 with the Néel temperature T N = 56 K has been investigated by means of a neutron diffraction technique using a powder sample in the temperature range between 5 and 65 K. The antiferromagnetic (AF) diffraction peaks are clearly observed below T N and can be indexed with the propagation vector ##IMG## [http://ej.iop.org/images/0953-8984/25/28/286005/cm470956ieqn42.gif] {$boldsymbolk=(1,frac13,frac12})$ . Based on the results of both a representational analysis and a Rietveld refinement of the magnetic peaks, the AF spin structure is determined as an A-type AF structure with m l = m 0 cos(2π k ⋅ l ), where m l is the moment at the l th Ni 3+ site and m 0 = (0.31,0,0.65) μ B at 5 K.
    Journal of Physics: Condensed Matter. 01/2013; 25(28):286005.
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    ABSTRACT: A quite unusual diffuse scattering phenomenology was observed in the single-crystal x-ray diffraction pattern of cubic perovskite BMT (BaMg1/3Ta2/3O3). The intensity of the scattering is parametrized as a set of cubelike objects located at the centers of reciprocal space unit cells, resembling very broad and cubic-shaped (1/2,1/2,1/2) satellites. BMT belongs to perovskites of formula AB1/3′B2/3′′O3 (A=Mg, B′=Ta, B′′=Mg). The cubes of the intensity can be attributed to the partial correlations of the occupancies of the B site. The pair correlation function is the Fourier transform of the diffuse scattering intensity and the latter's idealized form yields the unusual property of a power-law correlation decay with distance. Up to now this has been observed only in a few exotic instances of magnetic order or nematic crystals. Therefore it cannot be classified as a short-range order phenomenon, as in most situations originating diffuse scattering. A Monte Carlo search in configuration space yielded solutions that reproduce faithfully the observed diffuse scattering. Analysis of the results in terms of the electrostatic energy and the entropy points to this phase of BMT as a metastable state, kinetically locked, which could be the equilibrium state just below the melting point.
    Physical review. B, Condensed matter 09/2012; 86(10). · 3.77 Impact Factor
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    ABSTRACT: The magnetic properties of Fe1.03Te under hydrostatic pressure up to p ~ 5.7 GPa were investigated by means of muon spin rotation, dc magnetization, and neutron depolarization measurements. With increasing pressure the antiferromagnetic ordering temperature TN decreases continuously from 70 K at ambient pressure towards higher pressures. Surprisingly, the commensurate antiferromagnetic order of FeTe enters a region of incommensurate and dynamical magnetic order before at p ~ 1.7 GPa the system turns ferromagnetic. The ferromagnetic ordering temperature TC increases with increasing pressure.
    Physical review. B, Condensed matter 09/2012; 87(6). · 3.77 Impact Factor
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    ABSTRACT: We investigate the low-temperature state of the rare-earth pyrochlore Tb_{2}Ti_{2}O_{7} using polarized neutron scattering. Tb_{2}Ti_{2}O_{7} is often described as an antiferromagnetic spin liquid with spin correlations extending over lengths comparable to individual tetrahedra of the pyrochlore lattice. We confirm this picture at 20 K but find that at 0.05 K the data contain evidence of pinch-point scattering, suggesting that the low temperature state of Tb_{2}Ti_{2}O_{7} has power-law spin correlations.
    Physical Review Letters 07/2012; 109(1):017201. · 7.73 Impact Factor
  • Physical review. B, Condensed matter 07/2012; · 3.77 Impact Factor
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    ABSTRACT: Neutron scattering measurements have been used to study the phase transition of SBN-61 (Sr0.61Ba0.39Nb2O6) at a temperature Tc ~ 350 K. The transverse acoustic branch propagating along the [1, 1, 0] direction was measured. The results showed that this branch is not temperature dependent near the phase transition. Additional diffuse scattering, elastic within the resolution, was observed at lower energies than the transverse acoustic mode. Unexpectedly, on cooling from a temperature well above the phase transition, the intensity of the quasi-elastic scattering increased whereas the width did not change much. Below the phase transition it became very difficult to separate the Bragg reflection from the diffuse scattering. Assuming that this scattering is caused by the domain walls, it was possible to describe the data. The results show that the thickness of the domain walls steadily increases below Tc. We found no evidence that electric random-fields produce pronounced effects on the scattering properties of Sr0.61Ba0.39Nb2O6.
    06/2012;
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    ABSTRACT: The structure of BaMg1/3Ta2/3O3 (BMT) has been studied using X-ray scattering. The phonons have been measured and the results are similar to those of other materials with the perovskite structure such as PbMg1/3Nb2/3O3 (PMN). The acoustic and lowest energy optic branches were measured but it was not possible to measure the branches of higher energy, possibly this is because they largely consist of oxygen motions. High-resolution inelastic measurements also showed that the diffuse scattering was strictly elastic and not directly related to the phonon spectra. A diffuse scattering was observed in BMT near the (H\pm1/2, K\pm1/2, L\pm1/2) points in the Brillouin zone and this had a characteristic cube shape. This arises from ordering of the B-site ions in BMT. Additional experiments revealed a diffuse scattering in BMT similar in shape to Bragg reflections at wave-vectors of the form (H\pm1/3, K\pm1/3, L\pm1/3). Such reflections were also observed by Lufaso [Chem. Matt. 16 (2004) 2148] from powders and suggest that this structure of BMT consists of 4 differently oriented domains of a trigonal structure and results from a different ordering of the B-site ions from that responsible for the scattering at the (H\pm1/2, K\pm1/2, L\pm1/2) points. The results lead us to suggest that for BMT single crystals the bulk has the properties of a cubic perovskite, whereas the surface may have quite different structure from that of the bulk. This difference resembles the behaviour of cubic relaxors like PMN and PMN doped by PbTiO3, where significant surface effects have been reported.
    Journal of Physics Condensed Matter 04/2012; 24(45). · 2.22 Impact Factor
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    ABSTRACT: A quite unusual diffuse scattering phenomenology was observed in the single-crystal X-ray diffraction pattern of cubic perovskite BMT ($\mathrm{BaMg}_{1/3}\mathrm{Ta}_{2/3}\mathrm{O}_3$). The intensity of the scattering is parametrized as a set of cube-like objects located at the centers of reciprocal space unit cells, resembling very broad and cubic-shaped (1/2,1/2,1/2)-satellites. BMT belongs to perovskites of formula AB$'_{1/3}$B$"_{2/3}$O$_{3}$ (A=Mg, B$'=$Ta, B$"=$Mg). The cubes of the intensity can be attributed to the partial correlations of the occupancies of the B site. The pair correlation function is the Fourier transform of the diffuse scattering intensity and the latter's idealized form yields the unusual property of a power-law correlation decay with distance. Up to now this is observed only in a few exotic instances of magnetic order or nematic crystals. Therefore it cannot be classified as a short-range order phenomenon, as in most situations originating diffuse scattering. A Monte-Carlo search in configuration space yielded solutions that reproduce faithfully the observed diffuse scattering. Analysis of the results in terms of the electrostatic energy and the entropy point to this phase of BMT as a metastable state, kinetically locked, which could be the equilibrium state just below the melting point.
    04/2012;
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    ABSTRACT: The olivine compound Mn(2)GeO(4) is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity, and a canted commensurate order leads to weak ferromagnetism. Symmetry suggests that the direct coupling between the ferromagnetism and ferroelectricity is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multicomponent magnetic structures found in magnetically frustrated materials like Mn(2)GeO(4) provide a new route towards functional materials that exhibit coupled ferromagnetism and ferroelectricity.
    Physical Review Letters 02/2012; 108(7):077204. · 7.73 Impact Factor
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    ABSTRACT: We have studied the high temperature multiferroic cupric oxide using polarized neutron diffraction as a function of temperature and applied electric field. We find that the chiral domain population can be varied using an external electric field applied along the b axis. Using representation analysis we derive the incommensurate magnetic structure in the multiferroic phase. The origin of the magnetoelectric coupling is consistent with models based on the inverse Dzyaloshinskii-Moriya interaction, but is different from the simple cycloidal mechanism.
    Physical review. B, Condensed matter 01/2012; 85(13). · 3.77 Impact Factor
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    ABSTRACT: The dynamical properties of the nearly stoichiometric LiTaO 3 crystals are studied by neutron scattering and light scattering. The neutron scattering shows the high value of the Curie temperature Tc which is the typical feature of near stoichiometry. In the broadband Brillouin scattering the intense quasielastic scattering is observed in the vicinity of Tc and the critical slowing down is clearly observed in the vicinity of Tc. As to the inelastic neutron scattering in the vicinity of Tc, we found that it behaves as expected for the critical scattering at a phase transition. In addition, we observed scattering with complex shape in the wavevector-energy space which is present in a very broad temperature and energy transfers range.
    Ferroelectrics. 10/2011; 354(September 2007):192-197.
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    ABSTRACT: The multiferroic material BaMnF(4) has been investigated with unpolarized and polarized neutron diffraction. The structure has been shown to be antiferromagnetic. The magnetic moments are aligned at 12° to the b direction in the bc plane, 3° different from the previously determined value. The ferromagnetic component that is indicative of the linear magnetoelectric effect was not observed.
    Journal of Physics Condensed Matter 07/2011; 23(26):266004. · 2.22 Impact Factor
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    ABSTRACT: Relaxors have a broad temperature and frequency-dependent peak in the dielectric permittivity that is not necessarly linked to a structural phase transition. A model relaxor is PbMg1/3Nb2/3O3 (PMN) doped with PbTiO3 (PT). We report neutron studies of the low-energy spectra of (1-x)PMN-xPT crystals. Apart from phonons which do not show a soft mode, there are two components of the diffuse scattering: one is quasi-elastic (QE) and the other static. The energy width of the QE scattering decreases as the peak of the susceptibility is approached. The static component behaves like an order parameter. In the crystals that become ferroelectric it is maximal at the ferroelectric phase transition, but in PMN it steadily increases on cooling. We discuss previously reported and new results in terms of a random-field model of the cubic crystal.
    03/2011;
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    Peter Böni, Bertrand Roessli, Klaudia Hradil
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    ABSTRACT: Neutrons and X-rays are powerful probes for studying magnetic and lattice excitations in strongly correlated materials over very wide ranges of momentum and energy transfers. In the focus of the present work are the incommensurate magnetic systems MnSi and Cr. Under application of a magnetic field, helically ordered MnSi transforms into a weak itinerant ferromagnet. Using polarized neutrons we demonstrate that the Stoner excitations are spin flip excitations. The amplitude (longitudinal) fluctuations associated with the magnon modes are already strong far away from T_C. Interestingly, even the non spin flip excitations associated with the Stoner modes are observable. In Cr, we have observed Kohn anomalies in the phonon spectrum at those incommensurate positions in reciprocal space, where the spin density wave is observed. The corresponding phonon and magnon modes are not coupled. In addition, an anomalous softening of a transverse phonon branch along the N-H zone boundary line is observed that is caused by strong electron phonon coupling. High resolution neutron scattering indicate that the low energy Fincher-Burke excitations may rather correspond to localized modes in momentum and energy and not to propagating collective modes. Finally, we demonstrate that in the near future it may become feasible to investigate excitations in very small samples thus allowing to measure the dynamics of strongly correlated materials under extreme conditions and in the vicinity of quantum phase transitions.
    Journal of Physics Condensed Matter 03/2011; 23(25). · 2.22 Impact Factor
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    ABSTRACT: We construct an effective spin Hamiltonian for YMn2O5 through analysis of first-principles density-functional theory results. Using our first-principles-derived spin Hamiltonian we calculate the magnon dispersion of this compound and compare with the experimentally measured spectra probed using inelastic neutron scattering. Our first-principles-derived set of magnetic interaction parameters show the importance of the Mn1-Mn2 interaction along the b axis, which was ignored in the literature. We provide justification in favor of our first-principles-derived parameters.
    Physical Review B 01/2011; 84(5). · 3.66 Impact Factor
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    ABSTRACT: Relaxor ferroelectrics were discovered in the 1950s but many of their properties are not understood. In this review, we shall concentrate on materials such as PMN (PbMg1/3Nb2/3O3), which crystallize in the cubic perovskite structure but with the Mg ion, charge 2+, and the Nb ion, charge 5+, randomly distributed over the B site of the perovskite structure. The peak of the dielectric susceptibility for relaxors is much broader in temperature than that of conventional ferroelectrics, while below the maximum of the susceptibility most relaxors remain cubic and show no electric polarization, unlike that observed for conventional ferroelectrics. Because of the large width of the susceptibility, relaxors are often used as capacitors. Recently, there have been many X-ray and neutron scattering studies of relaxors and the results have enabled a more detailed picture to be obtained. An important conclusion is that relaxors can exist in a random field state, as initially proposed by Westphal, Kleemann and Glinchuk, similar to that which has been studied for diluted antiferromagnets. If a relaxor is cooled from a high temperature, then the Burns temperature is a measure of when slow fluctuations become evident. These fluctuations are connected with the disorder and are known as nano-domains. The Burns temperature is not a well-defined transition temperature. At a lower temperature, there is a well-defined boundary to a so-called random field state when the nano-domains become static but there is no long-range periodic order. This phase may have both history-dependent properties and a skin effect in which the surface of the sample is different from that of the bulk material, as also found in experiments on magnetic systems. Section 1 is an introduction to the review, to ferroelectricity and to relaxors. Section 2 gives a description of the results obtained by dielectric, optical, specific heat and other macroscopic properties. These long-wavelength properties give a variety of different characteristic temperatures and do not directly probe the random field state. In Section 3, we describe the results of neutron and X-ray scattering and show that they strongly support the interpretation that relaxors have a random field state. In Section 4, we briefly describe the results for other relaxor systems such as (PMN)1−x(PT)x for which PMN is mixed with different amounts of the ferroelectric lead titanate (PT), and we show that the existence of a random field state enables us also to describe the experimental results for these mixed materials. We hope that this review will inspire further theoretical and experimental work to understand the nature of the random field states and to compare the experimental results more satisfactorily with theory.
    Advances In Physics 01/2011; 60(2):229-327. · 34.29 Impact Factor

Publication Stats

2k Citations
480.38 Total Impact Points

Institutions

  • 1970–2014
    • Paul Scherrer Institut
      • Laboratory for Neutron Scattering (LNS)
      Aargau, Switzerland
  • 2010
    • University Joseph Fourier - Grenoble 1
      Grenoble, Rhône-Alpes, France
    • University of Oxford
      • Department of Physics
      Oxford, ENG, United Kingdom
  • 2006
    • Institute for Transuranium Elements
      Carlsruhe, Baden-Württemberg, Germany
  • 2005
    • Ioffe Physical Technical Institute
      Sankt-Peterburg, St.-Petersburg, Russia
  • 1994–2001
    • Institut Laue-Langevin
      Grenoble, Rhône-Alpes, France
  • 1999
    • Psi Chi
      Chattanooga, Tennessee, United States
    • Tohoku University
      • Department of Physics
      Sendai-shi, Miyagi-ken, Japan
  • 1998
    • ETH Zurich
      • Laboratory for Solid State Physics
      Zürich, ZH, Switzerland