175 research outputs found

    The magnetic exchange parameters and anisotropy of the quasi-two dimensional antiferromagnet NiPS3_3

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    Neutron inelastic scattering has been used to measure the magnetic excitations in powdered NiPS3_3, a quasi-two dimensional antiferromagnet with spin S=1S = 1 on a honeycomb lattice. The spectra show clear, dispersive magnons with a ∼7\sim 7 meV gap at the Brillouin zone center. The data were fitted using a Heisenberg Hamiltonian with a single-ion anisotropy assuming no magnetic exchange between the honeycomb planes. Magnetic exchange interactions up to the third intraplanar nearest-neighbour were required. The fits show robustly that NiPS3_3 has an easy axis anisotropy with Δ=0.3\Delta = 0.3 meV and that the third nearest-neighbour has a strong antiferromagnetic exchange of J3=−6.90J_3 = -6.90 meV. The data can be fitted reasonably well with either J1<0J_1 < 0 or J1>0J_1 > 0, however the best quantitative agreement with high-resolution data indicate that the nearest-neighbour interaction is ferromagnetic with J1=1.9J_1 = 1.9 meV and that the second nearest-neighbour exchange is small and antiferromagnetic with J2=−0.1J_2 = -0.1 meV. The dispersion has a minimum in the Brillouin zone corner that is slightly larger than that at the Brillouin zone center, indicating that the magnetic structure of NiPS3_3 is close to being unstable.Comment: 21 pages, 7 figures, 33 reference

    Strong (π,0)(\pi,0) spin fluctuations in β\beta-FeSe observed by neutron spectroscopy

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    We have performed powder inelastic neutron scattering measurements on the unconventional superconductor β\beta-FeSe (Tc≃8 KT_{\rm c} \simeq 8\,\mathrm{K}). The spectra reveal highly dispersive paramagnetic fluctuations emerging from the square-lattice wave vector (π,0)(\pi,0) extending beyond 80 meV in energy. Measurements as a function of temperature at an energy of ∼13 meV\sim 13\,\mathrm{meV} did not show any variation from TcT_{\rm c} to 104 K104\,\mathrm{K}. The results show that FeSe is close to an instability towards (π,0)(\pi,0) antiferromagnetism characteristic of the parent phases of the high-TcT_{\rm c} iron arsenide superconductors, and that the iron paramagnetic moment is neither affected by the orthorhombic-to-tetragonal structural transition at Ts≃90 KT_{\rm s} \simeq 90\,\mathrm{K} nor does it undergo a change in spin state over the temperature range studied.Comment: Revised version, includes Supplementary Materia

    HORACE: software for the analysis of data from single crystal spectroscopy experiments at time-of-flight neutron instruments

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    The HORACE suite of programs has been developed to work with large multiple-measurement data sets collected from time-of-flight neutron spectrometers equipped with arrays of position-sensitive detectors. The software allows exploratory studies of the four dimensions of reciprocal space and excitation energy to be undertaken, enabling multi-dimensional subsets to be visualized, algebraically manipulated, and models for the scattering to simulated or fitted to the data. The software is designed to be an extensible framework, thus allowing user-customized operations to be performed on the data. Examples of the use of its features are given for measurements exploring the spin waves of the simple antiferromagnet RbMnF3_{3} and ferromagnetic iron, and the phonons in URu2_{2}Si2_{2}.Comment: 14 pages, 6 figure

    Absence of strong magnetic fluctuations in the iron phosphide superconductors LaFePO and Sr2ScO3FeP

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    We report neutron inelastic scattering measurements on polycrystalline LaFePO and Sr2ScO3FeP, two members of the iron phosphide families of superconductors. No evidence is found for any magnetic fluctuations in the spectrum of either material in the energy and wavevector ranges probed. Special attention is paid to the wavevector at which spin-density-wave-like fluctuations are seen in other iron-based superconductors. We estimate that the magnetic signal, if present, is at least a factor of four (Sr2ScO3FeP) or seven (LaFePO) smaller than in the related iron arsenide and chalcogenide superconductors. These results suggest that magnetic fluctuations are not as influential on the electronic properties of the iron phosphide systems as they are in other iron-based superconductors.Comment: 7 pages, 5 figure

    Spin excitations used to probe the nature of the exchange coupling in the magnetically ordered ground state of Pr0.5_{0.5}Ca0.5_{0.5}MnO3_{3}

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    We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite Pr0.5_{0.5}Ca0.5_{0.5}MnO3_{3}, in its magnetic and orbitally ordered phase. The data, which cover multiple Brillouin zones and the entire energy range of the excitations, are compared with several different models that are all consistent with the CE-type magnetic order, but arise through different exchange coupling schemes. The Goodenough model, i.e. an ordered state comprising strong nearest neighbor ferromagnetic interactions along zig-zag chains with antiferromagnetic inter-chain coupling, provides the best description of the data, provided that further neighbor interactions along the chains are included. We are able to rule out a coupling scheme involving formation of strongly bound ferromagnetic dimers, i.e. Zener polarons, on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e. recovery of the Goodenough model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping, and provide a recipe for how to interpret future measurements away from half-doping, where degenerate double exchange models predict more complex ground states.Comment: 14 pages, 11 figure

    Spin resonance in the superconducting state of Li1−x_{1-x}Fex_{x}ODFe1−y_{1-y}Se observed by neutron spectroscopy

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    We have performed inelastic neutron scattering measurements on a powder sample of the superconductor lithium iron selenide hydroxide Li1−x_{1-x}Fex_{x}ODFe1−y_{1-y}Se (x≃0.16,y≃0.02x \simeq 0.16, y \simeq 0.02, Tc=41T_{\rm c} = 41\,K). The spectrum shows an enhanced intensity below TcT_{\rm c} over an energy range 0.64×2Δ<E<2Δ0.64\times2\Delta < E < 2\Delta, where Δ\Delta is the superconducting gap, with maxima at the wave vectors Q1≃1.46Q_1 \simeq 1.46\,\AA−1^{-1} and Q2≃1.97Q_2 \simeq 1.97\,\AA−1^{-1}. The behavior of this feature is consistent with the spin resonance mode found in other unconventional superconductors, and strongly resembles the spin resonance observed in the spectrum of the molecular-intercalated iron selenide, Li0.6_{0.6}(ND2_{2})0.2_{0.2}(ND3_{3})0.8_{0.8}Fe2_{2}Se2_{2}. The signal can be described with a characteristic two-dimensional wave vector (π,0.67π)(\pi, 0.67\pi) in the Brillouin zone of the iron square lattice, consistent with the nesting vector between electron Fermi sheets

    Excitonic transverse and amplitude fluctuations in the noncollinear and charge-ordered RbFe2+^{2+}Fe3+^{3+}F6_{6}

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    RbFe2+^{2+}Fe3+^{3+}F6_{6} is an example of an antiferromagnet with charge ordering of the octahedrally coordinated Fe2+^{2+} and Fe3+^{3+} ions. As well as different spin values, Fe2+^{2+} (S=2S=2) and Fe3+^{3+} (S=52S={5\over2}) possess differing orbital ground states with Fe2+^{2+} having an orbital degeneracy with an effective orbital angular momentum of l=1l=1. The resulting low temperature magnetic structure is non collinear with the spins aligned perpendicular to nearest neighbors (S. W. Kim \textit{et al.} Chem. Sci. {\bf{3}}, 741 (2012)). The combination of an orbital degeneracy and non collinear spin arrangements introduces the possibility for unusual types of excitations such as amplitude modes of the order parameter. In this paper we investigate this by applying a multi-level analysis to model neutron spectroscopy data (M. Songvilay \textit{et al.} Phys. Rev. Lett. {\bf{121}}, 087201 (2018)). In particular, we discuss the possible origins of the momentum and energy broadened continuum scattering observed in terms of amplitude fluctuations allowed through the presence of an orbital degree of freedom on the Fe2+^{2+} site. We extend previous spin-orbit exciton models based on a collinear spin structure to understand the measured low-energy excitations and also to predict and discuss possible amplitude mode scattering in RbFe2+^{2+}Fe3+^{3+}F6_{6}.Comment: 17 pages and 7 figure

    Incommensurate antiferromagnetic fluctuations in single-crystalline LiFeAs studied by inelastic neutron scattering

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    We present an inelastic neutron scattering study on single-crystalline LiFeAs devoted to the characterization of the incommensurate antiferromagnetic fluctuations at Q=(0.5±δ,0.5∓δ,ql)\mathbf{Q}=(0.5\pm\delta, 0.5\mp\delta, q_l). Time-of-flight measurements show the presence of these magnetic fluctuations up to an energy transfer of 60 meV, while polarized neutrons in combination with longitudinal polarization analysis on a triple-axis spectrometer prove the pure magnetic origin of this signal. The normalization of the magnetic scattering to an absolute scale yields that magnetic fluctuations in LiFeAs are by a factor eight weaker than the resonance signal in nearly optimally Co-doped BaFe2_2As2_2, although a factor two is recovered due to the split peaks owing to the incommensurability. The longitudinal polarization analysis indicates weak spin space anisotropy with slightly stronger out-of-plane component between 6 and 12 meV. Furthermore, our data suggest a fine structure of the magnetic signal most likely arising from superposing nesting vectors.Comment: 9 pages, 8 figure
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