175 research outputs found
The magnetic exchange parameters and anisotropy of the quasi-two dimensional antiferromagnet NiPS
Neutron inelastic scattering has been used to measure the magnetic
excitations in powdered NiPS, a quasi-two dimensional antiferromagnet with
spin on a honeycomb lattice. The spectra show clear, dispersive magnons
with a 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 NiPS has an easy axis anisotropy with meV and
that the third nearest-neighbour has a strong antiferromagnetic exchange of
meV. The data can be fitted reasonably well with either
or , however the best quantitative agreement with high-resolution data
indicate that the nearest-neighbour interaction is ferromagnetic with meV and that the second nearest-neighbour exchange is small and
antiferromagnetic with 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 NiPS is close to being
unstable.Comment: 21 pages, 7 figures, 33 reference
Strong spin fluctuations in -FeSe observed by neutron spectroscopy
We have performed powder inelastic neutron scattering measurements on the
unconventional superconductor -FeSe ().
The spectra reveal highly dispersive paramagnetic fluctuations emerging from
the square-lattice wave vector extending beyond 80 meV in energy.
Measurements as a function of temperature at an energy of did not show any variation from to
. The results show that FeSe is close to an instability
towards antiferromagnetism characteristic of the parent phases of the
high- iron arsenide superconductors, and that the iron paramagnetic
moment is neither affected by the orthorhombic-to-tetragonal structural
transition at 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
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 RbMnF and ferromagnetic
iron, and the phonons in URuSi.Comment: 14 pages, 6 figure
Absence of strong magnetic fluctuations in the iron phosphide superconductors LaFePO and Sr2ScO3FeP
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 PrCaMnO
We have used time-of-flight inelastic neutron scattering to measure the spin
wave spectrum of the canonical half-doped manganite
PrCaMnO, 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 LiFeODFeSe observed by neutron spectroscopy
We have performed inelastic neutron scattering measurements on a powder
sample of the superconductor lithium iron selenide hydroxide
LiFeODFeSe (, \,K). The spectrum shows an enhanced intensity below over an
energy range , where is the
superconducting gap, with maxima at the wave vectors \,\AA and \,\AA. 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,
Li(ND)(ND)FeSe. The signal can
be described with a characteristic two-dimensional wave vector
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 RbFeFeF
RbFeFeF is an example of an antiferromagnet with charge
ordering of the octahedrally coordinated Fe and Fe ions. As well
as different spin values, Fe () and Fe ()
possess differing orbital ground states with Fe having an orbital
degeneracy with an effective orbital angular momentum of . 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 Fe 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
RbFeFeF.Comment: 17 pages and 7 figure
Incommensurate antiferromagnetic fluctuations in single-crystalline LiFeAs studied by inelastic neutron scattering
We present an inelastic neutron scattering study on single-crystalline LiFeAs
devoted to the characterization of the incommensurate antiferromagnetic
fluctuations at . 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
BaFeAs, 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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