125 research outputs found
Magnetic chirality as probed by neutron scattering
We review the concept of chirality, at first briefly in a general context
then in the specific framework of the spin networks. We next discuss to what
extent neutron scattering appears as an unconvertible tool to probe magnetic
chirality in the static and dynamical regimes of the spins. The remarkable
chiral ground state and excitations of the Fe-langasite compound finally serves
to illustrate the use of neutron polarimetry in the experimental studies of the
magnetic chirality.Comment: 32 page
Spin liquid correlations in Nd-langasite anisotropic Kagom\'e antiferromagnet
Dynamical magnetic correlations in the geometrically frustrated
NdGaSiO compound were probed by inelastic neutron scattering
on a single crystal. A scattering signal with a ring shape distribution in
reciprocal space and unprecedented dispersive features was discovered.
Comparison with calculated static magnetic scattering from models of correlated
spins suggests that the observed phase is a spin liquid inherent to an
antiferromagnetic kagom\'e-like lattice of anisotropic Nd moments.Comment: 4 page
Magnetic order in the frustrated Ising-like chain compound SrNiIrO
We have studied the field and temperature dependence of the magnetization of
single crystals of Sr3NiIrO6. These measurements evidence the presence of an
easy axis of anisotropy and two anomalies in the magnetic susceptibility.
Neutron powder diffraction realized on a polycrystalline sample reveals the
emergence of magnetic reflections below 75 K with magnetic propagation vector k
~ (0, 0, 1), undetected in previous neutron studies [T.N. Nguyen and H.-C zur
Loye, J. Solid State Chem., 117, 300 (1995)]. The nature of the magnetic ground
state, and the presence of two anomalies common to this family of material, are
discussed on the basis of the results obtained by neutron diffraction,
magnetization measurements, and symmetry arguments
Chemical decoration in cubic approximant and quasicrystal in the Al-Cu-Fe system
International audienceThe local order in the Al--Cu--Fe quasicrystal and in two of its approximants has been investigated by extended X-ray absorption fine structure (EXAFS) studies at the Cu and Fe Kedges. The chemical occupation of the crystallographic sites in the 1/1 cubic α--Al55Si7Cu25.5Fe12.5 phase is revisited. From these results, a model for the chemical short--range order in the Al62.5Cu25.5Fe 12.5 quasicrystal is proposed
Phase diagram of multiferroic KCuAsO(OD)
The layered compound KCuAsO(OD), comprising distorted kagome
planes of Cu ions, is a recent addition to the family of type-II
multiferroics. Previous zero field neutron diffraction work has found two
helically ordered regimes in \kns, each showing a distinct coupling between the
magnetic and ferroelectric order parameters. Here, we extend this work to
magnetic fields up to ~T using neutron powder diffraction, capacitance,
polarization, and high-field magnetization measurements, hence determining the
phase diagram. We find metamagnetic transitions in both low temperatures
phases around ~T, which neutron powder diffraction reveals
to correspond to a rotation of the helix plane away from the easy plane, as
well as a small change in the propagation vector. Furthermore, we show that the
sign of the ferroelectric polarization is reversible in a magnetic field,
although no change is observed (or expected on the basis of the magnetic
structure) due to the transition at ~T. We finally justify the temperature
dependence of the polarization in both zero-field ordered phases by a symmetry
analysis of the free energy expansion
Parity Broken Chiral Spin Dynamics in BaNbFeSiO
The spin wave excitations emerging from the chiral helically modulated
120 magnetic order in a langasite BaNbFeSiO
enantiopure crystal were investigated by unpolarized and polarized inelastic
neutron scattering. A dynamical fingerprint of the chiral ground state is
obtained, singularized by (i) spectral weight asymmetries answerable to the
structural chirality and (ii) a full chirality of the spin correlations
observed over the whole energy spectrum. The intrinsic chiral nature of the
spin waves elementary excitations is shown in absence of macroscopic time
reversal symmetry breaking
THz Magneto-electric atomic rotations in the chiral compound BaNbFeSiO
We have determined the terahertz spectrum of the chiral langasite
BaNbFeSiO by means of synchrotron-radiation measurements.
Two excitations are revealed that are shown to have a different nature. The
first one, purely magnetic, is observed at low temperature in the magnetically
ordered phase and is assigned to a magnon. The second one persits far into the
paramagnetic phase and exhibits both an electric and a magnetic activity at
slightly different energies. This magnetoelectric excitation is interpreted in
terms of atomic rotations and requires a helical electric polarization
Vibronic collapse of ordered quadrupolar ice in the pyrochore magnet TbTiO
While the spin liquid state in the frustrated pyrochlore
TbTiO has been studied both experimentally and
theoretically for more than two decades, no definite description of this
unconventional state has been achieved. Using synchrotron based THz
spectroscopy in combination with quantum numerical simulations, we highlight a
significant link between two previously unrelated features: the existence of a
quadrupolar order following an ice rule and the presence of strong
magneto-elastic coupling in the form of hybridized Tb crystal-field and
phonon modes. The magnitude of this so-called vibronic process, which involves
quadrupolar degrees of freedom, is significantly dependent on small
off-stoichiometry and favors all-in all-out like correlations between
quadrupoles. This mechanism competes with the long range ordered quadrupolar
ice, and for slightly different stoichiometry, is able to destabilize it.Comment: Main text: 7 pages, 3 figures ; Supplemental Material: 6 pages, 2
figure
Helical bunching and symmetry lowering inducing multiferroicity in Fe langasites
International audienceThe chiral Fe-based langasites represent model systems of triangle-based frustrated magnets with a strong potential for multiferroicity. We report neutron scattering measurements for the multichiral Ba3MFe3Si2O14 (M = Nb, Ta) langasites revealing new important features of the magnetic order of these systems: the bunching of the helical modulation along the c-axis and the in-plane distortion of the 120° Fe-spin arrangement. We discuss these subtle features in terms of the microscopic spin Hamiltonian, and provide the link to the magnetically-induced electric polarization observed in these systems. Thus, our findings put the multiferroicity of this attractive family of materials on solid ground
Lattice dynamics and spin excitations in the metal-organic framework [CHNH][Co(HCOO)]
In metal-organic-framework (MOF) perovskites, both magnetic and ferroelectric
orderings can be readily realized by compounding spin and charge degrees of
freedom. The hydrogen bonds that bridge the magnetic framework and organic
molecules have long been thought of as a key in generating multiferroic
properties. However, the underlying physical mechanisms remain unclear. Here,
we combine neutron diffraction, quasielastic and inelastic neutron scattering,
and THz spectroscopy techniques to thoroughly investigate the dynamical
properties of the multiferroic MOF candidate [CHNH][Co(HCOO)]
through its multiple phase transitions. The wide range of energy resolutions
reachable by these techniques enables us to scrutinize the coupling between the
molecules and the framework throughout the phase transitions and interrogate a
possible magnetoelectric coupling. Our results also reveal a structural change
around 220 K which may be associated with the activation of a nodding donkey
mode of the methylammonium molecule due to the ordering of the CH groups.
Upon the occurrence of the modulated phase transition around 130 K, the
methylammonium molecules undergo a freezing of its reorientational motions
which is concomitant with a change of the lattice parameters and anomalies of
collective lattice vibrations. No significant change has been however observed
in the lattice dynamics around the magnetic ordering, which therefore indicates
the absence of a substantial magneto-electric coupling in zero-field
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