21 research outputs found
Change in the magnetic structure of (Bi,Sm)FeO3 thin films at the morphotropic phase boundary probed by neutron diffraction
We report on the evolution of the magnetic structure of BiFeO3 thin films
grown on SrTiO3 substrates as a function of Sm doping. We determined the
magnetic structure using neutron diffraction. We found that as Sm increases,
the magnetic structure evolves from a cycloid to a G-type antiferromagnet at
the morphotropic phase boundary, where there is a large piezoelectric response
due to an electric-field induced structural transition. The occurrence of the
magnetic structural transition at the morphotropic phase boundary offers
another route towards room temperature multiferroic devices
Analytical method to correct the neutron polarisation for triple-axis data
Polarised neutron scattering is the method of choice to study magnetism in
condensed matter. Polarised neutrons are yet typically very low in flux and
complex experimental configurations further reduce the count rate, neutron
polarisation corrections would therefore be needed. Here we analytically derive
formulae of the corrected partial differential scattering cross sections. The
analytical method is designed for the longitudinal polarisation analysis, and
the correction generally holds for time-independent polarised neutrons with a
triple-axis spectrometer. We then applied the correction to recent results of
our experiment on YFeO. Although there is a difficulty
with experimental determination of inefficiency parameters of neutron spin
polarisers and flippers, the correction appears to work properly.Comment: 6 pages, 2 figure
Quenched chirality in RbNiCl
The critical behaviour of stacked-triangular antiferromagnets has been
intensely studied since Kawamura predicted new universality classes for
triangular and helical antiferromagnets. The new universality classes are
linked to an additional discrete degree of freedom, chirality, which is not
present on rectangular lattices, nor in ferromagnets. However, the theoretical
as well as experimental situation is discussed controversially, and generic
scaling without universality has been proposed as an alternative scenario. Here
we present a careful investigation of the zero-field critical behaviour of
RbNiCl, a stacked-triangular Heisenberg antiferromagnet with very small
Ising anisotropy. From linear birefringence experiments we determine the
specific heat exponent as well as the critical amplitude ratio
. Our high-resolution measurements point to a single second order
phase transition with standard Heisenberg critical behaviour, contrary to all
theoretical predictions. From a supplementary neutron diffraction study we can
exclude a structural phase transition at T. We discuss our results in the
context of other available experimental results on RbNiCl and related
compounds. We arrive at a simple intuitive explanation which may be relevant
for other discrepancies observed in the critical behaviour of
stacked-triangular antiferromagnets. In RbNiCl the ordering of the
chirality is suppressed by strong spin fluctuations, yielding to a different
phase diagram, as compared to e.g.\@ CsNiCl, where the Ising anisotropy
prevents these fluctuations
Low dimensional ordering and fluctuations in methanol--hydroquinone-clathrate studied by X-ray and neutron diffraction
Methanol--hydroquinone-clathrate has been established as a model
system for dielectric ordering and fluctuations and is conceptually close to
magnetic spin systems. In X-ray and neutron diffraction experiments, we
investigated the ordered structure, the one-dimensional (1D) and the
three-dimensional (3D) critical scattering in the paraelectric phase, and the
temperature dependence of the lattice constants. Our results can be explained
by microscopic models of the methanol pseudospin in the hydroquinone cage
network, in consistency with previous dielectric investigations
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
Reduction of the spin susceptibility in the superconducting state of Sr2RuO4 observed by polarized neutron scattering
Recent observations [A.~Pustogow et al. Nature 574, 72 (2019)] of a drop of
the O nuclear magnetic resonance (NMR) Knight shift in the
superconducting state of SrRuO challenged the popular picture of a
chiral odd-parity paired state in this compound. Here we use polarized neutron
scattering to show that there is a % drop in the magnetic
susceptibility at the ruthenium site below the superconducting transition
temperature. Measurements are made at lower fields
than a previous study allowing the suppression to be observed. Our results are
consistent with the recent NMR observations and rule out the chiral odd-parity
state. The observed susceptibility
is consistent with several recent proposals including even-parity and
odd-parity helical states.Comment: New version with Supplementary Material discussing orbital
contributions to the susceptibility, Fermi liquid corrections and a two fluid
mode
Field-induced States and Excitations in the Quasicritical Spin-1/2 Chain Linarite
The mineral linarite, PbCuSO(OH), is a spin 1/2 chain with
frustrating nearest neighbor ferromagnetic and next-nearest neighbor
antiferromagnetic exchange interactions. Our inelastic neutron scattering
experiments performed above the saturation field establish that the ratio
between these exchanges is such that linarite is extremely close to the quantum
critical point between spin-multipolar phases and the ferromagnetic state.
However, the measured complex magnetic phase diagram depends strongly on the
magnetic field direction. The field-dependent phase sequence is explained by
our classical simulations of a nearly critical model with tiny orthorhombic
exchange anisotropy. The simulations also capture qualitatively the measured
variations of the wave vector as well as the staggered and the uniform
magnetizations in an applied field
Neutrons and magnetism
Neutron scattering is a unique technique in magnetism, since it measures directly the Fourier transform of the time-dependent magnetic pair correlations. The neutron interacts only weakly with matter, so that each neutron normally only scatters once in the sample volume. The dynamic pair correlation functions are thus probed in the whole volume of the sample. Here we summarize the relevant formalism and approximations for neutron scattering on magnetic materials, focusing on the investigation of single crystals. The lecture aims to stimulate – not to replace – the study of the relevant literature [1–9]