46 research outputs found
Magnetoelastic effects in multiferroic YMnO
We have investigated magnetoelastic effects in multiferroic YMnO below
the antiferromagnetic phase transition, K, using neutron
powder diffraction. The lattice parameter of the hexagonal unit cell of
YMnO decreases normally above , but decreases anomalously below T,
whereas the lattice parameter increases with decreasing temperature and
then increases anomalously below T. The unit cell volume also undergoes an
anomalous contraction below . By fitting the background thermal expansion
for a non-magnetic lattice with the Einstein-Gr\"uneisen equation, we
determined the lattice strains , and due to the
magnetoelastic effects as a function of temperature. We have also determined
the temperature variation of the ordered magnetic moment of the Mn ion by
fitting the measured Bragg intensities of the nuclear and magnetic reflections
with the known crystal and magnetic structure models and have established that
the lattice strain due to the magnetoelastic effect in YMnO couples with
the square of the ordered magnetic moment or the square of the order parameter
of the antiferromagnetic phase transition
Magnetic structures in the rich magnetic phase diagram of HoRhIn
The magnetic phase diagram of the tetragonal HoRhIn compound has
similar features to many related systems, revealing a zero magnetic field AF1
and a field-induced AF2 phases. Details of the magnetic order in the AF2 phase
were not reported yet for any of the related compounds. In addition, only the
HoRhIn phase diagram contains a small region of the incommensurate
zero-field AF3 phase. We have performed a number of neutron diffraction
experiments on single crystals of HoRhIn using several diffractometers
including experiments in both horizontal and vertical magnetic fields up to 4
T. We present details of the magnetic structures in all magnetic phases of the
rich phase diagram of HoRhIn. The Ho magnetic moments point along the
tetragonal axis in every phase. The ground-state AF1 phase is characterized
by propagation vector = (1/2, 0, 0). The more complex
ferrimagnetic AF2 phase is described by four propagation vectors
= (0, 0, 0), = (1/2, 0, 0),
= (0, 1/2, 1/2), = (1/2, 1/2, 1/2). The magnetic structure in
the AF3 phase is incommensurate with = (0.5, , 0).
Our results are consistent with theoretical calculations based on crystal field
theory.Comment: submitted to PR
Neutron Diffraction Study on Single-crystalline UAuSi
Magnetic structure of tetragonal UAuSi was investigated by
single-crystal neutron diffraction experiments. Below = 20 K it
orders antiferromagnetically with a propagation vector of and
magnetic moments of uranium ions pointing along the tetragonal -axis. Weak
signs of the presence of a ferromagnetic component of magnetic moment were
traced out.Taking into account a group theory calculation and experimental
results of magnetization and Si-NMR, the magnetic structure is
determined to be a squared-up antiferromagnetic structure, with a stacking
sequence () of the ferromagnetic -plane sheets along the -axis.
This result highlights similar magnetic correlations in UAuSi and
isostructural URuSi.Comment: 7 pages, 7 figure
Magnetic superelasticity and inverse magnetocaloric effect in Ni-Mn-In
Applying a magnetic field to a ferromagnetic NiMnIn
alloy in the martensitic state induces a structural phase transition to the
austenitic state. This is accompanied by a strain which recovers on removing
the magnetic field giving the system a magnetically superelastic character. A
further property of this alloy is that it also shows the inverse magnetocaloric
effect. The magnetic superelasticity and the inverse magnetocaloric effect in
Ni-Mn-In and their association with the first order structural transition is
studied by magnetization, strain, and neutron diffraction studies under
magnetic field.Comment: 6 pages, 8 figures. Published in the Physical Review
Magnetic superelasticity and inverse magnetocaloric effect in Ni-Mn-In
Applying a magnetic field to a ferromagnetic NiMnIn
alloy in the martensitic state induces a structural phase transition to the
austenitic state. This is accompanied by a strain which recovers on removing
the magnetic field giving the system a magnetically superelastic character. A
further property of this alloy is that it also shows the inverse magnetocaloric
effect. The magnetic superelasticity and the inverse magnetocaloric effect in
Ni-Mn-In and their association with the first order structural transition is
studied by magnetization, strain, and neutron diffraction studies under
magnetic field.Comment: 6 pages, 8 figures. Published in the Physical Review
Magnetic structures of non-cerium analogues of heavy-fermion Ce2RhIn8: case of Nd2RhIn8, Dy2RhIn8 and Er2RhIn8
R2RhIn8 compounds (space group P4/mmm, R is a rare-earth element) belong to a
large group of structurally related tetragonal materials which involves several
heavy-fermion superconductors based on Ce. We have succeeded to grow single
crystals of compounds with Nd, Dy and Er and following our previous bulk
measurements, we performed neutron-diffraction studies to determine their
magnetic structures. The Laue diffraction experiment showed that the
antiferromagnetic order below the N\'eel temperature is in all three compounds
characterized by the propagation vector k = (1/2, 1/2, 1/2). The amplitude and
direction of the magnetic moments, as well as the invariance symmetry of the
magnetic structure, were determined by subsequent experiments using two- and
four-circle diffractometers. The critical exponents were determined from the
temperature dependence of the intensities below TN.Comment: 10 pages, 7 figures, submitted to PR
Incommensurate magnetic structure of CrAs at low temperatures and high pressures
The magnetic structure of chromium arsenide CrAs is studied with neutron powder diffraction at ambient pressure in the temperature range 1.5–300 K as well as with neutron single-crystal diffraction at 2 K and 0.12 GPa. The material undergoes an anti-isostructural phase transition at TN = 267 K and atmospheric conditions, in which both orthorhombic phases have the same space-group symmetry (Pnma, Z = 4) but different distortions of the parent hexagonal structure of the NiAs type (P63/mmc, Z = 2). The magnetic structure below TN is incommensurate with the propagation vector k = (0, 0, kc). At ambient pressure, the component kc decreases from kc = 0.3807 (7) at 260 K to kc = 0.3531 (6) at 50 K. Below this temperature, it is basically constant. With increasing pressure at 2 K, kc is also constant within standard uncertainties [kc = 0.353 (2)]. For the analysis of the magnetic structure, a group-theoretical approach based on the space group of the nuclear structure and its subgroups is used. To avoid falling into false minima in the refinements, a random search for magnetic moments in the models is implemented. In the literature, the magnetic structure has been determined on the basis of powder diffraction data as a double helix propagating along the c axis. Although this double-helical model leads to satisfactory agreement factors for our powder data, it does not reproduce the intensities of the magnetic satellite reflections measured on single-crystal data in a satisfactory way and can therefore be discarded. Instead, several other models are found that lead to better agreement. Each of them is spiral-like with directional components in all three directions and with no spin-density wave character that would cause a non-constant magnetic moment. In all these models, the ordering of the spins is neither a pure helix nor a pure cycloid. Instead, the unit vectors of the spin rotation planes make an angle α, 0° < α < 90°, with respect to the c* direction. The model in superspace group P21.1′(α0γ)0s yields the best agreement factors in the refinements of the neutron single-crystal and powder diffraction data. This model is unique as it is the only one in which all the magnetic moments rotate with the same chirality
Magnetic and electronic structure of the topological semimetal YbMnSb
The antiferromagnetic (AFM) semimetal YbMnSb has recently been identified
as a candidate topological material, driven by time-reversal symmetry breaking.
Depending on the ordered arrangement of Mn spins below the N\'{e}el
temperature, = 345 K, the electronic bands near the Fermi energy
can ether have a Dirac node, a Weyl node or a nodal line. We have investigated
the ground state magnetic structure of YbMnSb using unpolarized and
polarized single crystal neutron diffraction. We find that the Mn moments lie
along the axis of the space group and are arranged in a C-type AFM
structure, which implies the existence of gapped Dirac nodes near the Fermi
level. The results highlight how different magnetic structures can critically
affect the topological nature of fermions in semimetals
Anomaly in structural noncentrosymmetry around TN n bulk and nanoscale BiFeO3
Using high resolution powder neutron diffraction data, we show that there is a distinct anomaly in the structural noncentrosymmetry around the magnetic transition point TN for bulk and nanoscale BiFeO3. It appears that the structural noncentrosymmetry - which gives rise to the ferroelectric polarization - is suppressed anomalously by ~1% (of the average noncentrosymmetry at above the magnetic transition) in the bulk sample and by ~12% in the nanoscale sample as the magnetic transition is approached from higher temperature. This observation shows that the multiferroic coupling improves in the nanoscale sample which is expected to brighten the application prospects of nanoscale BiFeO3 in nanospintronics-based sensor devices
Développement de l'analyse quantitative de texture utilisant des détecteurs bidimensionnels (application à la texture magnétique)
La détermination de la structure cristalline d'un échantillon nécessite la prise en compte de son caractère anisotrope et du même coup l'analyse quantitative de texture devient de plus en plus importante. Ce type d analyse a recours à la mesure de figures de pôles par diffraction de RX ou de neutrons, et à l'affinement des ODF. Couplée à d'autres types d'analyse (microstructure, contraintes résiduelles ) utilisant les mêmes diagrammes de diffraction, l'analyse du profil global incluant l'analyse texturale à pris le nom d'analyse combinée. Ce travail détaille les mesures d'analyse combinée, pour des expériences de diffraction X et de neutrons. Nous appliquons l'analyse combinée par diffraction X à la détermination des vitesses de propagation d'ondes élastiques générées par excitation photoacoustique picoseconde, dans des films d'or texturés. L évolution technologique a permit le développement de détecteurs 2D, réduisant considérablement les temps de mesures neutroniques. Nous développons l'analyse combinée et la calibration associée sur le détecteur CAPS de l'instrument D19, qui réduit les temps d acquisition et permet de développer l analyse quantitative de texture magnétique, pour mettre en évidence la réorientation des moments magnétiques sous champ modéré (~0,3 T). Ce mémoire est finalement consacré à l étude de l analyse MQTA, qui caractérise les matériaux magnétiques en termes de distribution macroscopique de l orientation des moments, et étudie comment le signal magnétique résultant est lié aux cristallites et aux microstructures de l échantillon. Nous détaillons ici l'aspect théorique de l'analyse MQTA, et illustrons son application sur un échantillon de fer doux.The structural determination of a sample requires to take account of its anisotropic nature and quantitative texture analysis (QTA) then becomes increasingly important. QTA relies on pole figure measurements, using X-ray or neutron diffraction, and on ODF refinement. It is resolved ideally using Whole-Powder-Pattern analyses, including microstructure, residual stresses, and structure, called "Combined Analysis".The present work is devoted to the study of Magnetic Quantitative Texture Analysis (MQTA), which characterizes magnetic moment distributions of magnetic materials, and investigates the links between magnetic signals, crystallites and microstructures. We first present how Combined Analysis can be used efficiently to determine the elastic wave velocities generated by picoseconds photoacoustic excitation, in textured gold films, using CPS detectors and x-rays. However, to target MQTA, weak neutron magnetic difference signals have to be measured using more efficient strategies. We then developed MQTA using the 2D Curved CPS detector of D19, which reduces greatly neutron acquisition times. In a second part we calibrate the new line on standards, and treat localization corrections. The third part is the core of the work and corresponds to MQTA. A magnetic sample holder allowing a fixed applied magnetic field with respect to the sample (which rotates in the Eulerian cradle) has been developed, using as for testing the methodology only a developed field of ~0.3 T at the sample. MQTA theory is then developed, with specific emphasis on total magnetic scattering pole figures and polarization pole figures. Illustration of MQTA application on a soft iron sample is shown.CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF