Polarized Neutron Diffraction Experiments Research Articles

Electric-Field Manipulation of Magnetic Chirality in a Homo-Ferro-Rotational Helimagnet.

Ferro-rotational (FR) materials, renowned for their distinctive material functionalities, present challenges in the growth of homo-FR crystals (i.e., single FR domain). This study explores a cost-effective approach to growing homo-FR helimagnetic RbFe(SO4)2 (RFSO) crystals by lowering the crystal growth temperature below the TFR threshold using the high-pressure hydrothermal method. Through polarized neutron diffraction experiments, it is observed that nearly 86% of RFSO crystals consist of a homo-FR domain. Notably, RFSO displays remarkable stability in the FR phase, with an exceptionally high TFR of ≈573K. Furthermore, RFSO exhibits a chiral helical magnetic structure with switchable ferroelectric polarization below 4K. Importantly, external electric fields can induce a single magnetic domain state and manipulate its magnetic chirality. The findings suggest that the search for new FR magnets with outstanding material properties should consider magnetic sulfates as promising candidates.

The first polarized neutron diffraction experiment at the time-of-flight single crystal neutron diffractometer SENJU at J-PARC

The 3He spin filter has been developed to use pulsed neutrons effectively at J-PARC MLF. In this study, the 3He spin filter was installed in the single crystal neutron diffractometer (BL18 SENJU) at J-PARC MLF as a polarizer of the incident beam, and Half-polarized neutron diffraction experiments were performed. The 3He spin filter was used the ”drop-in cell” method with cells that had a relaxation time of 150 hours in the past experiments. The sample was a single crystal of Heusler crystal Cu2MnAl and the sample magnetic field was 0.3 T. The intensities of (111) Bragg peaks of Cu2MnAl were drastically changed with the polarity of the incident neutron beam, and the flipping ratio of 21.7 at 3.6 Å was observed. The experimental setup and the performance of the 3He spin filter used in this experiment are described in detail in this paper.

Loop currents in quantum matter

In many quantum materials, strong electron correlations lead to the emergence of new states of matter. In particular, the study in the last decades of the complex phase diagram of high temperature superconducting cuprates highlighted intra-unit-cell electronic instabilities breaking discrete Ising-like symmetries, while preserving the lattice translation invariance. Polarized neutron diffraction experiments have provided compelling evidences supporting a new form of intra-unit-cell magnetism, emerging concomitantly with the so-called pseudogap state of these materials. This observation is currently interpreted as the magnetic hallmark of an intra-unit-cell loop current order, breaking both parity and time-reversal symmetries. More generally, this magneto-electric state is likely to exist in a wider class of quantum materials beyond superconducting cuprates. For instance, it has been already observed in hole-doped Mott insulating iridates or in the spin liquid state of hole-doped 2-leg ladder cuprates.

Magnetic order and multipoles in the 5d2 rhenium double perovskite Ba2YReO6

${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ is a double perovskite material where the ${\mathrm{Re}}^{5+}$ (${d}^{2}$) ions occupy a frustrated face-centered cubic lattice. Despite strong antiferromagnetic interactions between the Re ions, as indicated by a large negative Weiss constant $\ensuremath{\theta}=\ensuremath{-}616$ K, spin freezing in ${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ only occurs at 45 K. Since no long-range order of magnetic dipoles has previously been found in either muon spin rotation or neutron diffraction experiments below this temperature, it has been assumed that the low-temperature state is a spin glass. In stark contrast with these findings, we here show that ${\mathrm{Ba}}_{2}\mathrm{Y}\mathrm{Re}{\mathrm{O}}_{6}$ does in fact order with a strongly reduced dipole moment ${\ensuremath{\mu}}_{0}=0.29--0.42\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ via a polarized neutron diffraction experiment on a powder sample. Using the symmetries of the two likeliest magnetic structures and the properties of the $5{d}^{2}$ configuration in the presence of strong spin-orbit coupling and strong crystal fields, we use a recently derived single-ion wave function for ${\mathrm{Re}}^{5+}$ to predict a large quadrupolar moment based on the experimental ${\ensuremath{\mu}}_{o}$.

Diffraction by multipoles in a 5d2 rhenium double perovskite

A recent polarized neutron diffraction experiment on the $5{d}^{2}$ rhenium double perovskite ${\mathrm{Ba}}_{2}\mathrm{YRe}{\mathrm{O}}_{6}$ held at a low temperature uncovered weak magnetic diffraction peaks. Data analysis inferred a significantly reduced Re dipole moment, and long-range order compatible with an antiferromagnetic, noncollinear motif. To interpret the experimental findings, we present a model wave function for Re ions derived from the crystal field potential, Coulomb interaction, and spin-orbit coupling that fully respects the symmetry of the low-temperature ordered state. It is used to calculate in analytic form all multipole moments visible in neutron and resonance enhanced x-ray diffraction. A minimal model consistent with available neutron diffraction data predicts significant multipolar moments up to the hexadecapole and, in particular, a dominant chargelike quadrupole moment. Calculated diffraction patterns embrace single crystal x-ray diffraction at the Re $L$ edge, and renewed neutron diffraction, to probe the presumed underlying multipolar order.

Neutron diffraction and short range ordering study in multiferroic Bi2Fe4O9

We present the evidence of room temperature short range ordering (SRO) in the Cairo pentagonal lattice type Bi2Fe4O9 (BFO) using neutron diffraction study. The signature of this SRO is observed in the form of a broad diffuse magnetic peak at Q ∼ 1.3 Å−1 in the neutron diffraction pattern at room temperature. A transition from SRO into a long range magnetic order is observed in subsequent low temperature neutron diffraction patterns, which corresponds to a magnetic ordering regime arising from local spin arrangements of Fe ions. At T = 2 K, magnetic moments for Fe1 (octahedral) and Fe2 (tetrahedral) atoms are 3.78 ± 0.16 μB and 3.37 ± 0.13 μB, respectively. The nature of SRO at room temperature is found to be magnetic as determined from detailed polarized neutron diffraction experiment. Substantial variation in the lattice parameters, O–Fe–O bond angles, Fe–O bond lengths and the effective magnetic moment per Fe-site near Néel temperature (TN) is evidenced post neutron diffraction analysis. To further apprehend the origin of SRO, first principle calculation is explored and is correlated with structural as well as superexchange mechanisms of BFO.

Helimagnetic Chirality in Chiral Helimagnetic CsCuCl<sub>3</sub> Probed by Polarized Neutron Diffraction Experiments

Helimagnetic Chirality in Chiral Helimagnetic CsCuCl<sub>3</sub> Probed by Polarized Neutron Diffraction Experiments

Orientation of the intra-unit-cell magnetic moment in the high- Tc superconductor HgBa2CuO4+δ

Polarized-neutron diffraction experiments (PND) have revealed that the pseudogap state of the cuprates exhibits unusual intra-unit-cell (IUC) magnetism. At a qualitative level, the data indicate a moment direction that is neither perpendicular nor parallel to the CuO2 layers, yet an accurate measurement of a structurally simple compound has been lacking. Here we report PND results with unprecedented accuracy for the IUC magnetic order in the simple-tetragonal single-CuO$_2$-layer compound HgBa2CuO$_{4+{\delta}}$. At the transition temperature, we find evidence for magnetic critical scattering. Deep in the ordered state, we determine the moment direction to be 70{\deg} $\pm$ 10{\deg} away from the normal to the CuO$_2$ layers, which rules out both purely planar loop currents and high-symmetry Dirac multipoles, the two most prominent theoretical proposals for the microscopic origin of the IUC magnetism. However, the data are consistent with Dirac multipoles of lower symmetry or, alternatively, with a particular configuration of loop currents that flow on the faces of the CuO$_6$ octahedra.

Crystallographic and Magnetic Chirality in Chiral Helimagnetic CsCuCl<sub>3</sub>

We report enantiopure crystal growth and chiral helimagnetic ordering in chiral inorganic compounds CsCuCl3 with a chiral space group of right-handed P6122 or left-handed P6522. Our 2-step crystallization process makes it possible to grow centimeter sized enantiopure single crystals. Polarized neutron diffraction experiments probed that crystallographic chirality of CsCuCl3 strongly combined with helimagnetic chirality via Dzyalloshinskii-Moriya interaction.

Comment on “No evidence for orbital loop currents in charge-ordered YBa2Cu3O6+x from polarized neutron diffraction”

Intra-unit cell magnetic order has been observed in four different families of high-temperature superconductors from polarized neutron diffraction experiments and supported by several other techniques. That order, which does not break translation symmetry, is consistent with the predicted orbital moments generated by two microscopic loop currrents in each CuO$_2$ cell. Recently, using polarized neutron diffraction, Croft {\it et al} [Phys. Rev. B 96, 214504 (2017)] claim to find no evidence for such orbital loop currents in charge ordered ${\rm YBa_2Cu_3O_{6+x}}$. Their experiment is done with detwinned samples at least 100 times smaller than in previous experiments without counting much longer. We show by a detailed quantitative analysis of their data that contrary to their conclusion, the magnetic signal falls below their threshold of detection. None of the data reported by Croft {\it et al} challenge the universality of the intra-unit cell order in cuprates.

Effect of lattice distortion on uranium magnetic moments in U4Ru7Ge6 studied by polarized neutron diffraction

Small spontaneous lattice distortion of a cubic ferromagnet at temperatures below $T_{\mathrm{C}}$ is expected, but usually a neglected phenomenon. This study is focused on such effect on the example of $\mathrm{U_{4}Ru_{7}Ge_{6}}$. We propose the lattice distortion from the cubic space group to a lower symmetry as a result of our DFT calculations. The strong spin-orbit coupling on the U site plays the most essential role, that should lower the symmetry of the system and give rise to the two different U sites (U1 and U2). We bring convincing experimental evidence for this splitting resulting from our polarized neutron diffraction experiment. Proper treatment of the flipping ratios collected in the ordered state and in the paramagnetic state reveals dramatically different magnetic moments on the U1 and U2 sites. We have reached good agreement of the results from the MAXENT method and from direct fitting of the data. The ratio of the orbital and spin component on the U2 site is lower than for the free $\mathrm{U^{3+}}$ or $\mathrm{U^{4+}}$ ion and points to a strong hybridization between the U $5f$ and Ru $4d$ wave functions. The same ratio on the U1 site is unexpectedly low. This would mean that its orbital component is almost quenched. We have observed the absence of a magnetic moment on the Ru1 site, but a rather large induced moment on the Ru2 site. Very similar results were obtained also in the paramagnetic state in the regime induced by the magnetic field. It points to the intimate coupling of the magnetic ordering and structural distortion as it is possible to split the U site in to two different polarizing paramagnetic U moments above $T_{\mathrm{C}}$ by a strong magnetic field. We propose that the difference between the magnetic moment on the U sites is caused by the change of local point symmetry of the sites that is tightly bound to the role of the strong spin-orbit interaction.

Polarized neutron diffraction using a novel high-T c superconducting magnet on the single-crystal diffractometer POLI at MLZ

The polarized single-crystal diffractometer POLI is the first neutron scattering instrument routinely using 3He spin filters both to produce and to analyse neutron polarization. The instrument, with a non-magnetic goniometer, was designed to perform two types of polarized neutron diffraction experiment: spherical neutron polarimetry, also known as full three-dimensional polarization analysis in zero magnetic field, and classical polarized neutron diffraction, also called the flipping-ratio (FR) method, in high applied magnetic fields. Reported here is the implementation of the FR setup for short-wavelength neutrons on POLI using a new high-T c superconducting magnet with a maximal field of 2.2 T. The complete setup consists of a 3He polarizer, a nutator, a Mezei-type flipper, guide fields and dedicated pole pieces, together with the magnet. Each component, as well as the whole setup, was numerically simulated, optimized, built and finally successfully tested under real experimental conditions on POLI. The measured polarized neutron spin transport efficiency is about 99% at different wavelengths, e.g. as short as 0.7 Å, and up to the maximal available field of the magnet. No further depolarization of the 3He cells due to stray fields of the magnet occurs. The additional use of the available 3He analyser allows uniaxial polarization analysis experiments in fields up to 1.2 T. The results of the first experiment on the field-dependent distribution of the trigonal antiferromagnetic domains in haematite (α-Fe2O3) are presented and compared with the literature data.

Neutron scattering study of spin density wave and weak ferromagnetic orders in [formula omitted

Microscopic coexistence between weak ferromagnetism (WFM) and superconductivity (SC) in Tb0.47Y0.53Ni211B2C was investigated. Magnetization measurements indicate that antiferromagnetic (AFM), WFM and SC transitions occur in this material. Unpolarized neutron diffraction experiments revealed that the AFM order is best described by a longitudinal spin density wave with a propagation vector of q=0.550a∗. Polarized neutron diffraction experiments further confirmed that the WFM order is originated in Tb0.47Y0.53Ni211B2C.

Nanoscopic length scale dependence of hydrogen bonded molecular associates' dynamics in methanol.

In a recent paper [C. E. Bertrand et al., J. Chem. Phys. 145, 014502 (2016)], we have shown that the collective dynamics of methanol shows a fast relaxation process related to the standard density-fluctuation heat mode and a slow non-Fickian mode originating from the hydrogen bonded molecular associates. Here we report on the length scale dependence of this slow relaxation process. Using quasielastic neutron scattering and molecular dynamics simulations, we show that the dynamics of the slow process is affected by the structuring of the associates, which is accessible through polarized neutron diffraction experiments. Using a series of partially deuterated samples, the dynamics of the associates is investigated and is found to have a similar time scale to the lifetime of hydrogen bonding in the system. Both the structural relaxation and the dynamics of the associates are thermally activated by the breaking of hydrogen bonding.

Magnetic properties and crystal field inPr2Zr2O7

In this work, we revisit the crystal field acting on the non-Kramers Pr3+ ion (4f2) in the quantum spin ice candidate Pr2Zr2O7 using both a standard calculation restricted to the ground spin-orbit multiplet and intermediate coupling states in the full basis of the f 2 configuration. Analysis of the thermal variation of the polycrystal magnetic susceptibility and of the local susceptibilities χ⊥ and χ// determined by means of polarized neutron diffraction experiments reveals that the effective antiferromagnetic exchange is strongly depleted at low temperature with respect to its high-temperature value.We then discuss the influence of crystal field imperfections arising from residual strains, which are especially important for a non-Kramers ion. We find that they are an essential ingredient to account for the very low temperature M(H) magnetization curves, showing that the saturation is not achieved even at 8 T. Furthermore, as possible candidates to qualitatively understand the Curie-like behavior observed below 0.5 K, we discuss the influence of the magnetic hyperfine interaction.

Purely organic magnets: How to get ferromagnetic interactions

Purely organic magnets, materials with spontaneous magnetization despite no containing magnetic ions, are very promising for technological applications due to their peculiar properties as flexibility, lightness or even biocompatibility. Most of the published purely organic magnets are free radical compounds. One of the most important difficulties in order to get spontaneous magnetization in such materials is that magnetic interactions among free radicals are usually antiferromagnetic and, in addition, ferromagnetic interactions are weaker than the antiferromagnetic ones. A possible strategy to overcome this issue is the use of triradical molecules with total spin S=1/2. In this case, an adequate packing of the triradical molecules can give place to antiferromagnetic interactions between regions with positive spin density and regions with negative spin density of two close molecules. This antiferromagnetic interaction between regions with opposite spin density would result in an overall ferromagnetic interaction between the two close triradicals. With this idea in mind we have performed an study of the spin density distribution and of the intramolecular and intermolecular magnetic interactions of the triradical compound 2-[3',5'-bis(Ntert-butylaminoxyl)phenyl]-4,4,5,5-tetramethyl-4,5-dihydro-1-Himidazol-1-oxyl-3-oxide, containing two N-tert-butyl aminoxyls and a nitronyl nitroxide groups. Combination of experimental data from a polarized neutron diffraction experiment and ab initio calculations (DFT) has allowed us to obtain the spin density distribution. In addition, the intramolecular and intermolecular magnetic interactions have been computed by ab initio quantum chemistry methods. The values for the intramolecular interactions confirm the S=1/2 ground state of the triradical. As for the intermolecular interactions, the two strongest ones are ferromagnetic, what is in agreement with the overlapping of regions with opposite spin density of the two interacting triradicals. These results support the strategy of using triradical molecules for obtaining purely organic magnets with higher magnetic transition ordering temperatures since is easier to obtain ferromagnetic interactions between the radicals and these interactions, having an antiferromagnetic origin, can be stronger than typical ferromagnetic interactions between radicals.

An unique model for joint refinement of (polarised) neutron and X-ray data

A new charge and spin density model and the corresponding refinement software were recently developed to combine X-ray and polarised neutron diffraction experiments [1,2]. This joint refinement procedure allows for an access to both the charge and spin densities but also to spin up ( ) and spin down ( ) electron distributions. These two quantities ( and ) were thus separately modelled and for the first time it was possible to compare them with theoretical results. The first part of the presentation will introduce the refinement procedure and describe its application to the case of an end-to-end azido double bridged copper(II) complex[3]. The results of this joint refinement of X-ray and polarized neutron diffraction data will be compared to theoretical calculations. The second part will be devoted to recent applications to other materials including a purely organic radical.

Joint Refinement of Charge and Spin Densities

A new charge and spin density model and the corresponding refinement software were recently developed to combine X- ray and polarized neutron diffraction experiments [1] . This joint refinement procedure allows getting access to both charge and spin densitiy distributions by refining both spin up and down parameters for magnetic atoms. The paper is focused on the refinement procedure and its application to the case of an end-to-end azido double bridged copper(II) complex. The results of this joint refinement of X-ray and polarized neutron diffraction data are presented.

Polarized neutron diffraction at a spallation source for magnetic studies

The availability of high-power spallation neutron sources, along with advances in the development of coupled moderators and neutron polarizers, has made it possible to use polarized neutrons on time-of-flight diffractometers forin situstudies of phenomena contributing to field-induced magnetization of a material. Different electronic and structural phenomena that contribute to the overall magnetization of a material can be studied and clearly identified with polarized neutron diffraction measurements. This article reports the first results from polarized neutron diffraction experiments on a time-of-flight instrument at a spallation source. Magnetic field-induced rotation of electron spins in an Ni–Mn–Ga single crystal was measured with polarized neutron diffraction at the MAGICS reflectometer at the Spallation Neutron Source at Oak Ridge National Laboratory. The difference in intensities measured with spin-up and spin-down polarized neutrons is proportional to the field-induced magnetization of the crystal. The polarized neutron measurements indicate that the magnetic form factor for the 3delectrons of Mn in Ni–Mn–Ga is lower than the value reported earlier for an ideal spherical symmetry of electronic distribution. Future experiments for studying field-induced magnetization in materials following the current methodology are outlined.

Contributions of the electronic spin and orbital current to the ${\rm CoCl}_4^{2-}$ CoCl 42− magnetic field probed in polarised neutron diffraction experiments

Polarised neutron diffraction experiments conducted at 4.2 K on Cs(3)CoCl(5) crystals have been analysed by using a four-dimensional model Hilbert space made of ab initio n-electron wave functions of the CoCl(4)(2-) molecular ion. Two spin-orbit mixing coefficients and several configuration interaction coefficients have been optimized by fitting calculated magnetic structure factors to experimental ones, to obtain the best ensemble density operator that is representable in the model space. A goodness of fit, χ(2), less then 1 has been obtained for the first time for the two experimental data sets available. In the present article, the optimized density operators are used to calculate the magnetic field densities that are the genuine observables probed in neutron diffraction experiments. Density maps of such observables are presented for the first time and numerical details are provided. The respective contributions of spin density and orbital current to the magnetic field density are analyzed.