XYZ Polarization Analysis Research Articles

New design of a magnetic device for wide-angle XYZ polarization analysis PASTIS-3, from the concept to first tests with thermal neutrons

We present the new concept and neutron test results of the third generation PASTIS device for XYZ wide-angle polarization analysis for thermal neutrons. PASTIS-3 uses two polarized 3He Neutron Spin filter Cells (NSF) for polarizing and analyzing neutrons spins, flipping the incident neutron beam polarization by reversing the 3He spin state of the polarizing entrance cell. We detail a new coil design for high spatial homogeneity of the magnetic field in any field direction, and the calculation method employed to optimize the geometry and to characterize the field gradients. Various components of the apparatus are described with particular attention to their performance and impact on the relaxation time of the NSF cells. Finally, we present test results of the new device tested on the thermal triple-axis spectrometer IN20 at the ILL.

Long-range magnetic ordering in Li2MnGeO4 and precursor short-range spin correlations

Long-range magnetic ordering and short-range spin correlations in layered noncentrosymmetric orthogermanate ${\mathrm{Li}}_{2}\mathrm{MnGe}{\mathrm{O}}_{4}$ were studied by means of polarized and unpolarized neutron scattering. The combined Rietveld refinement of synchrotron and neutron powder diffraction data at room temperature within the $Pmn{2}_{1}$ space group allowed us to specify the details of the crystal structure. According to the additional Bragg peaks in low-temperature neutron diffraction patterns a long-range antiferromagnetic ordering with the propagation vector $k=\phantom{\rule{0.16em}{0ex}}(1/2\phantom{\rule{0.16em}{0ex}}1/2\phantom{\rule{0.16em}{0ex}}1/2)$ has been found below ${T}_{N}\ensuremath{\approx}8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Symmetry analysis revealed the model of the ground state spin structure within the ${C}_{a}c$ (no. 9.41) magnetic space group. It is represented by the noncollinear ordering of manganese atoms with a refined magnetic moment of $4.9\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/{\mathrm{Mn}}^{2+}$ at 1.7 K, which corresponds to the saturated value for the high-spin configuration $S=5/2$. Diffuse magnetic scattering was detected on the neutron diffraction patterns at temperatures just above ${T}_{N}$. Its temperature evolution was investigated in detail by polarized neutron scattering with the following XYZ-polarization analysis. Reverse Monte Carlo simulation of diffuse scattering data showed the development of short-range ordering in ${\mathrm{Li}}_{2}\mathrm{MnGe}{\mathrm{O}}_{4}$, which is symmetry consistent on a small scale with the long-range magnetic state below ${T}_{N}$. The reconstructed radial spin-pair correlation function $S(0)\phantom{\rule{0.16em}{0ex}}S(r)$ displayed the predominant role of antiferromagnetic correlations. It was found that spin correlations are significant only for the nearest magnetic neighbors and almost disappear at $r\ensuremath{\approx}12\phantom{\rule{0.16em}{0ex}}\AA{}$ at 10 K. Temperature dependence of the diffuse scattering implies short-range ordering long before the magnetic phase transition. Besides, the spin arrangement was found to be similar in both cases above and below ${T}_{N}$. As a result, an exhaustive picture of the gradual formation of magnetic ordering in ${\mathrm{Li}}_{2}\mathrm{MnGe}{\mathrm{O}}_{4}$ is presented.

Critical size limits for collinear and spin-spiral magnetism in CoCr2O4

The multiferroic behavior of CoCr$_2$O$_4$ results from the appearance of conical spin-spiral magnetic ordering, which induces electric polarization. The magnetic ground state has a complex size dependent behavior, which collapses when reaching a critical particle size. Here, the magnetic phase stability of CoCr$_2$O$_4$ in the size range of 3.6 - 14.0 nm is presented in detail using the combination of neutron diffraction with XYZ polarization analysis and macroscopic magnetization measurements. We establish critical coherent domain sizes for the formation of the spin spiral and ferrimagnetic structure and reveal the evolution of the incommensurate spin spiral vector with particle size. We further confirm the presence of ferroelectric polarization in the spin spiral phase for nanocrystalline CoCr$_2$O$_4$.

Spin Correlations and Short‐Range Magnetic Order in the Honeycomb‐Layered Na2Ni2TeO6

An experimental study of long‐range magnetic order formation mechanisms in a layered structure with a honeycomb arrangement of the magnetic atoms Na2Ni2TeO6 is conducted. For the first time, the strong spin correlations are directly observed above the Neel temperature TN that is manifested in the presence of broad diffuse peaks on neutron diffraction patterns obtained with the XYZ polarization analysis. Due to the possibility of separating the magnetic, nuclear incoherent, and nuclear coherent contributions to the total neutron scattering cross section, it is unequivocally established that the observed diffuse scattering has magnetic nature. The spin‐pair correlation function is reconstructed by modeling diffuse neutron scattering on Na2Ni2TeO6 with reverse Monte Carlo method. The obtained results indicate 2D nature of the magnetic correlations, and moreover, the symmetry of short‐range magnetic state corresponds to long‐range zigzag‐type magnetic order in the honeycomb net, which is established earlier based on the theoretical calculations.

Noncollinear magnetism in nanosized cobalt chromite

Using a combination of neutron diffraction with XYZ polarization analysis and magnetization measurements, the noncollinear magnetism in nanosized cobalt chromite, a potential multiferroic material, is revealed. For noninteracting 26.9(1) nm nanoparticles, a bulklike behavior is identified, including a ferrimagnetic Curie temperature of 99 K and a transition to the spin spiral magnetic phase at 27 K with a temperature-dependent, incommensurate propagation vector. A lock-in transition towards a commensurate propagation vector is not observed. Much smaller, 3.1(2) nm, nanoparticles reveal a strong cluster glass behavior, characterized by ferrimagnetic behavior below the Curie temperature of 43 K and a transition to asperomagnetic behavior at 18 K, with the absence of any magnetic reflections at a base temperature of 5 K.

Investigation of TbMn2O5 by polarized neutron diffraction

In order to make a new approach to the elucidation of the microscopic mechanisms of multiferroicity in the RMn2O5 family, experiments with different methods of polarized neutrons scattering were performed on a TbMn2O5 single crystal. We employed three different techniques of polarized neutron diffraction without the analysis after scattering, the XYZ-polarization analysis, and technique of spherical neutron polarimetry (SNP). Measurements with SNP were undertaken both with and without external electric field. A characteristic difference in the population of ‘right’ and ‘left’ helix domains in all magnetically ordered phases of TbMn2O5, was observed. This difference can be controlled by an external electric field in the field-cooled mode. The analysis of the results gives an evidence that antisymmetric Dzyaloshinsky-Moria exchange is effective in all the magnetic phases in TbMn2O5.

Off-axis spin orientation in goethite nanoparticles

Neutron diffraction is a powerful technique for determining the magnetic structure of antiferromagnetic materials. However, for some of these, determining the detailed magnetic structure remains a challenge. In goethite ($\ensuremath{\alpha}$-FeOOH) the antiferromagnetic unit cell coincides with the chemical unit cell and, consequently, nuclear and magnetic diffraction peaks occur at the same positions. Analysis of diffraction data from goethite is further complicated by finite-size peak broadening, resulting from goethite commonly occurring in nanocrystalline form. For these reasons, determining the magnetic structure of goethite has been challenging, and few detailed studies have been published. Even today, not all aspects of the magnetic structure are well established. Here, we investigate the magnetic structure of three samples of goethite nanoparticles with polarized neutron powder diffraction (xyz-polarization analysis). Two samples consist of acicular goethite particles that are approximately 40 nm long and with different thicknesses, and one sample consists of pseudo-spherical particles with a diameter of approximately 5 nm. The larger particles consist of several crystallites whereas the 5-nm particles are mostly single crystalline. The polarization analysis enables us to separate magnetic scattering from nuclear and spin-incoherent scattering, resulting in data that can readily be analyzed. For the two samples with the larger particle size, we find nuclear correlation lengths in the [100] direction that are approximately 3 nm longer than the magnetic correlation lengths, indicating a magnetically disordered layer perpendicular to the antiferromagnetic modulation direction. We find no evidence of a magnetically disordered surface layer in the 5-nm particles. We find the magnetic structure to be antiferromagnetic but, in contrast to most previous studies, we find the spin orientation in all three samples to make an angle of 28-${30}^{\ensuremath{\circ}}$ with respect to the crystallographic $b$ axis.

Magnetic structure of the noncentrosymmetric heavy-fermion superconductor CePt3Si

We have studied the magnetic structure of the noncentrosymmetric heavy-fermion superconductor CePt3Si using elastic polarized neutron scattering with field-projected (longitudinal) XYZ polarization analysis. We find that the magnetic moment has components both in the tetragonal basal plane and along the tetragonal four-fold axis. Such a magnetic structure is not symmetry-allowed if the magnetic phase transition is second order, and may imply that CePt3Si has lower symmetry in the superconducting state than originally thought.

PASTIS2 and CROCODILE: XYZ-wide angle polarisation analysis for thermal neutrons

We present a wide-angle device for inelastic neutron scattering with XYZ-polarisation analysis (PASTIS2). PASTIS2 employs a banana-shaped Si-walled 3He-filter for the polarisation analysis and allows pillar-free neutron scattering for horizontal scattering angles 0–100◦. The guide field direction at the sample can be chosen vertical or with 45◦ incremental steps in the horizontal scattering plane. When PASTIS2 is implemented on a polarised neutron beam, the incident neutron spin can be flipped with an easy-to-optimise broad-band adiabatic resonant flipper (CROCODILE) independent of the guide field direction at the sample position. We have tested the performance of this new device on the polarised thermal triple-axis spectrometer IN20 at the Institut Laue-Langevin, equipped with Heusler monochromator and the FlatCone multi-analyser, and discuss its potential for future instruments.

Recent on-beam tests of wide angle neutron polarization analysis with a 3He spin filter: Magic PASTIS on V20 at HZB

A complete XYZ polarization analysis solution is under development for the new thermal time of flight spectrometer TOPAS [1], to be operated in the coming east neutron guide hall at the MLZ. Polarization Analysis Studies on a Thermal Inelastic Spectrometer, commonly called PASTIS [2], is based on polarized 3He neutron spin filters and an XYZ field configuration for the sample environment and a polarization-preserving neutron guide field. The complete system was designed to provide adiabatic transport of the neutron polarization to the sample position while maintaining the homogeneity of the XYZ field. This system has now been tested on the polarized time-of-flight ESS test beam line V20 at HZB [3]. Down to the minimum wavelength of 1.6 Å on the instrument, the magnetic configuration worked ideally for neutron spin transport while giving full experimental freedom to change between the X, Y or Z field configuration. The 3He cell used was polarized at the 3He lab of the JCNS at the MLZ in Garching and transported to HZB in Berlin via car showing that such a transport is indeed feasible for such experiments. We present results of this test and the next steps forward.

First result from the magic-PASTIS using large 3He SEOP-polarized GE180 doughnut cell

We report on the first results of the newly proposed and prototyped PASTIS coil set, enabling for XYZ polarization analysis on the future thermal time-of flight spectrometers. Our setup uses a wide-angle banana shaped 3He Neutron Spin Filter cell (NSF) to cover a large range of scattering solid angle. The design assures relative magnetic field gradients < 10-3 cm-1 and large solid angle areas not interrupted by either coils or supports. In the vertical direction nearly 40° are open and the blind spots in the horizontal scattering plane comprise only 3° in 180° due to the square X and Y compensation coils. We present the first results of the field mapping and relaxations time measurements using a large 3He SEOP polarized GE180 doughnut cell.

Magnetic ordering in NdMn2O5 studied by the neutron diffraction

The magnetic ordering in NdMn2O5 single crystal has been studied by the neutron scattering, including XYZ-polarization analysis. At TN≈30K the onset of the long-range magnetic order with incommensurate wave vector k=(0.5 0 ~0.36) is observed. Below 30K two magnetic phases are observed with propagation vectors close to each other: k1=(0.5 0 kz1), k2=(0.5 0 kz2). kz1, kz2 components increase monotonously with temperature decrease down to 25K, then remain to be constant with values kz1=0.384(2), kz2=0.395(2). Two reorientation transitions were observed at ~20K and at ~5K.

An approach to the magnetic ground state of the molecular magnet {Mo72Fe30}

The nature of the magnetic ground state of the spin-frustrated molecular magnet {Mo72Fe30} is studied using polarized neutron scattering and specific heat methods. The magnetic scattering cross section up to 2.30 Å−1 has been determined by means of diffuse neutron scattering with xyz polarization analysis at temperatures from 1.5 to 100 K. The spin correlations observed at 1.5 K agree well with the simulation based on the three-sublattice spin configuration model for {Mo72Fe30}. The specific heat of {Mo72Fe30} has been measured from 60 mK to 15 K under zero external magnetic field. The energy gaps, 0.09(1) and 0.64(1) meV, identified by the Schottky-type anomalies in the specific heat data are in reasonable agreement with the low-lying magnetic excitations predicted by the quantum rotational band model for the three-sublattice spin configuration model. Therefore, our experimental study provides strong support that the three-sublattice spin configuration model is a good approach to the magnetic ground state of {Mo72Fe30}.

Antiferromagnetic spin correlations in MnO nanoparticles

We have investigated nearly monodispersive MnO nanoparticles of 10 nm diameter by polarised neutron diffraction with XYZ-polarisation analysis. We found no long-range ordering down to 3.5 K. A broad magnetic peak appeared close to Q= (1/2,1/2,1/2) signifying short-range antiferromagnetic correlations. The correlation length was found to be about 2.4 nm at T=3.5 K. The correlation length decreases rapidly with increasing temperature and becomes about 0.7 nm at T=250 K.

XYZ-polarisation analysis of diffuse magnetic neutron scattering from single crystals

Studies of diffuse magnetic scattering largely benefit from the use of a multi-detector covering wide scattering angles. Therefore, the different contributions to the diffuse scattering that originate from magnetic, nuclear coherent, and nuclear spin-incoherent scattering can be separated by the so-called XYZ-polarization analysis. In the past this method has been successfully applied to the analysis of diffuse scattering by polycrystalline samples of magnetic disordered materials. Single crystal studies that exploit the vector properties of spin correlations are of particular interest for furthering our understanding of frustration effects in magnetism. Based on the symmetry properties of polarised scattering a suitable extension of the conventional XYZ method has been derived, which allows for the complete separation and the analysis of features of diffuse magnetic scattering from single crystals.

The assets of crystal monochromator-Fermi chopper time-of-flight on continuous sources - potential for high efficiency PASTIS operation

Crystal monochromator-Fermi-chopper (FC) (X-TOF) instruments provide a high count-rate with flexible resolution optimization. We show that the most advantageous range of operation for this class of instruments falls in the thermal and sub-thermal range where focusing Bragg optics can be used to full benefit. In this spectral window, an X-TOF instrument at a high-flux reactor can match a chopper spectrometer on a MW power short pulse spallation source as can be shown by comparing the optimized concepts. The high count-rate makes this type of instrument highly attractive for polarized neutron operation. Recent experiments performed using a new large solid-angle 3He neutron spin-filter represent a significant breakthrough in the field of neutron polarization analysis studies on multi-detector spectrometers, forming the basis of the proposed PASTIS project which enables xyz-polarization analysis studies on a thermal TOF spectrometer (Stride et al., Stewart et al.). Outlook for implementation of PASTIS as a th...

Neutron polarization analysis study of crystal field excitations in CePtSn

We present the study of the Kondo antiferromagnet CePtSn, whose magnetic properties at T> T N can be explained by a strong anisotropy of the crystal-field (CF) potential on the Ce sites, which have an orthorhombic point-group symmetry. Applying a microscopic model with parameters deduced from magnetization and inelastic neutron scattering (INS) data, we predict a strong magnetic dipole moment parallel to the a-axis in the ground state. The dramatically increased count rate at the upgraded ILL three-axis spectrometer IN20 permits now to use the polarized beam even with a small single crystal sample (0.2 cm 3) to separate the CF signal from phonon background and to employ the XYZ polarization analysis to confirm directly this previously deduced anisotropy of the local CF Hamiltonian.

Optimising polarised neutron scattering measurements—XYZ and polarimetry analysis

The analytic optimisation of neutron scattering measurements made using XYZ polarisation analysis and neutron polarimetry techniques is discussed. Expressions for the “quality factor” and the optimum division of counting time for the XYZ technique are presented. For neutron polarimetry the optimisation is identified as analogous to that for measuring the flipping ratio and reference is made to the results already in the literature.

Polarized neutron scattering study of the kagome antiferromagnet SrCr 8Ga 4O 19

We present a polarized neutron scattering study on the magnetic fluctuations on a powder sample of the SrCr 8Ga 4O 19 compound. We use the D7 instrument at the ILL with the classical xyz-polarization analysis as well as time-of-flight analysis with Δ E≈1 meV energy resolution. We extract the true magnetic scattering over a broader temperature range than that earlier examined in other studies. The results show that the broadly peaked Q-dependence observed in the elastic response at low T⩽2 K persists in the dynamic signal up to T⩽50 K. At higher temperatures the peak broadens further into a structureless, paramagnetic like Q-dependence. Our data shows that the short-ranged dynamic AF correlations prevail up to a much higher temperature range than what had been earlier observed.