Polarized proton spin filter for epithermal neutrons based on dynamic nuclear polarization using photo-excited triplet electron spins

To polarize neutrons with energy beyond 0.1 eV, we developed a novel polarized proton spin filter based on dynamic nuclear polarization using photo-excited triplet electron spins. The spin filter consists of a single crystal of naphthalene doped with deuterated pentacene and has a size of $\varnothing15\times4 \, {\rm mm}^3$, allowing it to cover a wide beam diameter. It was operated in 0.35 T and at 90 K. We succeeded in polarizing neutrons in the energy range 0.1–10 eV using a RIKEN accelerator-driven compact neutron source. The averaged values of the proton and neutron polarization were $0.250\pm0.050$ and $0.076\pm0.015$, respectively.

In-plane uniaxial pressure-induced out-of-plane antiferromagnetic moment and critical fluctuations in BaFe2As2

A small in-plane external uniaxial pressure has been widely used as an effective method to acquire single domain iron pnictide BaFe2As2, which exhibits twin-domains without uniaxial strain below the tetragonal-to-orthorhombic structural (nematic) transition temperature Ts. Although it is generally assumed that such a pressure will not affect the intrinsic electronic/magnetic properties of the system, it is known to enhance the antiferromagnetic (AF) ordering temperature TN ( < Ts) and create in-plane resistivity anisotropy above Ts. Here we use neutron polarization analysis to show that such a strain on BaFe2As2 also induces a static or quasi-static out-of-plane (c-axis) AF order and its associated critical spin fluctuations near TN/Ts. Therefore, uniaxial pressure necessary to detwin single crystals of BaFe2As2 actually rotates the easy axis of the collinear AF order near TN/Ts, and such effects due to spin-orbit coupling must be taken into account to unveil the intrinsic electronic/magnetic properties of the system.

First Experiment of Spin Contrast Variation Small-Angle Neutron Scattering on the iMATERIA Instrument at J-PARC

Recently, we have developed a novel dynamic nuclear polarization (DNP) apparatus with a magnetic field of 7 T and a sample temperature of 1 K. High proton spin polarizations from −84% to 76%, for TEMPO doped polystyrene samples, have been demonstrated. This DNP apparatus satisfies the simultaneous requirement for quick and easy sample exchange and high DNP performance. On the iMATERIA (BL20) instrument at J-PARC, the first beam experiment using this DNP apparatus has been performed. For this experiment, the beamline was equipped with a supermirror polarizer. The stray magnetic field due to the superconducting magnet for DNP was also evaluated. The stray magnetic field plays an important role for in maintaining the neutron polarization during the transportation from the polarizer to the sample. The small-angle neutron scattering (SANS) profiles of silica-filled rubber under dynamically polarized conditions are presented. By applying our new analytical approach for SANS coherent scattering intensity, neutron polarization (PN) as a function of neutron wavelength was determined. Consequently, for the neutron wavelength, range from 4 Å to 10 Å, |PN| was sufficient for DNP-SANS studies.

The Interaction Potential

The interaction potentials and their spatial Fourer transforms are derived for nuclear and magnetic scattering, as well as for interactions with atomic electric fields. For the nuclear interaction, the Fermi pseudopotential is introduced and the scattering length operator is defined. The neutron spin dependence of the nuclear and magnetic interaction is calculated, and general expressions for spin-dependent scattering are obtained. The longitudinal and XYZ polarization analysis methods are described, and the technique of spherical neutron polarimetry is explained. The Blume-Maleev equation which gives the final neutron polarization for an arbitrary incident polarization are derived.

Magnetic Diffraction

The basic concepts of magnetic order in crystals are reviewed, including magnetic unit cells, propagation vectors and magnetic domains. Some commonly-occuring magnetic structures are discussed, such as ferromagnets, antiferromagnets, ferrimagnets, and noncollinear and incommensurate magnetic structures. The differential cross-section for neutron diffraction from a magnetic structure is derived, and the magnetic structure factor is defined. The use of neutron polarization analysis, including spherical neutron polarimetry, in the determination of magnetic structures and of the spatial distribution of magnetization is described in detail. Diffuse magnetic scattering due to magnetic frustration and magnetic phase transitions is discussed, and the relevance of the static approximation is explained. Neutron diffraction studies of nuclear spin order are described.

Principles of Neutron Scattering from Condensed Matter

The book contains a comprehensive account of the theory and application of neutron scattering for the study of the structure and dynamics of condensed matter. All the principal experimental techniques available at national and international neutron scattering facilities are covered. The formal theory is presented, and used to show how neutron scattering measurements give direct access to a variety of correlation and response functions which characterize the equilibrium properties of bulk matter. The determination of atomic arrangements and magnetic structures by neutron diffraction and neutron optical methods is described, including single-crystal and powder diffraction, diffuse scattering from disordered structures, total scattering, small-angle scattering, reflectometry, and imaging. The principles behind the main neutron spectroscopic techniques are explained, including continuous and time-of-flight inelastic scattering, quasielastic scattering, spin-echo spectroscopy, and Compton scattering. The scattering cross-sections for atomic vibrations in solids, diffusive motion in atomic and molecular fluids, and single-atom and cooperative magnetic excitations are calculated. A detailed account of neutron polarization analysis is given, together with examples of how polarized neutrons can be exploited to obtain information about structural and magnetic correlations which cannot be obtained by other methods. Alongside the theoretical aspects, the book also describes the essential practical information needed to perform experiments and to analyse and interpret the data. Exercises are included at the end of each chapter to consolidate and enhance understanding of the material, and a summary of relevant results from mathematics, quantum mechanics, and linear response theory, is given in the appendices.

BornAgain: software for simulating and fitting grazing-incidence small-angle scattering

BornAgain is a free and open-source multi-platform software framework for simulating and fitting X-ray and neutron reflectometry, off-specular scattering, and grazing-incidence small-angle scattering (GISAS). This paper concentrates on GISAS. Support for reflectometry and off-specular scattering has been added more recently, is still under intense development and will be described in a later publication. BornAgain supports neutron polarization and magnetic scattering. Users can define sample and instrument models through Python scripting. A large subset of the functionality is also available through a graphical user interface. This paper describes the software in terms of the realized non-functional and functional requirements. The web site https://www.bornagainproject.org/ provides further documentation.

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

The polarized single-crystal diffractometer POLI is the first neutron scattering instrument routinely using3He 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-Tcsuperconducting magnet with a maximal field of 2.2 T. The complete setup consists of a3He 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 the3He cells due to stray fields of the magnet occurs. The additional use of the available3He 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.