Multi-wire proportional chambers filled with the He + CF4 gas mixture with a delay line readout are widely used for thermal neutron detection, especially in Small-Angle Neutron Scattering (SANS) instruments [1–3]. The purpose of the discussed work was to find a set of strongly interrelated detector design parameters with the aim to improve the detector performance. After careful consideration of important detector parameters, such as the space resolution, the efficiency and differential and integral nonlinearity, a satisfactory solution was found. In order to guarantee a reliable and optimal operation of the neutron detector, a wide range of scientific and technological investigations was carried out to: • improve and optimize the detector mechanical characteristics, simulations of the detector pressure behavior up to 10 bar were performed; • minimize the detector outgassing, a new technology for fabrication of the detector electrodes was successfully developed. It has opened a way to improve the gas purity by a few orders of magnitude; • minimize the gas leakage to a level smaller than 0.03 % per day, the detector gas sealing technology was improved. This paper describes general approaches and technological solutions that have allowed us to develop thermal neutron detectors for the SANS diffractometers “Vector” and “Membrana-2” at the VVR-M reactor of PNPI. The required parameters of the detector are listed in Table 1.

Spin dynamics in Fe65Ni35 Invar alloy has been studied by left–right asymmetry of small-angle polarized neutron scattering below TC=485 K in external magnetic fields of H=0.05–0.25 T inclined relative to the incident beam. The spin-wave stiffness D and the damping Γ were obtained by fitting the antisymmetrical contribution to the scattering. The spin-wave stiffness extrapolated by a (T/TC)5/2 law to T=0 K is D0=117±2 meVÅ2, which is somewhat smaller than the spin-wave stiffness obtained by triple-axis spectrometry.

Представлены результаты исследования спин-волновой динамики классического инварного сплава Fe65Ni35 методом малоуглового рассеяния поляризованных нейтронов при наклонной геометрии магнитного поля в широком температурном диапазоне (TTC)Метод основан на анализе лево-правой асимметрии в магнитном рассеянии поляризованных нейтронов, возникающей в случае, когда вектор намагниченности в образце наклонен по отношению к волновому вектору падающего пучка. Спин-волновое рассеяние сконцентрировано, в основном, вблизи критического угла c который зависит от магнитного поля как (H) = -(g BH) 0/E, где , H – величина внешнего магнитного поля, E – начальная энергия нейтрона, D – постоянная жесткости спиновой волны, mn – масса нейтрона. В окрестности критического угла рассеяние размыто спин-волновым затуханием. Постоянная жесткости спиновой волны D была получена в результате сравнения антисимметричного вклада в рассеяние с модельной функцией. Температурная зависимость D=D(T) хорошо описывается выражением D=D0| |x, где , x =0.47±0.01, D0=137±3 мэВ · A2 для всего диапазона температур ?>0.1. Данный метод позволяет с высокой точностью и малым шагом построить температурную зависимость постоянной жесткости спиновой волны.

The magnetic state of Y 1 − x Tb x Mn 6 Sn 6 (with x ≤ 0.25) alloy is studied at different temperatures and magnetic fields by neutron diffraction. The alloy with x c = 0.22 exhibits an incommensurate-to-commensurate structure phase transition. The transition is accompanied by a decrease in the unit-cell volume. The weak dependence of the intensity of satellites on the x concentration and the monotonic displacement of their angular positions with increasing x allow us to conclude that the antiferromagnet → ferrimagnet transition occurs through the uniform magnetic state. The incommensurate phase in YMn 6 Sn 6 exhibits four satellites, which are observed in the low-angle range of X-ray diffraction pattern and demonstrate different behavior with changing temperature. At 293 K, the antiferromagnet → ferromagnet transition in the Y 1 − x Tb x Mn 6 Sn 6 compound can be induced by applied magnetic field, in particular, the field μ 0 H = 0.3 T induces the transition in Tb 0.15 Y 0.85 Mn 6 Sn 6 .

Three series of anodic alumina membranes have been studied using the small-angle diffraction of neutrons and synchrotron radiation. The samples are obtained by the oxidation of aluminum wafers by sulfuric and oxalic acids at various anodization voltages and differences in the distance between the pores. Using experiments on small-angle diffraction, a linear dependence between the average grain size of an initial aluminum wafer and the average rectilinearity of pores of the synthesized membranes is established. We suggest that the observed correlation is caused by the influence of the crystallographic orientation of grains of an initial aluminum wafer on the growth of a porous oxide film.

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic cobalt nanowires embedded into Al2O3 matrix have been studied using polarized small-angle neutron scattering. A comprehensive analysis of contributions to the scattering intensity was carried out, including nonmagnetic (nuclear) contribution, magnetic contribution depending on the magnetic field, and nuclear–magnetic interference indicating the correlation between the magnetic and nuclear structures. Experiments have revealed an anomalously low value of the magnetic contribution as compared to the nuclear one. This behavior is interpreted in terms of low coherence of the magnetic structure caused by the anisotropy of Co crystallites as compared with the large coherency of nuclear structure of nanowires.

Data on the experimental detection and use of three-particle (chiral) spin dynamic correlations in ferromagnets are presented. The oblique-geometry method for investigating polarized neutron small-angle scattering is described, which gives the dependence that the scattering asymmetry has on the polarization P signs and the scattering angle θ. The following results of the dynamics investigation in the critical and ferromagnetic phases in the magnetic field are presented: the temperature dependence of the critical field H c , the factorization of the momentum transfer dependence of three-particle vertices, the corroboration of the “hard” version of the dipole critical dynamics, and the dynamics of amorphous magnets and invars.

The critical small-angle scattering of polarized neutrons from spin fluctuations in a nickel single crystal with a special inclined geometry of the magnetic field has been studied. The method of inclined geometry makes it possible to investigate not only two-particle spin correlations but also three-particle spin correlations that determine the polarization-dependent contribution to scattering, which is asymmetric with respect to the momentum transfer q . This contribution depends on the momentum transfer q as 1/( q 2 + ξ −2 ) 5/2 , where f is the neutron scattering correlation length; it linearly increases with an increase in the magnetic field H in the low-field range and then reaches saturation. The results obtained are in good agreement with the similarity theory.

Structural and magnetic properties of two-dimensional spatially ordered system of ferromagnetic cobalt nanowires embedded into Al2O3 matrix have been studied using polarized small-angle neutron scattering. The small-angle scattering pattern exhibits many diffraction peaks (up to the third reflection order), which corresponds to the scattering from highly correlated hexagonal structure of pores and magnetic nanowires. A comprehensive analysis of contributions to the scattering intensity, including nonmagnetic (nuclear) contribution, magnetic contribution depending on the magnetic field, and nuclear-magnetic interference indicating the correlation between the magnetic and nuclear structures was carried out. A detailed pattern of the remagnetization of an ordered array of the magnetic nanowires has been obtained. It has been illustrated that polarized neutron scattering provides unique information inaccessible by the standard magnetometry techniques.

The structural and magnetic characteristics of two-dimensional spatially ordered arrays of magnetic nickel nanowires embedded in the anodized alumina template have been investigated. It has been shown using small-angle polarized-neutron diffraction that, there exists the samples under investigation, in a highly ordered hexagonal structure of pores and magnetic nanowires separated by a characteristic distance d = 106 ± 2 nm. An analysis has been made of different contributions to neutron scattering, such as the nonmagnetic (nuclear) contribution, the magnetic contribution dependent on the magnetic field, and the interference contribution indicating a correlation between the magnetic and nuclear structures. The performed analysis of the results obtained has demonstrated that, when the magnetic field is applied perpendicular to the longitudinal axis of the nanowire in a completely magnetized sample, there arise demagnetizing fields around each nanowire that form a regular hexagonal lattice.

Inverse photonic nickel-based crystal films formed by electrocrystallization of metal inside the voids of polymer artificial opal have been studied using the microradian X-ray diffraction. Analysis of the diffraction images agrees with an face-centred cubic (FCC) structure with the lattice constant a0 = 650 ± 10 nm and indicates two types of stacking sequences coexisting in the crystal (twins of ABCABC... and ACBACB... ordering motifs), the ratio between them being 4:5 The transverse structural correlation length Ltran is 2.4 ± 0.1 μm, which corresponds to a sample thickness of 6 layers. The in-plane structural correlation length Llong is 3.4 ± 0.2 μm, and the structure mosaic is of order of 10°.

Certainty in the spin wave stiffness D 0 was achieved for the first time in the problem of invars under Fe-Ni alloy polarized neutron scattering. A temperature power dependence of stiffness D = D 0 Τ X , with x ≅0.5, was obtained for the entire ferromagnetic region Τ = 1 — T/T c , from Τ = 0.1to Τ = 0.9. The spin stiffness was found to be D 0 = 137 meVÅ 2 . Possible causes of scaling dependence CD(T) are discussed.

The type and degree of imperfection for opal-like photonic crystals on conducting substrates have been investigated using synchrotron small-angle X-ray scattering with a microradian resolution. It has been demonstrated that self-assembly of poly(styrene) spheres by the vertical deposition method leads to the formation of a face-centered cubic structure on a mica/Au substrate and a random hexagonal close packing on a glass substrate with the In2O3(SnO2) conducting coating.

The merits and demerits of the methods for reversing neutron polarization and of the devices (flippers) for neutron spin flipping relative to the magnetic field are discussed. A flipper capable of operating under problem conditions—when installed on a neutron guide and in vacuum—is described.

Magnon Bose condensation (BC)in the symmetry breaking magnetic field is a result of unusual form of the Zeeman energy, which has terms linear in the spin-wave operators and terms mixing excitations differ in the Wave-vector of the magnetic structure. The following examples are considered: simple easy-plane tetragonal antiferromagnets (AF), frustrated AF family$R_2Cu O_4$ where $R=Pr,Nd$ etc. and cubic magnets with the Dzyaloshinskii-Moriya interaction ($Mn Si$ etc.). In all cases the BC becomes important when the magnetic field becomes comparable with the spin-wave gap. The theory is illustrated by existing experimental results.

Below T-C = 29 K the weak itinerant ferromagnet MnSi becomes ordered in a left- handed spin helical structure as a result of the Dzyaloshinskii - Moriya ( DM) interaction. We give a recipe for calculating the orientation of the helix under an applied field. The recipe is derived on a basis of a theory recently developed for cubic magnets with DM interaction. The theory evaluates the ground state energy and the spin wave spectrum. It is shown that in zero field the orientation of the helix depends solely on the anisotropic exchange interaction and cubic anisotropy. Under an applied field the helix possesses two types of magnetic susceptibility: one parallel and another perpendicular to the applied magnetic field. The perpendicular susceptibility is related to the fact that the helical structure itself is unstable with respect to the small magnetic field H applied perpendicularly to the wavevector k. The spin wave gap Delta provides the stability of the spin wave spectrum of the helix structure and its presence may be revealed in the magnetic field behaviour. Our calculations show the essence of the field- induced transformations of the magnetic structure related to the spin wave gap. The experimental data provide the evidence for its existence. On the basis of our findings we discuss a possible scenario for the quantum phase transition in MnSi.

The magnetic structure of Fe1-xCoxSi single crystals with x=0.10,0.15,0.20,0.50 has been studied by small angle polarized neutron diffraction and superconducting quantum interference device measurements. Experiments have shown that in zero field the compounds with x=0.1,0.15 have a well-defined tendency to order in the one-handed spiral along [100] axes due to the anisotropic exchange, that, however, decreases with increasing Co concentration x. The magnetic structure of Fe1-xCoxSi with x=0.2,0.5 consists of spiral domains with randomly oriented spiral wave vector k. The applied magnetic field produces a single domain helix oriented along the field. The process of the reorientation starts at the field H-C1. Further increase of the field leads to a magnetic phase transition from a conical to a ferromagnetic state near H-C2. In the critical range near T-C the integral intensity of the Bragg reflection shows a well-pronounced minimum at H-fl attributed to a k flop of the helix wave vector. On the basis of our experiments we built the H-T phase diagram for each compound. It is shown that the same set of the parameters governs the magnetic properties of these compounds k, H-C1, H-fl, and H-C2. Our experimental findings are well interpreted in the framework of a recently developed theory [Phys. Rev. B 73, 174402 (2006)] for cubic magnets with Dzyaloshinskii-Moriya (DM) interaction. In particular, the theory suggests an additional quantum term in the magnetic susceptibility caused by the DM interaction which is in good agreement with the experiment.

The spiral structure of the single crystal MnSi has been studied by SANS near TC=29K in the magnetic field. When the field (H>20mT) is applied along one of the 〈111〉 axes, it produces a single domain sample with the helix wave vector along the field. The 90°-reorientation of the spin spiral from the [111] axis to [1−10] axis is observed in the field range from 130 to 180mT in close vicinity to TC. Further increase of the field above H=180mT, restores the original orientation of the helix and leads to the induced ferromagnetic state at H=350mT. The SANS experiments demonstrate that not a new phase, but helix reorientation occurs in the narrow domain of the H–T phase diagram near TC.