S.V. Grigoriev

Spin-echo small angle neutron scattering in Delft

We describe two spin-echo instruments for neutron small angle scattering, which have been installed at the reactor institute in Delft. The first setup is using a monochromatic beam and magnetized foils as spin flippers, while the second uses resonant spin flippers in a pulsed neutron beam. The components that play an essential role for operation are described in some detail. Each setup has specific advantages in its range of spin-echo lengths that covers the range of correlation lengths that could be measured. This is demonstrated in a comparative measurement, the setup with magnetized foils measuring at spin-echo-lengths up to 20 μm and the setup with resonant flippers measuring in the range up to 0.5 μm.

Peculiarities of the construction and application of a broadband adiabatic flipper of cold neutrons

Abstract An available mathematical model for calculation of a broadband adiabatic flipper for cold polarized neutrons is suggested. This model of the sine-cosine modulation of an effective field enables one to optimize and to simplify the calculation for constructing the flipper. A functional relation between spin flip probability and the main parameters of the device such as the length of the flipper, the neutron wavelength and both form and size of the magnetic fields have been found. Tests of flippers constructed on the base of this model has been successfully carried out.

Elastic Neutron Scattering Measurements Using Larmor Precession of Polarized Neutrons

An overview will be given of new instruments using Larmor precession of polarized neutrons in precession regions with inclined front and end faces. These instruments concern small angle scattering, neutron reflectometry and high-resolution diffraction. The advantages of the first application, spin echo small angle scattering (SESANS), with respect to conventional SANS, is the range of applicability and orders of magnitude higher available intensity. The reflectometry application makes it possible to measure the momentum dependent intensity without hindrance of the waviness of the sample also with the high intensity of SESANS. The high resolution application enables one to measure very high resolution diffraction (10−5 relative in momentum space) without angular of wavelength confinement of the beam, thus with very high intensity.

Test of adiabatic spin flippers for application at pulsed neutron sources

Abstract Experimental results on the flipping efficiency are shown for a set of 2 V -coils as spin flipper and for a high-frequency flipper with adiabatic transition. The influence of the adiabaticity parameter is discussed. The merits of these adiabatic flippers are compared with the use of “monochromatic” flippers, when operated in a beam from a pulsed neutron source. It is concluded that for “long pulse” sources adiabatic flippers will be superior.

Technical Aspects of Larmor Precession with Inclined Front and End Faces

Some technical and physical features of Larmor precession techniques will be discussed. Various options to encode the transmission angle of the neutron beam by inclined front and end faces using DC fields are considered, under which magnetized foils and wedge shaped precession regions. The use of shaped pole faces as precession regions to avoid material in the transmitted beam are considered together with correction methods for the inhomogeneous field line integrals accompanied by those magnetic fields. It appears that the use of π flippers as occurring in the resonance method are of great advantage.

Three-dimensional magnetic spin-echo small-angle neutron scattering and neutron depolarization: A comparison

The recently developed magnetic spin-echo small-angle neutron scattering (SANS) technique provides unique information about the distance correlation of the local vector magnetization as a function of the spin-echo length within a magnetic material. The technique probes the magnetic correlations on a length scale from 10nm up to 10μm within the bulk of a magnetic material by evaluating the Larmor precession of a polarized neutron beam in a spin-echo setup. The characteristics of the spin-echo SANS technique are discussed and compared to those of the more conventional neutron depolarization technique. Both of these techniques probe the average size of the magnetic inhomogeneities and the local magnetic texture. The magnetic spin-echo SANS technique gives information on the size distribution of these magnetic inhomogeneities perpendicular to the beam and, in principle, independent on the local magnetic induction. This information is not accessible by the neutron depolarization technique that gives the average size parallel to the beam multiplied with the square of the local magnetic induction. The basic possibilities of the magnetic spin-echo SANS technique are demonstrated by experiments on samples with a strong magnetic texture.The recently developed magnetic spin-echo small-angle neutron scattering (SANS) technique provides unique information about the distance correlation of the local vector magnetization as a function of the spin-echo length within a magnetic material. The technique probes the magnetic correlations on a length scale from 10nm up to 10μm within the bulk of a magnetic material by evaluating the Larmor precession of a polarized neutron beam in a spin-echo setup. The characteristics of the spin-echo SANS technique are discussed and compared to those of the more conventional neutron depolarization technique. Both of these techniques probe the average size of the magnetic inhomogeneities and the local magnetic texture. The magnetic spin-echo SANS technique gives information on the size distribution of these magnetic inhomogeneities perpendicular to the beam and, in principle, independent on the local magnetic induction. This information is not accessible by the neutron depolarization technique that gives the averag...

Zero-field precession induced by adiabatic RF spin flippers

Abstract A neutron “precession device” consisting of two adiabatic gradient field RF spin flippers was built. The “zero field precession” (ZFP) is demonstrated over the path length between them in a spin echo experiment in which a DC coil is used for compensation. Signals as a function of this DC field are shown for a single wavelength and for a “white” neutron spectrum. This development extends the use of ZFP to white neutron beams.

Polarized small-angle neutron scattering study of two-dimensional spatially ordered systems of nickel nanowires

The magnetic and structural properties of two-dimensional spatially ordered systems of ferromagnetic nickel nanowires embedded into an Al2O3 matrix have been studied using polarized small-angle neutron scattering (polarized SANS). We measured the total (nuclear and magnetic) scattering I(q) as a polarization-independent scattering, the field-dependent scattering as IH(q) = I(q, H) − I(q, 0), where H is the magnetic field, and the nuclear-magnetic interference as a polarization-dependent (P) scattering ΔI(q, P). A typical scattering pattern is composed of the diffuse small-angle scattering and the Bragg peak. It is shown that the introduction of Ni into the matrix does not change the position of the Bragg peak but results in an increase of the scattering intensity both in the small-angle region and at the Bragg positions. An external magnetic field was applied perpendicular or parallel to the long dimension of the nanowires in order to reveal the anisotropic properties of the magnetic system. It is shown that, firstly, the magnetic-field-dependent scattering IH(q) provides new and principally different information as compared with the interference term ΔI(q). Secondly, two contributions to the interference term ΔI(q) (ascribed to the diffuse scattering and to the diffraction peaks) have different signs indicating different origins of the scattering objects. Thirdly, polarized SANS gives a detailed picture of the magnetization process, which could not be obtained by methods of standard magnetometry.

SANS study of new magnetic nanocomposites embedded into the mesoporous silica

Abstract Magnetic nanocomposites embedded into mesoporous silica have been studied by Small Angle Polarized Neutron Scattering. It is well known that mesoporous silica represents a highly regular hexagonal structure of nanotubes. For the samples under study a diffraction peak in SANS at q c ≈1.7 nm −1 is observed which corresponds to a hexagonal structure with periodicity a≈3.5 nm . We found that the intercalation of iron into the silica matrix leads to some changes of the matrix itself. Additionally, the nuclear–magnetic interference in scattering of polarized neutrons was investigated. The interference reveals the absence of a periodical magnetic structure at room temperature which would be consistent with the hexagonal structure of nanotubes. The correlation between magnetic nanoparticles and pores of the matrix is revealed in the interference scattering at q⪡qc.

Spectrometer combining time-of-flight and Larmor modulation

Abstract Neutron beam instruments applying Larmor modulation for scattered, and time-of-flight for incoming wavelength measurement (or vice-versa) in principle yields quasi- and inelastic spectra. Since there is no selection of neutron wavelengths for either the incoming or scattered beam, much of the available flux is used and there is no a priori selection of the energy transfer range and resolution. This type of instrument may be applied at the future ESS pulsed neutron source in order to complement current types of spectrometers.