Characterisation of novel magnetic materials using the USANSPOL technique

USANSPOL is a novel ultra-small-angle scattering technique with polarised neutrons for investigation of magnetic materials. It represents a polarised neutron extension to traditional USANS which works with unpolarised neutrons. The high angular resolution of this technique relies on the narrow reflection width of perfect crystal reflections and is employed in a double-crystal diffractometer. Corresponding to the μrad resolution of the set-up, micro-structures of the order of a few tenths of a micrometre up to a few tens of micrometres may be investigated. Neutron polarisation is achieved by insertion of birefringent magnetic prisms between the monochromator crystal and the sample. Rocking the analyser crystal produces a scattering pattern for both neutron spin states in a single measurement but well separated in reciprocal space. By this technique, we have recently studied various amorphous Galfenol soft-magnetic ribbons which were produced by spinning from melt at different manufacturing conditions. USANSPOL allows for a determination of domain sizes of the non-magnetised samples and a study of the growing of magnetically homogeneous regions with increasing externally applied magnetic field. The manufacturing process of the ribbons is reflected in the magnetic micro-structure of the different specimens.     

Small-angle polarized neutron scattering study of spherical Fe16N2 nano-particles for magnetic recording tape

We have performed small-angle polarized neutron scattering experiments of spherical Fe₁₆N₂ nano-particles, which are potential candidate materials for high density magnetic recording tape. The results were analyzed based on a core-shell model, and we evaluated the magnetic structure of the particles. The correlation between the magnetic structure and magnetic stability of the particles were discussed in terms of high density recordings.     

A magnetised foil as inclined π-flipper for time-of-flight neutron beams

A prototype instrument with an inclined ferromagnetic foil was built which was capable of π-flipping over a broad wavelength range of polarised thermal neutrons in a time-of-flight beam. It was found that, for a single foil, polarised neutrons could be flipped in a wavelength range of 0.2-0.5 nm with an efficiency of 93%. With improved mechanical setup this range probably can even be extended to 0.1-1.0 nm. Foil flippers can have some major advantages for polychromatic SESANS over resonance flippers, but the technical realisation with four foils in series will be challenging.     

On the magnetic structure of magnetite/oleic acid/benzene ferrofluids by small-angle neutron scattering

Magnetic small-angle scattering of non-polarized and polarized neutrons from ferrofluds (magnetite covered by oleic acid in benzene) is analyzed. A complex correlation in the magnetic structure is observed. It takes place at the length of more than the particle size, and has a core-shell-like organization. Along with it no spatial interparticle correlation in the nuclear structure is detected.     

Counterion condensation in ionic micelles as studied by a combined use of SANS and SAXS

We report a combined use of small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) to the study of counterion condensation in ionic micelles. Small-angle neutron and X-ray scattering measurements have been carried out on two surfactants cetyltrimethylammonium bromide (CTABr) and cetyltrimethylammonium chloride (CTACl), which are similar but having different counterions. SANS measurements show that CTABr surfactant forms much larger micelles than CTACl. This is explained in terms of higher condensation of Br⁻ counterions than Cl⁻ counterions. SAXS data on these systems suggest that the Br⁻ counterions are condensed around the micelles over smaller thickness than those of Cl⁻ counterions.     

In situ SEOP polarised He neutron spin filter for incident beam polarisation and polarisation analysis on neutron scattering instruments

We discuss the development and characterisation of a new in situ spin exchange optical pumping (SEOP) based ³He neutron spin filter polarisation device. We present results from a recent test of the prototype system developed with the Institut Laue-Langevin. The polariser was installed on the polarised reflectometer CRISP at ISIS in the analyser position. The ³He was pumped continuously in situ on the beamline. The system also integrated a ³He adiabatic fast passage spin flipper that allowed reversal of the ³He and therefore neutron polarisation state, allowing for measurement of all four polarisation cross-sections. The system was run for a number of days reaching a ³He polarisation of 63%.     

A polarised SUSANS facility to study magnetic systems

Using a right-angled magnetic air prism, we have achieved a separation of ～10 arcsec between ～2 arcsec wide up and down-spin peaks of 5.4 Å neutrons. The polarised neutron option has thus been introduced into the SUSANS instrument. Strongly spindependent SUSANS spectra have been observed over ±1.3 × 10^?4 Å^?1 range for several magnetic alloy samples. Spatial pair-distribution functions for the up and down-spins as well as the nuclear and magnetic scattering length density distributions in the micrometer domain, have been deduced from these spectra.     

Small-angle neutron scattering from micellar solutions

Micellar solutions are the suspension of the colloidal aggregates of the sur-factant molecules in aqueous solutions. The structure (shape and size) and the interaction of these aggregates, referred to as micelles, depend on the molecular architecture of the surfactant molecule, presence of additives and the solution conditions such as temperature, concentration etc. This paper gives the usefulness of small-angle neutron scattering to the study of micellar solutions with some of our recent results.     

富勒烯-PVP聚合物链团结构的中子小角散射实验研究

应用中子小角散射技术研究了水溶液中富勒烯-PVP聚合物的链团结构及其大小以及它们在不 同富勒烯含量下的变化.结果表明：当加入富勒烯后，不论是PVP单体分子链还是大分子链团 ，其相关长度与纯PVP溶液相比均变小，且大分子链团的变化更为明显；在不同富勒烯含量 情况下，高富勒烯含量的富勒烯-PVP分子链团的体积更小. 关键词： 中子小角散射 富勒烯 PVP聚合物     

National facility for neutron beam research

In this talk, the growth of neutron beam research (NBR) in India over the past five decades is traced beginning with research at Apsara. A range of problems in condensed matter physics could be studied at CIRUS, followed by sophisticated indegenous instrumentation and research at Dhruva. The talk ends with an overview of current scenario of NBR world-wide and future of Indian activities.     

Search for spin-dependent macroscopic short-range interactions using neutron spin precession close to a mirror

Free neutrons can be employed as a sensitive probe to search for spin-dependent macroscopic short-range interactions induced by axion-like particles. In this Letter it is proposed to use pseudomagnetic precession of ultracold neutrons propagating close to a massive mirror of a trap. The method should be several orders of magnitude more sensitive than other methods proposed so far.     

Small-angle neutron scattering studies on water soluble complexes of poly(ethylene glycol)-based cationic random copolymer and SDS

The interaction of cationic random copolymers of methoxy poly(ethylene glycol) monomethacrylate and (3-(methacryloylamino)propyl) trimethylammonium chloride with oppositely charged surfactant, sodium dodecyl sulphate, and the influence of surfactant association on the polymer conformation have been investigated by small-angle neutron scattering. SANS data showed a positive indication of the formation of RCPSDS complexes. Even though the complete structure of the polyion complexes could not be ascertained, the results obtained give us the information on the local structure in these polymer-surfactant systems. The data were analysed using the log-normal distribution of the polydispersed spherical aggregate model for the local structure in these complexes. For all the systems the median radius and the polydispersity were found to be in the range of 20 ± 2 Å and 0.6 ± 0.05, respectively.     

Small-angle neutron scattering study of dynamically polarized polyethylenes

We carried out a small-angle neutron scattering (SANS) study of dynamically polarized polyethylene (PE) samples doped with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO). The transmission of the PE with almost fully polarized neutrons (98.5%) increased with increasing the proton polarization, P. The incoherent scattering cross section decreased with increasing P. The effect of P on the polarized neutrons’ transmission and the incoherent scattering cross section agreed well with the theory. The q-dependence of the coherent scattering, which reflects a two-phase structure of PE composed of crystalline and amorphous domains, was kept unchanged by the proton polarization, but the intensity increased by a factor of 3 and 6 for P=+23% and −23%, respectively. The results mean that the contrast between the two phases was successfully enhanced by a dynamic nuclear polarization (DNP) technique. However, the enhancement is only 1/13–1/16 of the enhancement calculated by assuming a homogeneous polarization through the PE sample. The discrepancy suggests that P in amorphous domains (25%) should be higher than that in crystalline domains (22%) by 3%, which in turn may suggest the partial depolarization of proton spins on the way of the spin diffusion from amorphous domains, where TEMPO radicals localize, to crystalline domains.     

Absence of voids in fast neutron irradiated zirconium by small angle neutron scattering

Abstract

The aim of this research was to resolve a difference of opinion in the literature on the presence of voids in fast neutron irradiated zirconium. There is a great interest in the study of zirconium, since zirconium and its alloys are used extensively in modern power reactors, for example in the fuel rods as a containment material for enriched uranium. A polycrystalline sample of zirconium was irradiated in the HERALD reactor at 40°C with 10²⁰ fast neutrons per cm^?2. The neutron scattering from irradiated and unirradiated standard samples was studied over a wide Q range from 0.001 to 1.12 Å^?1 on a D11 Spectrometer at the ILL (France). The defect cross-section (the difference between the scattering of the standard zirconium crystal and irradiated crystal) was nearly flat as a function of Q (momentum transfer vector) with an average value of 8.5 mb/Str/atom. This indicates a point defect concentration of about 1.8%. Thus the absence of any small angle (Q dependent) defect scattering indicates that large damage regions (e.g. voids) are not produced in zirconium by fast neutron irradiation.     

Tracking the evolution of magnetic ordering in Co/Ru multilayers with inhomogeneous interlayer coupling using polarised neutron reflectometry

We report the measurement of reflected neutron intensity “hysteresis loops” from Co/Ru multilayers that have both antiferromagnetically and ferromagnetically coupled regions. We show that by measuring the four neutron spin-resolved reflectivities at a particular value of wavevector transfer, the normalised value of the relevant magnetic order parameter may be determined. The response of that order parameter to an applied magnetic field may hence be tracked. We have benchmarked our results against conventional magnetometry and magnetotransport measurements.     

Small-angle neutron scattering study of structural evolution of different phases in protein solution

Small-angle neutron scattering (SANS) has been used to study the structural evolution of different phases in protein solution leading to crystallization, denaturation and gelation. The protein solution under crystallization mostly consists of monomers and dimers, and higher-mers are not observed as they are perhaps formed in very small numbers. The onset and the rate of crystallization strongly depend on the salt concentration. Protein denaturation on addition of surfactant occurs due to the formation of micelle-like clusters along the unfolded polypeptide chains of the protein. The structure of such protein-surfactant complex is found to be independent of the size of the micelles in their pure surfactant solutions. The structure of temperature-induced protein gels shows a fractal structure. Rheology of these gels shows a strong dependence on varying pH or protein concentration, whereas the structure of such gels is found to be similar.      

Small-angle neutron scattering studies of nonionic surfactant: Effect of sugars

Micellar solution of nonionic surfactantn-dodecyloligo ethyleneoxide surfactant, decaoxyethylene monododecyl ether [CH₃(CH₂)₁₁(OCH₂CH₂)₁₀OH], C₁₂E₁₀ in D₂O solution have been analysed by small-angle neutron scattering (SANS) at different temperatures (30, 45 and 60° C) both in the presence and absence of sugars. The structural parameters like micelle shape and size, aggregation number and micellar density have been determined. It is found that the micellar structure significantly depends on the temperature and concentration of sugars. The micelles are found to be prolate ellipsoids at 30° C and the axial ratio of the micelle increases with the increase in temperature. The presence of lower concentration of sugar reduces the size of micelles and it grows at higher concentration of sugar. The structure of micelles is almost independent of the different types of sugars used.     

Role of IUC-DAEF in promoting neutron beam research in India

Inter University Consortium for Department of Atomic Energy Facilities (IUC-DAEF) is an autonomous institute of the University Grants Commission and provides an interface between the university fraternity and the institutions of Department of Atomic Energy. Mumbai Centre of IUC-DAEF promotes and supports the use of neutron facilities at Dhruva reactor by the university scientists. To augment the existing neutron scattering facilities, IUC-DAEF has developed a neutron beam line at Dhruva reactor. The present paper gives a brief survey of the activities and achievements of Mumbai Centre of IUC-DAEF.     

Design of a pulsed spatial neutron magnetic spin resonator

It is well known that upon passage through a spatially alternating transverse magnetic field, produced by a meander-shaped conducting foil, in its rest frame each neutron creates its individual frequency which depends on its velocity and the period of the meander. A resonant spin flip process takes place, if this frequency equals the Larmor frequency determined by a homogeneous vertical field. Clearly, this effect can be used to monochromatise a polarised neutron beam. Here we propose a novel design of such a magnetic resonator consisting of a sequence of separate compact modules, which provide high homogeneity of the transversal field oscillations and allow rapid beam chopping since they meet the specifications of fast electronic switching. The wavelength resolution of this resonator device can be changed in an instant and likewise an arbitrary amplitude modulation of the transversal field can be established, which is required for an efficient suppression of subsidiary maxima of the wavelength-dependent spin flip probability.