C. G. Shull

The use of polarized neutrons in determining the magnetic scattering by iron and nickel

Abstract The polarized neutron-beam technique is discussed and utilized in determining the magnetic scattering by iron and nickel. Procedures for obtaining and using polarized beams of monochromatic neutrons are outlined, and experimental data are presented on the performance of two polarizing crystals, Fe 3 O 4 and 92 Co-8 Fe alloy. The reflectivity of neutrons of different polarization states by single crystals of iron and nickel has been studied and used for determining the magnetic scattering amplitudes at various scattering angles. Comparison is then made between the observed magnetic form values and theoretical values derived from calculated 3 d wave functions.

The magnetic structure of Mn2Sb

Abstract Neutron diffraction investigations on single crystals and powdered samples of Mn 2 Sb have determined the magnetic structure of this compound. The ferrimagnetic structure predicted by G uillaud was confirmed except for the exact values of the individual atomic moments of the manganese atoms. Values of +2.13 ±0.20 μ B and −3.87±0.40 μ B were obtained for the magnetic moments of the Mn atoms on the two sublattices from single crystal investigations, and these experimental values satisfactorily explain the scattering from powdered samples at temperatures from 78°K to about 800°K. Results from previous macroscopic magnetic experiments are also consistent with these values of the atomic magnetic moments.

The magnetic structure of Fe3Al

Abstract A polarized-neutron-beam spectrometer has been used to determine the room-temperature magnetic structure of ordered Fe3Al. The results show that the aluminum atoms possess no magnetic moment and that the iron moments differ for the two sublattice sites. On one sublattice, where the iron atoms are surrounded by both iron and aluminum nearest neighbors, the average moment/atom is found to be l.46±0.1μB. On the other sublattice, where the iron atoms have only iron atoms as nearest neighbors, the average moment/atom is found to be 2.14±0.1μB. The data taken on higher-order magnetic reflections also permitted a comparison of the magnetic form factors for each of the different sites with that of metallic iron. No difference was observed to within the ±2 per cent accuracy of the measurements on the form factor.

The crystal structure of thorium and zirconium dihydrides by X‐ray and neutron diffraction

Thorium forms a tetragonal lower hydride of composition ThH{sub 2}. The hydrides ThH{sub 2}, ThD{sub 2} and ZrD{sub 2} have been studied by neutron diffraction in order that hydrogen positions could be determined. The hydrides are isomorphous, and have a deformed fluorite structure. Metal-hydrogen distances in thorium hydride are unusually large, as in UH{sub 3}. Thorium and zirconium scattering amplitudes and a revised scattering amplitude for deuterium are reported.

Neutron Diffraction Studies

This document is reproduced as a project report and is without editorial preparation. The manuscript has been submitted to The Physical Review for possible publication. FILE COPY NAVY »WEAMH SECTION Bate Declassified, September 29, 1^| ^f** #* c Issuance of this document does not constitute authority for declassification of classified copies of the same or similar content and title and by the same authors.

Perfect crystals and imperfect neutrons

Neutron diffraction studies of perfect, non-absorbing crystals have been used to explore aspects of dynamical diffraction theory. The Pendellosung fringe patterns obtained from Laue transmitting crystals with full-spectrum incident radiation have been used to determine crystal structure factors with high precision. It has been established that spherical wave theory is called for, and small crystal curvature must be allowed for, in the interpretation of the fringe patterns. Novel information about the characteristics of the neutron wave packet becomes available from the experimental studies.

Magnetic‐Moment Distribution in Nickel Metal

The magnetic form factor for nickel metal has been determined for the first 27 Bragg reflections. Magnetic‐moment density maps have been obtained in three dimensions by Fourier inversion of the form factor. It is found that the moment density is quite asymmetric about the lattice sites and is negative in the region between lattice sites. The measured form factor agrees very well with a free‐atom form factor for Ni++ provided a uniform negative contribution is included in the moment density. From the comparison of the free‐atom and measured form factors we have found that 81%±1% of the 3d electrons occupy t2g orbitals compared to the 60% required for spherical symmetry. The size of the negative contribution needed for agreement between the measured and free‐ion form factors agrees very well with that given by the moment density maps, and the analysis of the data is consistent with the following model for the magnetization of nickel: 3d spin+0.656 μβ, 3d orbit+0.055 μβ, negative constant −0.105 μβ.

PHOTOGRAPHY OF NEUTRON DIFFRACTION PATTERNS

Photography of slow neutron radiation can be accomplished through use of fluorescent screens in which secondary radiation (beta particles, gamma rays, or visible light) is used to produce photographic action. Thus, as part of a comprehensive study on screen-film combinations, a study is made of the sensitivity and spatial resolution of Li/sup 6/ metal-zinc sulfide systems and Li/ sup 6/F-- ZnS systems in v-arious forms of combination. Hence, the relative neutron exposures required for the different screens to produce a photographic density of 0.5 on Tri-X film with monochromatic neutrons of wavelength 1.5 A are tabulated. It is seen that the homogeneously-mixed screen is more sensitive by an order of magnitude than the heterogeneous screens. Results are also given which show the screen response as a function of mixing ratio (ZnS to Li/sup 6/F) for different neutron wavelengths and an optimum weight ratio of about 4 is suggested. (N.W.R.)

Polarized neutron techniques for the observation of ferromagnetic domains

Simple domain structures were observed in iron‐silicon and cobalt‐iron single crystals by three techniques using polarized neutrons. The first method involves scanning the specimen with a fine beam and measuring the depolarization suffered by the beam after transmission: it can only detect regions where the magnetization direction deviates from the direction of polarization of the beam, and cannot differentiate domains with antiparallel magnetization. The second method, based on the determination of the ratio of the Bragg‐diffracted intensities when the polarization of a fine beam is reversed, provides, when the specimen is scanned, a map of the domains. The third technique, polarized neutron diffraction topography, provides photographs of the domain structure. The possibilities of these techniques for the observation of domains within bulk specimens are discussed.

Neutron Diffraction Effects with Moving Lattices

Observations are reported on the effect of specimen motion upon the neutron diffraction pattern of iron. Specimen speeds of 120 m/sec combined with a neutron speed of 3310 m/sec are found to produce easily measured angular shifts, integrated intensity changes, neutron wavelength alteration, and peak width changes all of which are dependent upon the direction of motion relative to the scattering vector. The effects can be conveniently interpreted with the reciprocal lattice construction and the calculated changes with motion agree satisfactorily with the observations.