E. O. Wollan

NEUTRON DIFFRACTION INVESTIGATION OF MAGNETIC ORDERING IN DYSPROSIUM

Neutron diffraction measurements on a single crystal of dysprosium show that the magnetic structure in the antiferromagnetic region between 179° and 87°K closely resembles a helical-type arrangement of the atomic moments. In this arrangement the moments within a hexagonal layer are aligned parallel and point in a direction perpendicular to the c axis of the crystal. The moment direction in adjacent layers is rotated by a specific angle which is dependent on the temperature of the sample. A slight modification of this structure exists below about 140°K, and a transition to ferromagnetism occurs at 87°K.

The hydrogen atom positions in face centered cubic nickel hydride

Abstract Neutron diffraction observations on the nickel-nickel hydride system show that the hydrogen atoms occupy the octahedral sites of the face centered cubic lattice. The hydrogen to nickel atom ratio in the hydride phase was found to be 0·6 ± 0·1. In these respects nickel and palladium behave similarly. The systems differ, however, in the one important detail that the palladium hydride is stable at room temperature whereas the nickel hydride is unstable.

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

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Some Magnetic Structure Properties of Terbium and of Terbium‐Yttrium Alloys

Neutron diffraction studies have been made on single‐crystal and polycrystalline specimens of terbium. Earlier magnetic and thermal measurements have indicated a transformation to an ordered magnetic state at approximately 230°K, and a subsequent order‐order transformation at approximately 220°K. The neutron measurements show that in the narrow antiferromagnetic region, the magnetic structure of terbium is a helical structure. The interlayer turn angle varies from 20.5° per layer at the Neel point to 18.5° per layer at the lower transition. At this lower temperature the structure transforms, in the absence of any external applied field, to a classical ferromagnetic structure in which the moments are in, or nearly in, the planes perpendicular to the hexagonal axis. At very low temperatures the magnetic moment per atom is very nearly 9.0 Bohr magnetons, the value expected for the ordered tripositive ion. Neutron diffraction measurements have also been made on a series of alloys of yttrium and terbium in ord...

Neutron Diffraction Study of Metallic Erbium

Neutron diffraction measurements were made on erbium single crystals in the temperature range 298°–4.2°K. The material is antiferromagnetic below 80°K and ferromagnetic below 20°K. In the antiferromagnetic region, the magnetic scattering consists of satellite reflections corresponding to a modulation of the magnetic scattering amplitude along the c axis. The spacing and intensity distribution of these satellites show two distinct subregions of antiferromagnetism. In the upper region, between 80° and 52°K, the data suggest a sinusoidal modulation of the magnitude of the c-axis component of magnetic moment with a period of 3.5 c0. Between 52° and 20°K the wavelength of the modulation varies continuously from 3.5° c0 to 4.0 c0. In addition, there is a squaring up of the modulation and a simultaneous ordering of the component of the moment normal to the c axis. Below 20°K the material is basically ferromagnetic with a moment of 7.2 μB directed parallel to the c axis.

Atomic Magnetic Moments in Dilute Iron—Palladium Alloys

Neutron scattering and magnetization measurements were made on dilute iron—palladium alloys to determine the existence and magnitude of the magnetic moments of the constituent atoms. This determination was made by the combination of the difference in the aligned magnetic moments obtained from the ferromagnetic diffuse neutron scattering and the average ferromagnetic moment per atom obtained from the saturation magnetization measurements. Data were taken on two alloys which contained 3 and 7 at. % Fe. The following results were obtained: Pd0.97Fe0.03:  3.0±0.2 μB/Fe,  0.15±0.01 μB/PdPd0.93Fe0.07:  3.0±0.2 μB/Fe,  0.27±0.02 μB/Pd.

Neutron Diffraction Investigations of Ferromagnetic Palladium and Iron Group Alloys

In order to account for the magnetic properties of alloys it becomes important to determine the individual magnetic moments of the constituent atoms. This determination can be accomplished by the combination of neutron diffraction and magnetic induction measurements. Such measurements were made on the following ferromagnetic alloys: Pd3Fe, PdFe, Pd3Co, PdCo, Ni3Co, and NiCo. The average moment values were obtained from magnetic induction measurements while the differences in the atomic moments were determined from either the ferromagnetic diffuse scattering of the disordered alloys or the superlattice reflections of the ordered alloys.

Neutron Diffraction Study of Magnetic Ordering in Thulium

Neutron diffraction measurements have been made on a single-crystal specimen of metallic thulium at temperatures ranging from 293° to 4.2°K. In the temperature range between 38° and 56°K the magnetic structure of thulium is similar to that observed for the high temperature form of erbium; namely, a simple oscillating z-component type antiferromagnetic structure. Below 38°K additional satellite reflections are observed in the diffraction patterns from which it is inferred that the sinusoidal modulation is modified below this temperature. At 4.2°K the normal lattice reflections show an increase in intensity in zero field which is consistent with a mean magnetic moment per thulium atom of approximately one Bohr magneton directed parallel to the c axis. The low temperature magnetic structure which is proposed for thulium is one in which the moments are parallel to the c axis of the crystal but change their orientations according to the sequence 4, 3, 4, 3.... The fundamental period of the modulation remains constant over the entire range of temperature at a value corresponding to 3.5 a 3 periods.

Neutron diffraction investigation of antiferromagnetism in CrCl3

Abstract Neutron diffraction observations have been made on powder and single crystal samples of anhydrous CrCl3 at temperatures from 298 to 4.2°K. This hexagonal layer-type crystal undergoes a transition at 16.8°K to an antiferromagnetic state in which the magnetic moments within each hexagonal layer of metal ions are aligned parallel but adjacent layers of moments are oppositely directed. The axis of spontaneous sublattice magnetization is closely perpendicular to the c-axis. Observations of the (003) antiferromagnetic reflection in an external magnetic field show that the antiferromagnetism can be destroyed with fields of only a few kilo-oersteds, and lead to the conclusion that a net magnetization can be produced with very small magnetic fields.