J. W. Cable

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.

Magnetic Structure of Neodymium

Neutron diffraction measurements have been made on a single crystal of neodymium at temperatures between 1.6° and 20°K. The onset of magnetic ordering occurs at 19°K and the magnetic structure changes at 7.5°K. The crystal structure is hexagonal with a four‐layer stacking sequence of type ABAC. Thus, alternate layers have different nearest‐neighbor environments, either hexagonal or face‐centered cubic. The main features of the diffraction pattern can be interpreted on the basis of an approximate model in which the hexagonal sites order at 19°K with an antiferromagnetic arrangement between alternate hexagonal layers and with a sinusoidal modulation within each layer. The propagation vector for this modulation and the direction of the magnetic moments are both in the basal plane along a b1 reciprocal lattice vector. The magnitude of the propagation vector changes from 0.13 b1 at 18°K to 0.11 b1 at 7.5°K. The cubic sites order at 7.5°K with a similar structure, except that the propagation vector is 0.15 b1 a...

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.

Magnetic rare-earth superlattices (invited)

The magnetic structures of several single‐crystal, magnetic rare‐earth superlattice systems grown by molecular‐beam epitaxy are reviewed. In particular, the results of recent neutron diffraction investigations of long‐range magnetic order in Gd‐Y, Dy‐Y, Gd‐Dy, and Ho‐Y periodic superlattices are presented. In the Gd‐Y system, an antiphase domain structure develops for certain Y layer spacings, whereas modified helical moment configurations are found to occur in the other systems, some of which are commensurate with the chemical superlattice wavelength. References are made to theoretical interaction mechanisms recently proposed to account for the magnetic states of these novel materials.

Magnetic Structure versus Electron Number for Some Rare‐Earth Intermetallic Compounds

Neutron diffraction measurements were made on a series of rare‐earth compounds in the Tb(Pd, Ag) and Tb(Ag, In) systems. Most of these compounds exhibit the CsCl type of crystal structure and, for these, magnetic structure determinations were made in order to relate the type of magnetic structure to the number of valence electrons. Two types of antiferromagnetic structures were observed; the (ππ0) type consisting of an antiparallel array of ferromagnetic (110) planes of moments, and the (00π) type with ferromagnetic (001) planes of moments alternating in direction. On the basis of 0, 1, 3, and 3 valence electrons for Pd, Ag, In, and Tb, respectively, the (ππ0) structure is found in the region of 3.5 to 4 valence electrons per unit cell and the (00π) structure in the region of 5 valence electrons per unit cell.

Experimental studies of random‐field effects in uniaxial random antiferromagnets (invited)

We discuss how random fields (RFs) are generated in uniaxial random antiferromagnets (URAFs) by applied fields and review the experiments that have been performed on these systems. They include direct and indirect specific heat measurements, neutron‐scattering experiments, and phase diagram studies. We compare the results of different experiments on different systems, discuss their implications on the theories, and suggest further experiments. A new explanation for the Lorentzian‐squared (LSQ) structure factor observed in the neutron‐scattering experiments is also given.

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.