W. C. Koehler

Magnetic Properties of Rare‐Earth Metals and Alloys

A review of neutron diffraction studies of the magnetic properties of rare‐earth metals and alloys is presented. For each of the pure metals Tb, Dy, Ho, Er, and Tm there is observed a transition at a temperature TN to an oscillatory antiferromagnetic configuration (helical or linear oscillator type). At lower temperatures, further transitions to ferromagnetic, ferromagnetic spiral, or antiphase domain‐type configurations are observed. For Gd, only the ferromagnetic configuration is found. The magnetic moments in the ordered configurations at low temperatures approach the values expected from the corresponding free tripositive ions. In the first half of the series only Nd has been studied by single‐crystal methods, but Ce, Pr, and Eu have now been investigated with polycrystalline samples. The data for Ce are complicated by the existence of several allotropic forms, but a complex antiferromagnetic structure appears to be associated with the hexagonal form at low temperatures. For Nd and Pr, oscillatory ant...

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.

Magnetic Structure of Rare‐Earth‐Cobalt (RCo2) Intermetallic Compounds

Saturation magnetization and neutron diffraction measurements have been performed on cubic Laves phase compounds RCo2 in which R is Nd, Tb, Ho, and Er. Neutron powder patterns obtained at room temperature and at 15°K allowed the determination of the magnetic structures. The low‐temperature patterns are of the ferromagnetic or ferrimagnetic type, with large magnetic intensities superimposed on the nuclear peaks. For the compounds of Tb, Ho, and Er, the rare‐earth atoms show nearly the full moment expected for the free tripositive ion and the cobalt moment is about one Bohr magneton. The rare‐earth moments are coupled parallel to each other, but antiparallel to all the cobalt moments. For NdCo2, the observed Nd moment of 2.6±0.2 Bohr magnetons is smaller than the free ion value of 3.27 Bohr magnetons, and it is coupled parallel to the cobalt moment of 0.8±0.2 Bohr magnetons. In the Nd ion, the spin is opposite to the moment (J = L−S), while for the heavier rare earths the spin is parallel to the total momen...

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 Form Factor of Chromium

Neutron‐diffraction measurements of the magnetic Bragg peaks from single crystals of chromium with small additions of manganese have been used to determine the magnetic form factor of chromium. The addition of manganese simplifies the experimental problem by stabilizing the simple antiferromagnetic structure. A single magnetic peak is observed at reciprocal lattice positions, rather than a group of six satellite peaks as in pure chromium. Also, manganese increases the Neel point so that the measurements were performed at room temperature. The results are in good agreement with free‐atom Hartree‐Fock calculations for the chromium 3d electrons. A definite indication of nonspherical symmetry was obtained by comparing the (221) and (300) reflections. These reflections come at the same scattering angle, yet the (221) intensity is larger by about a factor of six. This indicates a t2g population of 79%, compared to 60% for a spherical spin distribution. The results are independent of manganese concentration over...

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.

Induced magnetic form factor of Sm in mixed‐valence compounds

The induced moment form factor of Sm has been measured in a number of compounds using the polarized neutron technique. For integral‐valence compounds, experimental and theoretical evidence is presented showing that Sm2+ and Sm3+ usually have large differences in their form factors. For the mixed‐valence systems, SmS at high pressure and Sm0.76Y0.24S, the observed form factor is nearly identical to that of the 2+ ion. If we adopt the viewpoint that the Sm ions are fluctuating between 4f5 and 4f6 configurations, then our measurements imply that the induced moment from the 4f5 configuration is dominated by the Van Vleck contribution to the susceptibility arising from an admixture of states with different J values or that the 4f5 configuration makes a negligible contribution to the form factor. The suppression of the Curie (1/T) terms is consistent with low‐temperature susceptibility measurements on these systems. The implications of these observations are examined within the framework of a simple valence‐fluctuation model.The induced moment form factor of Sm has been measured in a number of compounds using the polarized neutron technique. For integral‐valence compounds, experimental and theoretical evidence is presented showing that Sm2+ and Sm3+ usually have large differences in their form factors. For the mixed‐valence systems, SmS at high pressure and Sm0.76Y0.24S, the observed form factor is nearly identical to that of the 2+ ion. If we adopt the viewpoint that the Sm ions are fluctuating between 4f5 and 4f6 configurations, then our measurements imply that the induced moment from the 4f5 configuration is dominated by the Van Vleck contribution to the susceptibility arising from an admixture of states with different J values or that the 4f5 configuration makes a negligible contribution to the form factor. The suppression of the Curie (1/T) terms is consistent with low‐temperature susceptibility measurements on these systems. The implications of these observations are examined within the framework of a simple valence‐flu...