D.W. Jones

The purification of the rare-earth metals: I. Solid state electrolysis of gadolinium

Abstract The utilisation of solid state electrolysis as a purification technique for the preparation of high-purity gadolinium is described and assessed. Using ultra high vacuum and a double cycle technique, the oxygen content in gadolinium has been reduced by two orders of magnitude and many other impurities reduced by significant amounts, yielding gadolinium of much higher purity and resistance ratio than previously reported. Additionally, large single crystals are often produced in the purified sample.

The purification of the rare earth metals: II. Solid state electrotransport processing of Terbium

Abstract Solid-state electrotransport processing has been demonstrated as a technique for purifying terbium. The results show that both oxygen and nitrogen migrate rapidly in the same direction as the electron flow. Although hydrogen contamination occurs on contact with air, terbium of better than 99.9 at.% has been prepared from commercially-available starting material. The preparation and characterisation of high-quality, single-crystal terbium specimens is also described.

Zone refining of rare earth metals: Lanthanum, cerium and gadolinium

Abstract Three rare earth metals were zone refined under low contamination conditions. Where possible, effective distribution coefficients for the major impurities were determined, and these were used to estimate the ultimate potential for zone refining as a purification technique for the rare earth metals. It is concluded that zone refining will be of most value for refining with respect to certain common metallic impurities. The metallic impurity levels at one end of a zone-refined gadolinium rod are believed to be the lowest ever recorded for a rare earth metal.

Preparation of rare earth materials, crystals and specimens

Abstract The reactive nature of rare earth metals is frequently noted but is usually only a problem during the preparative stages. In terms of processing of rare earth metals the dominant feature is their reactivity: vapour pressure and phase transformations are two other parameters that have a major influence. We present our state-of-art position in the preparation of specimens of rare earth materials. Production to a given materials requirement can involve various processes including refining, alloying and crystal growth, where the basic requirement is the use of a non-contaminating environment, e.g. ultra high vacuum. We have extended our refining studies to include vacuum re-melting, sublimation, zone-refining and solid state electrotransport. Considerable technical developments have also been achieved and our latest equipment includes, for instance, the capability of RF vertical float zoning in UHV and pressurised melt-growth. Following the processing stages we are concerned with actual specimen fabrication and report recent developments to improve surface preparation, particularly to deal with brittle materials.

Search for commensurate effects in the helimagnetic phase of holmium

Abstract Critical field measurements and the direct determination, through neutron diffraction, of the mean turn angle as a function of temperature in the helimagnetic phase of holmium have revealed small singularities in the region around 95.5 K where ultrasound anomalies have been detected.

Assessment of the crystal lattice perfection of rare earth and intermetallic single crystals by x-ray topography

Abstract Single crystals of a number of rare earths and intermetallics used for bulk magnetic measurements have been examined by X-ray diffraction topography using the Bragg geometry. Experiments were performed using both a conventional X-ray source and synchrotron radiation. Tb grown by solid state electrotransport (SSE) from high purity start material had the highest perfection as well as the highest residual resistance ratio. SSE refined Dy was of somewhat inferior perfection while Ho crystals cut from an ingot not SSE processed were of very much poorer quality. The reduction in lattice strain as cutting damage was removed was monitored. A significant decrease in perfection of the Tb was noted over several months following thermal cycling through the Curie temperature. Increase in surface strains due to oxidation was most marked in Gd and the Gd-Tb alloys. In contrast no polishing problems occurred with RAl 2 crystals grown by the Czochralski method. Clean images, showing little long range strain, were obtained from TbAl 2 , NdAl 2 and GdAl 2 .

Measurement of the spontaneous magnetostriction of terbium by x-ray diffractometry

Abstract The magnetostriction of high purity, SSE refined, single crystal terbium has been measured in zero applied magnetic field by X-ray diffractometry. This technique uses the splitting of the Bragg diffraction peaks due to the presence of ferromagnetic domains to determine the spontaneous strains. The measured gamma-strain is found to be much lower than that reported previously by various techniques but, unlike the high field data, it falls smoothly to zero at the Curie temperature. Reasons for the discrepancy are analysed and it is suggested that the incoherency of the domain boundary walls acts to reduce the strain within each domain. The effects on peak shape and position of slow and rapid cooling through the Curie temperature are discussed. A satisfactory fit to the experimental data is obtained using single ion theory plus a substantial term representing a two-ion interaction. This good fit can only be obtained when zero field magnetization data, derived from neutron diffraction measurements, are used.

Anomalies in the temperature dependence of the critical field in antiferromagnetic single crystal dysprosium

Abstract An intensive study of the temperature variation of the critical field, H c , in antiferromagnetic dysprosium, reveals small, abrupt changes in the value of H c at particular temperatures between the Neel temperature, T N , and Curie temperature T c . These measurements made on two single crystals of different quality, show that anomalies occur at ≈99, 114, 127, 131, 138, 146 and 153 K, and the jump in the critical field is ≈ 10 -2 T on each occasion. The anomalies are attributed to a distortion of the helical antiferromagnetic structure whenever the periodicity of the helix is commensurate with that of the hexagonal crystal lattice. Accompanying changes in the helical turn angle, Δω, are estimated and found to increase as the temperature approaches T c .

A high resolution neutron scattering study of single crystal neodymium

Abstract It is well known that neodymium develops a modulated antiferromagnetic structure below a first-order transition at T N = 19.9 K . In marked contrast to the other lanthanide metals, no well-defined magnetic excitations have previously been observed in Nd, either in the paramagnetic or magnetically ordered phases. A careful study of the inelastic scattering of neutrons from a new large single crystal of Nd (grown by the solid state electrotransport technique) has been carried out at the Institut Laue-Langevin. Quasielastic scattering which varies markedly with both wavevector and temperature has been found. This scattering has a maximum intensity around T N and exhibits a characteristic linewidth of some 0.4 meV. These results may be interpreted in terms of heavily-damped excitations within low-lying crystal field levels. At lower temperatures, and further from the satellite wavevectors, two weak but well-defined magnetic excitations have been observed.

Search of commensurate locking effects in the helical magnetic structures of Tb and Dy single crystals

Abstract We report a sensitive, high resolution, experimental method to investigate the possibility of small commensurate locking effects in helically modulated magnetic structures in rare earth metals, based on the fact that electrons moving along the c-axis strongly interact with the helical structure. By measuring a highly sensitive transport coefficient such as the temperature derivative of the c-axis electrical resistivity (dρ/dT) one may then see subtle effects related to the change in the turn angle (w) with temperature. This new method has been used to investigate the antiferromagnetic structure of Tb and Dy single crystals. Sharp and rather localized anomalies (Δ ⪅ 0.2 K in Tb; Δ ⪅ 0.6 K in Dy) were observed in dρ/dT when the expected commensurate helical turn angles of 40°, 42° occur in Dy, and 20° in Tb.