R.S. Craig

Magnetic and structural characteristics of some 2: 17 rare earth-cobalt systems

Abstract The R 2 Co 17 systems occuring in the Th 2 Ni 17 (hexagonal) structure have six magnetic sublattices - R in the 2b and 2d sites and Co in the 6g, 12j, 12k and 4f sites. The overall magnetic behavior is the result of a complex interplay between the several magnetic sublattices. In Er 2 Co 17 and Yb 2 Co 17 only the 2b and 12j sublattices lead to the uniaxial anisotropy which is desired for permanent magnet fabrication. The useful magnetic propties of these systems can be enhanced by partially replacing the rare earth with Pr and Co by Fe and/or Mn. Fe-containing systems are particularly interesting because they have large magnetizations, high Curie temperatures and relatively strong magnetic anisotropies. Inclusion of Pr in place of Er or Yb produces a substantial increase in magnetization plus a modest increase in anisotropy. The rise in anisotropy is attributed to preferential replacement of Er or Yb by Pr in the 2d sites. In these sites, theory shows Er and Yb to have planar and Pr axial anisotropy, and it is for this reason that the incorporation of Pr in the lattice increase the uniaxial anisotropy. The increase in anisotropy when Co is replaced by Fe and/or Mn seems to arise in two ways - (1) preferential substitution at the dumbell sites and (2) band structure effects.

Calorimetric enthalpies of formation and decomposition of hydrides of ZrMn2, ZrCr2, and related systems☆

Abstract Integral enthalpies of absorption and desorption of hydrogen by hyperstoichiometric ZrMn 2 T 0.8 ( T = Mn, Fe, Co, Ni, and Cu) and stoichiometric ZrMn 2 - and ZrCr 2 -based alloys have been determined. The measured enthalpies range from ∼24 to ∼41 kJ/mole H 2 . The ΔH values for hydrides formed by the series of metallic hosts ZrMn 2 T 0.8 are smaller than that for ZrMn 2 , accounting for the enhanced dissociation pressures of the ZrMn 2 T 0.8 hydrides. In the series of ZrMn 2 T 0.8 hydrides there is a pronounced minimum for hydride of ZrMn 2 Co 0.8 , accounting for the extraordinarily high decomposition pressure of this system. Site occupancies, provided by published neutron diffraction studies, were used to calculate configurational entropies of ZrCr 2 hydrides and related systems. Results obtained were in fair agreement with experiment.

Magnetic characteristics of R6Mn23 hydrides (R = Gd, Ho or Er)

Abstract R 6 Mn 23 systems, with R = Gd, Ho and Er, were hydrided to the composition R 6 Mn 23 H x where x ap; 22. Magnetic properties of these systems and the parent intermetallics were studied over the temperature range 4 to 300 K and at applied field up to 21 kOe. Since Y 6 Mn 23 H 25 was established earlier to exhibit only Pauli paramagnetism, the magnetism of the R 6 Mn 23 hydrides must originate with the rare earth sublattice. Gd 6 Mn 23 H 22 orders at ≈ 150 K, whereas ordering in Gd 6 Mn 23 occurs at 468 K. The moment measured at 4 K indicates a non-collinear structure, perhaps generated by competition involving exchange between nearest and next nearest neighbors. The hydrides involving Ho and Er appear to remain paramagnetic to the lowest temperatures studied, perhaps because the reduced de Gennes factor exchange is insufficient to produce magnetic ordering. The possibility cannot be excluded, however, that they are antiferromagnetic.

Magnetic and structural properties of Y6−xErxFe23 alloys and their hydrides☆

Abstract The magnetic characteristics and structural features of the Y6−xErxFe23 alloys and their hydrides are reported. The parent alloys all formed in a face-centered cubic structure. A pronounced minimum in the lattice parameter was observed for Er4Y2Fe23. A similar minimum was also observed in the 1 atm hydrogen capacity of the alloys. The hydrides of the Y-rich compounds were found to retain the cubic structure of the parent compounds, whereas the hydrides of Er5YFe23 and Er6Fe23 adopt tetragonally distorted structures. Both the intermetallics and their hydrides were observed to be ferrimagnetic, exchange-dominated systems. In all cases, absorption of hydrogen resulted in an increase in both the saturation magnetization and the Curie temperature.

Effect of Si addition on the valence state of Ce in CePd3

Abstract It is observed that Si can be added to CePd 3 , which has the AuCu 3 type cubic structure, to form alloys of the type CePd 3 Si x where 0 ≤ x ≤ 0.3. Addition of Si preserves the structure but results in an expansion of the cell volume. The expansion is much larger in CePd 3 than in LaPd 3 for the same silicon concentration. In CePd 3 , cerium is in a mixed-valent state and its susceptibility tends to a temperature independent value at low temperature. However, as Si is added to CePd 3 , the susceptibility progressively increases and shows a Curie-Weiss behavior with effective moment close to that of Ce 3+ ions. Thus from lattice parameters and susceptibility measurements, it is inferred that Si addition causes a change in the valence of Ce in CePd 3 from mixed valent to nearly trivalent.

Magnetostrictive strains in some light rare earth-aluminum laves phase compounds

Abstract Magnetostriction measurements of some RAl 2 (R = Pr, Nd and Sm) compounds were carried out from 4.2 K up to above their magnetic ordering temperature ( T c ) and in applied field up to 2.1 T. PrAl 2 exhibited a huge magnetostriction (≈10 −3 ) and well-defined saturation below 30 K and at about 0.8 T. For SmAl 2 no saturation was observed up to the highest available applied field. This implies that the material is highly anistropic. The strain for NdAl 2 exhibited a change is sign at higher fields. Variation of strain with temperature for PrAl 2 is found to be in good agreement with the single-ion magnetoelastic theory. Large volume strains are observed for the compounds studied. These strains are attributed to a volume dependence of the crystal field interaction.

Magnetic evidence for the existence of RCoz with z ≈ 7 (R = La, Ce, Pr and Nd)

Abstract RCo5 alloys (R = La, Ce, Pr, Nd, Sm and Y) in fine particle form were examined magnetically. The cast materials were observed to develop additional phases during comminution by ball milling. The compositions of the phases were inferred from the measured Curie temperatures. A component having a Curie temperature between those of RCo5 and R2Co17 was present in the fine powders and was identified as R3Co22 or R4Co27. A component in the La-Co system having Tc ≈ 825°C was identified as La2Co17. The R3Co22, R4Co27 and La2Co17 compounds, which have not been reported previously, are apparently formed from the RCo5 compounds during the comminution process. There is no evidence of the formation of these phases for R = Sm or Y.

Magnetic Specific Heat Anomaly in GdNi2

We have accurately determined the critical behavior of the magnetic contribution to the heat capacity in GdNi2, using both a phase sensitive A.C. technique in the critical region and a calorimetric technique from helium temperature up to room temperature. The deduced values of critical exponents are α = 0.36 and α′ = 0.026.

Hydrogenation kinetics and surface compositions of ErT2 systems (T ≡ Mn, Fe, Co, Ni)☆

Abstract The kinetics of absorption of hydrogen by intermetallic compounds is an important factor with regard to the use of these materials for hydrogen storage. The results of previous work in the field have suggested that the kinetics of absorption of hydrogen are more rapid for manganese and iron intermetallic compounds than for cobalt- and nickel-containing systems. To ascertain whether this trend is indeed correct, hydrogen absorption kinetic studies were performed on the intermetallic series ErT 2 with T ≡ Mn, Fe, Co, Ni. The rate of absorption follows the trend ErMn 2 > ErFe 2 > ErCo 2 > ErNi 2 with the rates differing by about a factor of 500. The kinetics results are correlated with the observed surface compositions determined by Auger electron spectroscopy. The surface enrichment by the d transition metal is greatest for ErMn 2 and least for ErNi 2 , but the difference is insufficient to account for the variation in kinetic behavior.

Influence of Crystal Field Interaction on the Thermal Behavior of ErAl2

Low temperature heat capacities were measured in an adiabatic calorimeter for ErAl2 and LuAl2 in the temperature range 4.2 – 300 K. ErAl2 exhibits a λ-type anomaly in the heat capacity vs. temperature curve peaking at 10.2 K and a Schottky anomaly at nearly 23 K. Heat capacities of LuAl2 were utilized to characterize the lattice and electronic contributions in ErAl2. The excess entropy calculated for ErAl2 is nearly R ln(2J+1) at room temperature and is suggestive of the fact that the degeneracy of the lowest ground state is completely lifted. Detailed calculations were performed to evaluate the influence of the cubic crystal field in ErAl2 on the 4I15/2 ground state multiplet of the Er3+ ion. Two sets of crystal field parameters which describe the heat capacity results satisfactorily were extracted from our study (B 4 0 = -1.53 × 10-7 ev, B 6 0 = -0.66 × 10-9 ev and B 4 0 = 0.48 × 10-7 ev, B 6 0 = -1.18 × 10-9 ev). It is concluded that magnetic ordering in ErAl2 takes place within the Γ8 3 quartet state. These results are discussed in the light of recent studies on other RAl2 compounds.