E.B. Boltich

Influence of hydrogen on structure and magnetic properties of Ho6Fe23 and Er6Fe23

Abstract The pressure composition isotherm of the Er6Fe23-H2 system studied at 0°C reveals the presence of three different hydride phases. Maximum absorption is typically 14 hydrogen atoms/formula unit in Er6Fe23 and Ho6Fe23 compared to about 22 in the isostructural Mn compounds. During hydrogenation Ho6Fe23 remains cubic with lattice parameter which increases from 12.034 A to 12.399 A. Upon hydrogenation Er6Fe23 exhibits the very surprising behavior of transforming from a cubic ( a = 12.004 A ) to a tetragonal structure ( a = 12.131 A , c = 12.601 A ). In spite of their contrasting crystallographic features both hydrides exhibit similar magnetic behavior, relative to the parent compounds, namely, lowering of both the saturation moment at 4.2 and the compensation temperature and an increase in the saturation moment at room temperature. The results of magnetic measurements may be understood by assuming an increase in the iron moment in these compounds on hydrogen absorption.

Magnetic behavior of the hydrides of Th7Fe3, Th7Co3 and Th7Ni3☆

Abstract Th7Fe3, Th7Co3 and Th7Ni3 exhibit Pauli paramagnetism and are superconductors with transition temperatures at about 2 K. Hydrogenation of the iron compound converts it into a ferromagnet with a Curie temperature of about 325 K and a magnetization of 1.4μB per iron atom. During hydrogenation vacancies are introduced into the d band by electron capture by hydrogen, and these vacancies give rise to ferromagnetism. Hydrogenation of Th7Co3 and Th7Ni3 produces a rise and a drop in susceptibility respectively, but no ferromagnetism. The behavior of Th7Co3 hydride indicates a barely filled d band or perhaps one with few vacancies. Th7Ni3 is unlike LaNi5, the magnetism of which is dominated by surface-precipitated nickel; oxidation of Th7Ni3 is slight and it contains only very small amounts of surface nickel. The fall in susceptibility is a bulk effect and probably arises from the fact that the Fermi level in the hydride falls at a low point on the density of states curve.

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.

Effect of absorbed hydrogen on magnetic ordering in Tb6Mn23 and Dy6Mn23

Abstract Magnetic data are presented for R 6 Mn 23 and R 6 Mn 23 H 23 ( R  Tb , Dy ). Exchange in these materials is weak compared with that in the host metal. Dy 6 Mn 23 H 23 remains paramagnetic to 4.2 K whereas the host material has a Curie temperature T c of 443 K. Hydrogenation of Tb 6 Mn 23 lowers its Curie temperature by about 230 K and decreases its magnetization by a factor of about 3. The lowered exchange is attributed to a diminished electron concentration brought about by electron capture by hydrogen. The reduced magnetization is thought to be a crystal field effect.

Itinerant magnetic ordering in EuRh3B2

Abstract The RRh 3 B 2 (R=La to Gd) compounds crystallize in the hexagonal CeCo 3 B 3 type structure. The unit cell volume of EuRh 3 B 2 follows the usual lanthanide contraction, indicating that Eu is in a trivalent state (electronic configuration 4f 6 ; 7 F 0 ) and hence is nonmagnetic. Magnetization studies on EuRh 3 B 2 , however, reveal that this compound orders magnetically with Curie temperature of 40 K and a moment of 0.55 μ B per formula unit at 4.2 K and 20 kOe applied field. The magnetism is presumed to be of the itinerant type arising from the Rhd band. Possibly the Eu 4f electrons also play a sugnificant role.

Magnetization studies on ErCo3-hydride and TmCo3-hydride

Abstract The compounds ErCo 3 and TmCo 3 are ferrimagnetic with Curie temperatures of 401 K and 370 K, respectively. These absorb hydrogen to form ErCo 3 H 4.3 and TmCo 3 H 3.3 . From magnetization studies on these as well as other RCo 3 hydrides in the temperature interval 4.2 to 300 K, it is inferred that hydrogen absorption leads to a reduction in magnetic moment on cobalt and a weakening of the R-Co interaction (R = rare earth). Except in the case of the GdCo 3 -hydride, saturation in magnetization is not achieved at 4.2 K in applied fields up to 21 kOe. This suggests the possibility of fanning of rare earth moments. The RCo 3 -hydrides investigated earlier with R = Gd, Dy and Ho and the ErCo 3 -hydride and TmCo 3 -hydride all appear to be magnetically ordered at room temperature.

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 hydrogen absorption-desorption cycle on the magnetic susceptibility of CeNi4Al☆

Abstract The effect of hydrogen on the magnetic susceptibility of CeNi 4 Al has been investigated. It is observed that following a hydrogen absorption-desorption cycle, without exposing the sample to air, the magnetic susceptibility of CeNi 4 AlH 0 (I) increases over that of CeNi 4 Al. If, after hydrogenation, the sample is kept in the air and then hydrogen removed, the increase in susceptibility of CeNi 4 AlH 0 (II) is even larger. Magnetic precipitation of Ni due to the presence of O 2 and H 2 O is thought to be responsible for this behavior. An increase in susceptibility of CeNi 4 AlH 3.7 over that of CeNi 4 Al is also observed and attributed partly to the tendency of valence change of cerium from 4+ toward 3+.

Magnetic behavior of lower hydrides of Y6Mn23 and Th6Mn23

Abstract Earlier studies have revealed dramatic changes in the magnetic behavior of Y6Mn23 and Th6Mn23 on hydrogen absorption. Y6Mn23 is ferrimagnetically ordered while Y6Mn23H≈22 exhibits antiferromagnetic ordering. In contrast, Th6Mn23 is a Pauli paramagnet while Th6Mn23H≈30 is ferrimagnetically ordered. We have now examined the magnetic behavior of lower hydrides of Y6Mn23 and Th6Mn23. It is observed that magnetic ordering, possibly of the ferrimagnetic type, still persists in Y6Mn23H≈10. This material also shows anomalies in magnetization at low temperature. Th6Mn23H17.6 is observed to be magnetically ordered with a low Curie temperature and a small magnetic moment per formula unit.

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.