S.K. Malik

Hydrogen induced magnetic ordering in Th6Mn23

Abstract The influence of absorbed hydrogen on the magnetic behavior of isostructural Y6Mn23 and Th6Mn23 has been investigated. The compound Y6Mn23 is a ferromagnet with Tc ≈ 500 K and a moment of 0.5μB/Mn. On hydrogen absorption it forms Y6Mn23H∼25 in which magnetic ordering is completely suppressed. In contrast Th6Mn23 is a Pauli paramagnet but Th6Mn23H∼30 exhibits ferromagnetic ordering with Tc ≈ 329 K and a moment of 0.8μB/Mn. This contrasting behavior of isostructural Y6Mn23 and Th6Mn23 seems to be unique so far and presumably arises because of the slightly different band structure of the two compounds, Y being trivalent while Th is quadrivalent.

Hydrogen absorption in RNi4Al (R = Rare Earth) ternary compounds☆

Abstract Hydrogen absorption in a series of eleven rare earth ternary intermetallic compounds, RNi4Al (where R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm) has been studied. All these compounds are found to crystallize in the hexagonal CaCu5-type structure and absorb a large quantity of hydrogen at moderate pressure and ambient temperature. In terms of equilibrium pressure, the hydrides of these ternaries are more stable than those of corresponding binary, RNi5, compounds. This is consistent with previous observations on the Th(Ni, Al)5 ternary system.

Crystal structure of RCu4Ag and RCu4Al (R = Rare Earth) intermetallic compounds

Abstract Crystallographic studies have been made on the ternary rare earth intermetallic compounds RCu4Al and RCu4Ag, where R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm. All RCu4Al compounds are found to crystallize in the hexagonal CaCu5 structure. However, in RCu4Ag series, compounds with R = Nd to Tm form in the MgSnCu4-type structure, while those with R = La, Ce, and Pr form only multiphase solids.

Magnetic properties of hydrides of the rare earths and rare earth intermetallics

Rare earth elements (R) react with hydrogen at elevated temperatures to form hydrides with compositions approaching RH3. Intermetallic compounds involving the rare earths in chemical union with Mn, Fe, Co or Ni absorb copious quantities of hydrogen rapidly and dissociatively at room temperature. In all the hydrides RH3 and in many of the hydrogenated intermetallics the proton density exceeds that in condensed elemental hydrogen. The presence of such large amounts of hydrogen significantly modifies various physical properties of the host metal. In the elemental rare earths hydrogenation produces a loss in metallic conduction and greatly weakens exchange. Magnetic ordering is suppressed by as much as 250 degrees. In contrast hydrogenation of intermetallics may either strengthen or weaken exchange, the latter predominating numerically in the systems studied to date. Contrasting behavior is exhibited by the isostructural pair of compounds Y6Mn23 and Th6Mn23. Th6Mn23 exhibits Pauli paramagnetism whereas Y6Mn23...

Magnetic behavior of Laves phase RCo2–xFex (R = Ho, Er) compounds and their hydrides

Abstract The effect of absorbed hydrogen on the magnetic behavior of Laves phase compounds HoCo2–xFex and ErCo2–xFex has been investigated. These compounds are found to absorb between 3.3 to 3.5 H/formula unit. The hydrogen absorption does not change the structure of these compounds but is accompanied by a large expansion in unit cell volume ranging from 20 to ≈35%. Magnetization studies in the temperature range 4.2 to 300 K reveal that hydrogen absorption leads to a decrease both in the Co moment and the Curie temperature of HoCo2 and ErCo2. The Fe moment in HoFe2 and ErFe2 as well as in pseudobinaries with Co appears not to be influenced much on hydrogen absorption. The magnetization of all the hydrides investigated shows a lack of saturation at 4.2 K and 21 kOe applied field, and in Fe-containing compounds the rare earth moment randomizes very rapidly at low temperatures while the Fe magnetization remains temperature-independent up to 300 K. These results are taken to imply a weakening of the R-Co and R-Fe exchange interactions on hydrogen absorption and a possible fanning of the rare earth moments.

Hydrogen absorption and its effect on structure and magnetic behavior of GdNi2

Abstract The compound GdNi2 having the cubic Laves phase structure absorbs hydrogen to from GdNi2H4. The x-ray diffraction pattern indicates that there is no long-range atomic ordering in the hydride. Magnetization studies have been carried out on GdNi2H4 and the results are compared with those on crystalline and amorphous GdNi2. The Curie temperature of GdNi2H4 is only 8 K, much smaller than 38 K for amorphous GdNi2 and 81 K for cyrstalline GdNi2.

Hydrogen induced change from superconducting to magnetically ordered state in Th7Fe3

Abstract The Pauli paramagnet Th 7 Fe 3 becomes a superconductor at temperatures below 1.86 K. 1 This compound readily hydrogenates to Th 7 Fe 3 H 30 , giving one of the densest hydrogen media known. Magnetization measurements on the hydride show that it is magnetically ordered. It has a Curie temperature above 300 K and a saturated moment measured at 4.2 K of 1.4 μ B /Fe. The differing behavior in the host metal and the hydride is ascribed, in the latter, to competition between H and Fe for the electrons provided by Th. In the host metal the Fe d-band is filled by electron transfer and the Fe moment is quenched. In the hydride some of the electrons provided by Th are absorbed by H, leaving vacancies in the Fe d-band. These, in turn, give rise to the magnetic order. The present instance in which a superconductor is converted into a ferromagnet appears to be unique as regards the effects of hydrogenation on the properties of metallic systems.

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 characteristics of RCo2−xFex hydrides (R ≡ Tb,Dy)

Abstract The Laves phase compounds RCo2−xFex (R ≡ Tb, Dy and x = 0, 0.6 and 2) are found to absorb about 3–3.5 hydrogen atoms per formula unit. The hydrogen absorption results in a large increase in cell volume (about 20%–25%) without a change in the structure. Detailed magnetization studies on the parent compounds and the hydrides have been carried out in the temperature interval 4.2–300 K. From these measurements it is inferred that the hydrogen absorption leads to a reduction in the cobalt magnetic moment in TbCo2 and DyCo2 but to an increase in the iron moment in TbFe2 and DyFe2 as well as in the cobalt-rich ternaries TbCo1.4Fe0.6 and DyCo1.4Fe0.6. Magnetization of all the hydrides at 4.2 K shows no tendency to saturate even in fields up to 20 kOe. This suggests a possible fanning of the rare earth moments in the hydrides due to the weakening of the Rue5f8Co and Rue5f8Fe exchange. A change from the first-order magnetic transition in DyCo2 at the Curie temperature to the second order, on hydrogenation, is observed.

Magnetization behavior of RFe3-hydrides (R = Tb, Er and Tm)

The RFe/sub 3/ compounds (R = heavy rare earth), crystallizing in rhombohedral PuNi/sub 3/ type structure, absorb typically three hydrogens per formula unit at room temperature. The hydrogen absorption does not change the structure of these compounds but merely causes an expansion of the lattice. Magnetization studies have been carried out on the hydrides in the temperature range 4.2 to 300 K and in applied fields up to 21 kOe. The parent compounds are known to be ferrimagnetically ordered with reasonably high Curie temperatures. The magnetization of the hydrides at 4.2 K is observed to be smaller than that of the corresponding parent compounds. The compensation temperature also decrease on hydrogen absorption. These results may be taken to imply either an increase in moment on iron and/or a decrease in rare earth moment. In ErFe/sub 3/-hydride, a sharp increase in magnetization is observed close to room temperature. This is attributed to the change in easy direction of magnetization.