K. S. V. L. Narasimhan

Magnetic properties of RMn2Ge2 compounds (R=La, Ce, Pr, Nd, Cd, Tb, Dy, Ho, Er, and Th)

Magnetic properties RMn2Ge2 compounds where R is La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Th have been investigated from 4.2 to 500 K. LaMn2Ge2, CeMn2Ge2, PrMn2Ge2, and NdMn2Ge2 are ferromagnetic with Curie temperatures of 306, 316, 334, and 334 K, respectively. The heavy rare‐earth compounds do not order ferromagnetically until low temperatures. GdMn2Ge2 shows a sharp magnetic transition at 97 K both in the magnetization‐vs‐temperature and resistivity‐vs‐temperature data. For this compound, it is proposed that above 97 K the Mn moments order antiferromagnetically among themselves while the Gd moments are disordered. Below 97 K the Mn moments couple ferromagnetically with each other but are opposed to the ordered Gd sublattice. Magnetic moment obtained at 4.2 K can be explained for all the compounds on the basis of an antiferromagnetic coupling between Mn and rare‐earth spins.

Control of the hydrogen absorption and desorption of rare‐earth intermetallic compounds

The rate at which rare‐earth intermetallic compounds absorb and desorb hydrogen can be controlled by ’’poisoning’’ the catalytic reaction, H2=2H. Specimens treated with SO2, a well‐known catalytic ’’poison’’, are prevented from absorbing hydrogen for extended periods. More surprisingly, SO2 treatment after hydriding prevents hydrogen desorption by specimens for weeks. The applications of this process to energy storage are numerous.

Electronic specific heat and high field magnetization studies on the Y6(Fe1−xMnx)23 system☆

Abstract Magnetizations at applied fields up to 120 kOe and electronic specific heats have been determined for ternaries represented by the formula Y 6 (Fe 1− x Mn x ) 23 . The Curie temperature and ordered magnetic moment of Y6Mn 23 and Y 6 Fe 23 decrease sharply when these materials are formed into solid solutions, being minimal in the vicinity of the equimolal solution. To test whether the reduction is a band effect electronic specific heats were measured; they are maximal where T c and the moment are minimal, indicating unimportance of band effects. Curie-Weiss behavior is exhibited at low fields, suggesting rather localized d -electrons. The nearest neighbor distances are such as to lead to antiferromagnetic Mn-Mn interactions. Y 6 Mn 23 is taken to be ferrimagnetic whereas Y 6 Fe 23 appears to be ferromagnetic. While the magnetic structure of the ternaries is yet to be fully clarified, it is clear that antiferromagnetic exchange is enhanced when either binary is formed into a ternary. It also appears that the Mn-Fe coupling is antiferromagnetic in the ternaries.

Magnetic properties of RT3−x′Nix compounds (R=Dy or Ho and T′=Fe or Co)

The effect of varying electron concentration on the magnetic ordering in the ternary intermetallic compounds DyCo3−xNix, DyFe3−xNix, HoCo3−xNix, HoFe3−xNix, with 0<x<3, has been investigated by saturation magnetization studies. The results show a filling of the Co and Fe bands and also a polarization of the 3d band of nickel. In the case of the Co compounds an additional contribution from the induced moment on cobalt by the rare‐earth exchange field is found to be important.

Magnetic properties of RMn2X2 compounds (R=Rare earth, Y or Th and X=Ge, Si)

Magnetic properties of RMn2Ged2 and RMn2Si2 compounds crystallizing in th BaAl4 type structure have been investigated over a wide temperature range (4.2 K to 1200 K). LaMn2Ge2, CeMn2Ge2, PrMn2Ge2, NdMn2Ge2 and LaMn2Si2 all possess Curie temperatures above 300 K and the easy direction of magnetization have been found to be along the c‐axis. When R is a heavy rare earth antiferromagnetic coupling was obtained at 4.2 K between the rare earth and manganese moments whereas when R is a light rare earth a ferromagnetic coupling of moments was obtained. Ymn2Si2, ThMn2Si2 and ThMn2Ge2 possess a small moment <⋅2μB at 4.2 K. High temperature susceptibilities on RMn2Si2 compounds indicate a Curie‐Weiss behavior.

Mössbauer and crystallographic study of DyFe3−x Nix compounds

Mossbauer and crystallographic investigations were carried out on DyFe3−x Nix compounds crystallizing in the PuNi3 type of structure. The hyperfine field observed at the iron sites shows a maximum at x =0.75. The 12‐line spectrum observed at room temperature corresponding to the two different kinds of iron in the structure collapsed into a single six‐line pattern at 4.2 K. Lattice parameter c shows a rapid decrease when x is increased beyond 2.0, but no such behavior was found in the a lattice parameter. The results of the x‐ray and Mossbauer data are explained in terms of site preference and nonlocalized transition‐metal moments, respectively.

Magnetic Properties of the Hydrides of Selected Rare‐Earth Intermetallic Compounds with Transition Metals

ErFe2, TmFe2 and HoFe2 have been found to form stable hydrides of composition ErFe2H3.9, TmFe2H4.3, and HoFe2H4.47, when exposed to hydrogen at 3.6 × 106 Pa at room temperature. X‐ray diffraction reveals an expansion of the ErFe2 lattice parameter from 7.282 ± 0.001 to 7.828 ± 0.001 A, the expansion of the TmFe2 lattice parameter from 7.238 ± 0.001 to 7.839 ± 0.001 A, and the expansion of the HoFe2 lattice parameter from 7.284 ± 0.001 to 7.880 ± 0.001 A with hydrogen absorption. Magnetization measurements as a function of temperature and field were made on these hydrides. The compensation temperature observed at 480 K for ErFe2 and 233 K for TmFe2 was shifted to 42 K and 8 K, respectively, upon hydriding. Magnetic moments measured at 4.2 K in an applied field of 120 kOe are 6.45 and 5.60 μB/FU for TmFe2 and HoFe2, respectively. These large moments arise due to the reduction of the iron moment. Results on hydrides of NdCo5 and DyCo5.2 are also presented.

Magnetic properties of DyFe3−xCox and HoFe3−xCox*

Abstract Magnetic moments, Curie and compensation temperatures were measured for DyFe 3−x Co x and HoFe 3−x Co x compounds. The variation of transition metal moment has been explained on the basis of the rigid band model.

Magnetic properties of Gd1−xThxFe2 and Gd1−xCexFe2

Abstract Saturation magnetization, magnetization vs temperature, Curie temperatures, and lattice parameters are presented for the ternary alloys Gd 1− x Th x Fe 2 and Gd 1− x Ce x Fe 2 . Quadrivalent Th and Ce were introduced into the lattice in an effort to induce ferromagnetic GdFe coupling. Experiment showed that the antiferromagnetic GdFe coupling in GdFe 2 is preserved in the ternaries. The Fe moment and Curie temperature decrease as the Gd content of the sample is decreased. This is ascribed to electron transfer from Th or Ce to the Fe d shell. Failure to achieve ferromagnetic coupling is ascribed to electron capture by iron, which prevents a rise in electron concentration as Gd is replaced by Ce or Th.

Magnetic properties of Ln2−xLn′xCo17 compounds (Ln = Gd, Dy, Ho, or Er, Ln′ = Th or Ce)

Abstract Magnetic and structural characteristics of the ternary systems Ln 2− x Th x Co 17 (Ln = Gd, Dy, Ho, and Er) and Ln 2− x Ce x Co 17 (Ln = Gd, Dy, and Ho) are presented. Incorporation of Th in the lattice stabilizes the Th 2 Zn 17 structure, whereas incorporation of Ce does not; if the binary system has the Th 2 Ni 17 structure the incorporation of Ce leaves the structure unchanged. The antiferromagnetic LnCo coupling observed in the Ln 2 Co 17 systems persists in the ternary alloys. The moment of the cobalt sublattice is decreased when more than half of the Ln is replaced by Th, suggesting that the extra electron contributed by Th enters the Co d -band or d -shell. The direction of easy magnetization is in the basal plane for all composition in the Gd, Dy, and Ho systems. In Er 2− x Th x Co 17 the easy direction is along the c -axis for x = 0 and 0.2, but is in the basal plane for higher thorium contents.