Inelastic scattering studies are also in progress.

Well-defined spin waves have been seen in the ordered state of the x = 0.85 system but not for x = 0.4 or 0.2. These results contrast wtih data on rare earthY alloys, in which spin wave dispersion relations have been measured 4

for very dilute systems, e. g. Ho.1 Yo.9, and Tbo.1 0.9· ́ This suggests

that the marked reduction in Fermi surface density of states Sc (as compared to Y and the other rare earths) may affect the spin-wave lifetimes.

1. Bozorth, R. M., J. Appl. Phys. 38, 1366 (1967).

2.

Sarkissian, B. V. B., Coles, B. R., Commun. Phys. 1 17 (1976) and Abbundi, R., et. al., Phys. Rev. B. 18 331 3 (1978).

3. Child, H. R., and Koehler, W. C., Phys. Rev. 174, 562 (1968). Wakabayashi, N. and Nicklow, R. M., Phys. Rev. B10, 2049 (1974).

4.

SUBLATTICE MAGNETIZATION AND STRUCTURE OF HYDRIDES OF THE LAVES PHASE
RARE EARTH COMPOUNDS RFe2

G. E. Fish and J. J. Rhyne

and

S. G. Sankar and W. E. Wallace

(University of Pittsburgh, Pittsburgh, PA)

and

T. Brun, P. J. Viccaro, D. Niarchos, B. D. Dunlap, and G. K. Shenoy (Argonne National Laboratory, Argonne, IL)

We have used neutron scattering to study the structure and magnetic ordering of a series of hydrides and deuterides of the rare earthiron compounds RFe2. The parent RFe2's have the cubic close packed Laves phase (C15) structure for R heavier than Nd and order ferrimagnetically

at ~600 K with the full free ion moment on each R and ~ 1.6μg/Fe (depressed

i

from the 2.2μg in Fe metal) at saturation.

The stable hydride phases with

~ 2 and 3.5 H(D) per formula unit are known from x-ray and neutron
diffraction to retain the C15 cubic structure, with lattice parameters
1,2
increased by 5% and 7%. The RFe2H4 phase is rhombohedrally dis-
torted and is not ordered at 4.2 K3.

Figure 1 shows the temperature dependences of magnetization of Er and Fe sublattices obtained by neutron scattering for ErFe2, ErFe2D2, and ErFe2D3.5 These data are typical of results summarized in table 1 for all the RFe2H2 and RF RFe2H3.5 's studied. The Fe moment either is unchanged or is raised by a slight amount (comparable to experimental error), while the coherent R moment drops substantially. The lowering of T in the Tc hydrides signals a reduction in the Fe-Fe exchange which is known to dominate in the parent RFe2's. The R-Fe exchange, weak in RFe2, is further lowered, particularly in the case of ErFe2D3.5, where the Er sublattice

C

The sizeable reduction of R moment seen in all the systems studied can arise in two ways. First, it is clear that introduction of H weakens the exchange interaction in these materials, resulting in lowered T and in the case of the ErFe2 series, complete disorder of the R spins well below T where the Fe sublattice disorders. Tc This may allow the crystal field to significantly perturb the exchange-split J states. Alternatively, the colinear spin ordering of R seen in the parent RFe2's may be broken

in the hydrides.

The sublattice moments have also been inferred from measurements 2,3 of hyperfine fields using Mössbauer spectroscopy. These experiments, which are sensitive to the local moment and not the coherent spatial average seen in neutron diffraction, give identical results for the Fe moment, but nearly the full free ion moment for R in both the RFe2D2 and REe2D. phases. This indicates that the crystal field does not significantly perturb the R wavefunctions.

2D3.5

Hence, we conclude that the rare earth spins in the RFe, hydrides are not ordered in a unique direction. The random hydrogen site occupancy

Figure 1.

Magnetization of Er and Fe sublattices in ErFe,, ErFe,D2,
and ErFe2D3,5°
Curie temperature (T) for EŕFe, and ErFe,D3.5
were determined by bulk magnetization. For ErFe22, desorption
begins below Te, so moments were determined using a smooth
extrapolation of diffraction peak intensity to the nuclear

limit.

C'

Figure 2.

Schematic diagram of spin ordering showing the colinear anti-
parallel arrangement in RFe, and the fanning of R spins upon
hydriding. The disorder increases with higher H content.
Application of a magnetic field й partially aligns the R spins.

(a)

Desorption of D begins at 330 K, so that T

cannot be determined.

C

(b)

Not measured

undoubtedly results in variation in the direction of the local anisotropy field, such as found in amorphous rare earth alloys, which leads to a "fanning" of the individual rare earth moments illustrated in figure 2.4

This would account for the reduction in overall sublattice magnetization and the incomplete saturation in applied fields 100 k0e.

5

1.

2.

Fish, G. E., Rhyne, J. J., Sankar, S. G., and Wallace, W. E., J. Appl. Phys. 50, 2003 (1979).

Viccaro, P. J., Friedt, J. M., Niarchos, D., Dunlap, B. D., Shenoy, G. K., Aldred, A. T., and Westlake, D. G., J. Appl. Phys. 50, 2051 (1979).

3. Viccaro, P. J., Shenoy, G. K., Dunlap, B. D., Westlake, D. G., and Miller, J. F., Journal de Phys. 40, C2-198 (1979).

4. Rhyne, J. J., Schelleng, J. H., and Koon, N. C., Phys. Rev. B10, 4672 (1974).

5.

Gualtieri, D. M., Narasimhan, K. S. V. L., and Takeshita, T., J.
Appl. Phys. 47, 3432 (1976).

LOW TEMPERATURE PHASE TRANSITION IN Cs2NaPrC16

G. E. Fish and J. J. Rhyne

and

J. W. Lynn

(University of Maryland, College Park, MD)

and

(National Bureau of Standards, Washington, DC)

and

H. H. Patterson

(University of Maine, Orono, ME)

A single crystal of Cs2NaPrC16 has been studied by neutron scattering to characterize the structural phase transition from the high temperature cubic phase (space group Fm3m) to a phase of lower