where the 4f shell is less than half filled, J = L - S > 0 and ferromagnetism results. It is clear, however, that if one takes the iron moment to be 1.6 μg (as obtained from amorphous GdFe2), in order to explain the observed net moment of 1.3 Hp/atom, a neodymium moment of only 0.7 g is required instead of the full Nd moment of 3.27 HB

3+

This implies a higher degree of disorder of the rare earth moment in NdFe2 than, for example, in TbFe2. In this case the disorder presumably comes from the combination of a very weak Nd-Fe exchange coupling and a strong random direction axial magnetic anisotropy which results in a significant "fanning" or local fluctuation of the moment direction about the direction of the average exchange field.

The high degree of disorder of the neodymium moments apparently does not affect the general features previously found for the heavy rare earth alloys, TbFe2, HoFe2, and Tb Tb.018Fe as enumerated above. particular, the arrested divergence of the correlation length when

.982

T→ T from above and inhomogeneous clustering of a complicated nature in the ordered state appear to be general features of all amorphous rare earth alloys with large rare earth orbital moments.

1.

2.

S. J. Pickart, J. J. Rhyne, H. A. Alperin, Phys. Rev. Letters 33, 424 (1974).

S. J. Pickart, J. J. Rhyne, H. A. Alperin, AIP Conf. Proc. 24, 117 (1975).

3. H. A. Alperin, J. R. Cullen, A. E. Clark, E. Callen, Physica 86-88B, 767 (1977).

4.

S. J. Pickart, H. A. Alperin, J. J. Rhyne, Phys. Letters 64A, 337 (1977).

5. H. A. Alperin, S. J. Pickart, J. J. Rhyne, J. Appl. Cryst. 11, 648 (1978).

In

MAGNETIC EXCITATIONS IN TьFe2

J. J. Rhyne

and

N. C. Koon

(Naval Research Laboratory, Washington, DC)

and

H. A. Alperin

(Naval Surface Weapons Center, White Oak, MD)

and

(National Bureau of Standards, Washington, DC)

1

2

Magnetic inelastic scattering studies have been performed as a function of temperature on ErFe2, HoFe2 and TbFe2. All the heavy rare earth iron compounds of composition RFe2 crystallize in the C15 Laves phase structure with lattice constants 7.3 A. They have

B'

Curie temperatures in the range 575 K to 700 K and exhibit a ferrimagnetic alignment of iron and rare earth spins. The iron moment is typically 1.5 μg, and the rare earth has the full free ion moment at 0 K. The six atoms in the primitive unit cell give rise to six groundstate spin wave modes, only three of which are at energies low enough to investigate by thermal neutron scattering. A linear spin wave model has been utilized1 for these compounds which accurately represents these three modes as shown in the previous work on HoFe2 and ErFe2. Figure 1 shows the results of this model calculation compared to the observed excitation groups in Tb Fe. The lower "acoustic" mode corresponds to an in-phase precession of all spins with a bandwidth determined principally by the rare earth-iron exchange. The flat mode which, in contrast to ErFe2 and HoFe2, has not been observed in this study of TbFe2, represents an out-of-phase precession of the rare earth spins. The highest steeplydispersive mode is an in-phase precession of the iron-spins. This mode, which was observed in the previous studies of HoFe2 and ErFe2, has a dispersion (w = Dq2) nearly identical to that in iron metal. The absence of scattering in TbFe2 corresponding to the two higher modes is not

2

understood, although it would be expected to be weak due to the small size of the crystal and the relatively higher energy of the modes in TbFe2. The values shown in the figure for the Fe and Tb angular momenta have been determined in a separate magnetic diffraction experiment. The terbium-iron exchange parameter was determined from fitting the model to the observed acoustic-mode data, while the remaining two exchange constants were fixed at values found from the previous HoFe2 and ErFe2

studies.

An anomalous broadening and decrease in intensity of the spin
-1

wave groups at all temperatures studied was observed for q> 0.15 A ̄1.

For q values larger than those shown, the spin waves were not resolvable.

Figure 1.

Inelastic magnetic scattering data and model calculation
for TbFe2.
Acoustic mode results are shown for both room
temperature and 4 K where the q = 0 gap is increased to 3.75
meV due to the crystal field anisotropy.

This effect may result from lifetime broadening or by spurious scattering from additional crystallites which effectively make q an invalid quantum number except for q≈ 0.

1.

2.

J. J. Rhyne, N. C. Koon, J. B. Milstein and H. A. Alperin, "Spin
Waves in ErFe,," Physica 86-88B, 149 (1977) and N. C. Koon and
J. J. Rhyne, "Excited State Spin Waves in ErFe2," Solid State
Commun. 26, 537 (1978).

J. J. Rhyne and N. C. Koon, "Magnetic Excitation in HoFe2," J.
Appl. Phys. 19, 2133 (1978).

MAGNETIC ORDERING AND DYNAMICS IN Tb-Sc ALLOYS

G. E. Fish and J. J. Rhyne

and

B. J. Beaudry and K. A. Gschneidner

(Ames Laboratory, Iowa State University, Ames, IA)

Alloys of the heavy rare earths with each other and with nonmagnetic La, Lu, and Y have been shown to obey a universal relationship between magnetic ordering temperature and averaged deGennes factor:

where C is the concentration of the i-th species and

i

1

for each ion with spin J and Landé g-factor gɲ• This behavior comes

from the indirect, long-range character of the RKKY exchange interaction coupling the magnetic atoms, and results in ordering even in magnetically dilute systems.

The system RSc1-x, where R is any heavy rare earth, is anomalous, however, in that x 0.25 is required to induce long range ordering. Previous magnetization and Mossbauer effect studies have suggested the possibility of a spin-glass transition for lower concentration. The

Tb_SC1-x
Tb Sc1-x system is ideal for study of these phenomena because of the

X

large free ion moment of Tb (9μg) and because large, high quality single crystals are available. Accordingly, we have taken both elastic and inelastic neutron scattering data on samples grown at the Ames Lab with 3 x = 0.2, 0.4, and 0.85. Previous work on polycrystalline samples and 0.85. We have confined previous work on polycrystalline samples indicated that for 0.25 x 1.0, Tb Sc1-x orders in a basal phase spiral < ≤ (see table 1). For pure Tb, there is also a ferromagnetic transition at

-

X

3

T= 221 K, but we saw no ferromagnetism at 4 K in the x = 0.85 sample.

c

For x = 0.2, there was no evidence of any long-range magnetic ordering, but weak, satellite peaks (001+) begin to develop.

From the breadth of these

satellites, we estimate that even at 4 K the magnetic correlation lengths are at most a few atomic distances. There is no evidence that the ordering

in the x 0.4 sample is anomalous in any way. We also attempted to induce ferromagnetic order in the x = 0.4 sample at T = 8 K with an applied field. For H60k0e, the spiral phase persists with slight decrease

in turn angle, and slight increase in the satellite peak width.