PERSPECTIVES FOR RARE EARTH RESEARCH - Rare Earth Magnetism: Structures and Excitations - page 376

extended to other branches, and the magnon energies should be mea-
sured in a single-domain crystal, in order to establish whether the modi-
fication of the hexagonal symmetry by the magnetic ordering is reflected
in the dispersion relations. It would be interesting to extend the mea-
surements of Nicklow et al. (1971b) on ferromagnetic Dy, and subject
it to an equally rigorous investigation, since the anisotropy parameters
are rather different from those in Tb, but its large hexagonal anisotropy
restricts the range of measurements in a field. Studies of ferromagnetic
Ho are even more constrained.
When the moments vary with position in a periodic structure, the
excitations become less amenable to study and the available informa-
tion on them is relatively sparse. Eu corresponds to Gd in the role of an
isotropic model system, but with a simple helical rather than a ferromag-
netic structure. It is unfortunate that its intractable neutron properties
(even the more favourable isotope absorbs inconveniently strongly) have
so far precluded any measurements of the spin waves. It would be par-
ticularly interesting to investigate the mode of wave-vector Q ,whose
energy is determined by the small anisotropy, and its dependence on
magnetic field. It is energetically favourable for the plane of the helix to
rotate so that it is normal to the field direction, and this would be ex-
pected to occur via a soft-mode transition analogous to that observed in
Ho at low temperature, with a similarly decisive influence of the dipo-
lar interactions. Apart from Pr, to which we return shortly, the only
observations of inelastic neutron scattering in the light rare earths are
preliminary studies of crystal-field excitations in the longitudinal-wave
structure of Nd by McEwen and Stirling (1982). It may be an advan-
tage to apply to these complex systems a magnetic field large enough to
induce ferromagnetic ordering, in order to decouple the complications of
periodic ordering from the problems of the interplay of crystal-field and
exchange interactions in the excitation spectrum.
A number of reasonably complete studies of the spin waves in the
c -direction have been made in incommensurable periodic phases of the
heavy rare earths, notably in the helical structure of Tb 90 Ho 10 and the
conical phase of Er. The former is a good example of how the mutual
solubility of the rare earths can be utilized in modifying the magnetic
properties in a convenient manner, in this case by extending the tem-
perature range over which the helix is stable. The effect of varying the
temperature has only been cursorily explored, however, and lifetimes
and field effects have not yet been investigated. Experimental spectra
for the longitudinal-wave structure of Er bear little resemblance to the
predictions of the theory, and both will presumably have to be improved
before they can be expected to converge. A fairly good understanding
has been attained of the excitations in the commensurable spin struc-