PERSPECTIVES FOR RARE EARTH RESEARCH - Rare Earth Magnetism: Structures and Excitations - page 377

tures of Tm and Ho, and the first effects of spin slips in introducing new
energy gaps within the zone have been observed in the latter. However,
no experiments have yet been carried out on the spin waves in the helifan
and fan structures which may be induced by a magnetic field, nor has
the soft mode which should accompany a second-order transition from
the latter to the ferromagnet been observed.
Such is the richness of the excitation spectrum of Pr that a num-
ber of experiments of fundamental importance remain to be performed,
despite the considerable efforts which have already been devoted to this
unique element. The magnetic excitons on the hexagonal sites and their
relationship to the process of magnetic ordering are well understood,
and the anisotropic exchange, magnetoelastic, and crystal-field inter-
actions have been measured with unprecedented accuracy. The

3 s >
level has not yet been detected directly, however, and a further study
of the lifetimes of the long-wavelength acoustic modes as a function of
magnetic field would give detailed information about their interaction
with the conduction electrons. The excitations on the cubic sites are
much less precisely described, and even though they are of lesser signifi-
cance, a full characterization of their energies and the underlying inter-
actions is essential for the explanation of the temperature dependence
of the magnetic properties. In particular, a measurement of the missing
branch of excitations polarized in the c -direction, its possible splitting
from the branches polarized in the plane, and of the field dependence
of the energies, would help to determine the remaining magnetoelastic
and crystal-field parameters, especially B 2 ( c ) which is usually assumed
to be zero, but is likely to be as large as the corresponding parameter
on the hexagonal sites.
The magnetically ordered state in Pr provides a new set of chal-
lenges. The mechanism of ordering by different perturbations requires
further investigation. The neutron-diffraction studies of the hyperfine-
coupling-induced collective state should be taken to lower temperatures,
and the electronic and nuclear components on the different sites disen-
tangled. The excitations of this state would also naturally be of interest.
The influence of magnetic impurities, such as Nd and Er, could be clari-
fied by further inelastic-scattering experiments in a field. The energy of
the

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+1 > state on the hexagonal sites of Pr can be reduced by a mag-
netic field in the c-direction, which should affect the quasielastic central
peak and precursor satellite. The precise nature of these unusual scat-
tering phenomena is still a mystery, which further measurements under
different constraints, especially of external pressure, could help to un-
ravel. The application of a uniaxial pressure in the a -direction creates in
effect a new magnetically ordered element, and one with very interesting
properties. Many informative results have already been obtained from

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