PERSPECTIVES FOR RARE EARTH RESEARCH - Rare Earth Magnetism: Structures and Excitations - page 372

related actinide compounds, and of the possible connection between
magnetism and superconductivity in heavy-fermion systems, is one of
the most fruitful areas of research in solid state physics, and one in
which great progress can be anticipated in the next few years.
We have emphasized the ecacy of neutron scattering for the ex-
perimental study of rare earth magnetism, but macroscopic measure-
ments provide an invaluable complement. The macroscopic magnetic
properties are frequently very sensitive to impurities, and many of the
parameters on which we rely for comparison with theory were measured
on crystals substantially less pure than can be prepared today. The mea-
sured value of the conduction-electron polarization in Gd, for instance,
increased significantly with crystal purity. The magnetic moment of the
conduction-electron gas is an important quantity, which gives valuable
information on the exchange interaction with the 4 f electrons, but since
it is typically an order of magnitude smaller than the ionic moment,
its accurate determination is not straightforward, either in the para-
magnetic or the ordered phases, and the values of Table 1.6 could in
many cases be improved upon. The anisotropy parameters, describing
the angular dependence of the free energy in a field, are also dicult to
determine precisely, paradoxically because they are frequently so large
in the rare earths. It may consequently be very dicult to pull the mo-
ments out of the easy direction, and the torque on a crystal in a field can
be huge. Apart from the question of sample purity, many of the values
in the literature therefore suffer from substantial uncertainties for tech-
nical reasons. The same may be said for the multifarious magnetoelastic
parameters, which characterize the dependence of the free energy of the
magnetic systems on the lattice strain. The temperature dependence
of this free energy is reflected in the heat capacity, which therefore in
principle contains useful information on the energetics of magnetic ma-
terials. However, the unscrambling of the nuclear, lattice, magnetic and
electronic components may be a formidable task, and small amounts of
impurity may make the results essentially useless. Nevertheless, it would
be worthwhile to attempt to improve upon the accuracy of the available
measurements, and the effect of an external magnetic field on the heat
capacity could be pursued further, since the few studies which have so
far been made have been very informative.
It was the revelation by neutron diffraction of the exotic magnetic
structures of the rare earths which initiated the revolutionary progress of
the 1960s, and since that time countless studies have been performed of
the patterns of the ordered moments. It is therefore remarkable that so
much remains to be done. The temperature dependences of the struc-
tures of the heavy rare earths have been determined in great detail,
although there is still scope for further study of some phases, for in-