PERSPECTIVES FOR RARE EARTH RESEARCH - Rare Earth Magnetism: Structures and Excitations - page 370

8
PERSPECTIVES FOR
RARE EARTH RESEARCH
During the last three decades, a remarkable transformation has occurred
in our understanding of rare earth magnetism. This development has
been described in the preceding chapters, and we conclude with a short
epilogue, in which we attempt briefly to summarize the status of the
field and the perspectives for future research in it. The close interplay
between experiment and theory is particularly pronounced in rare earth
research. It was measurements on pure materials and single crystals
which stimulated the early development of the subject, but the con-
struction of the standard model pointed the way to more refined and
varied experiments, which in turn required more sophisticated explana-
tions. At the moment, we appear to be in a period where the theory
is in the ascendancy; it is able to account for the great majority of the
observations, and also to suggest a wide variety of new investigations.
Much of the following will therefore be concerned with the indication of
promising directions for experimental study.
However, it is clear that the standard model is indeed a model, and
transforming it into a fundamental theory will require a deeper quan-
titative understanding of the magnetic interactions. The key to such
an understanding lies in the electronic structure. Band structure cal-
culations are able to predict the ground-state properties of solids with
impressive accuracy, and to the extent that comparisons with experi-
mental results exist for the rare earth metals, they are highly successful.
Nevertheless, this comparison between theory and experiment is incom-
plete for even the two most carefully studied examples, Gd and Pr. In
both cases, the Fermi surface has been measured in considerable detail
with the dHvA effect, but even though the general agreement with the
calculations is satisfactory for the larger sheets, the small areas are still
not fully accounted for. These discrepancies point to the necessity both
of more accurate measurements and of first-principles self-consistent cal-
culations, in which the spin-orbit coupling, the exchange splitting, and
the external field are rigorously incorporated. A more complete descrip-
tion of the conduction electrons in Gd would allow further progress in
the computation of the sf exchange, although it would not immediately
solve the most intractable part of the problem, the screening of the ex-
change field of the 4 f electrons, which reduces its effect by a large factor.