PERSPECTIVES FOR RARE EARTH RESEARCH - Rare Earth Magnetism: Structures and Excitations - page 373

stance in the intermediate temperature-range of Er. However, the effect
of other external constraints, especially magnetic fields and pressure,
have only been cursorily investigated. It is apparent, for instance, that
a variety of helifan and analogous structures may be produced by ap-
plying a field to periodic structures, especially if the interactions are
adjusted by taking advantage of the almost perfect mutual solubility
of the elements. Apart from their intrinsic interest, measurements of
the resistivity may provide a very sensitive method for determining the
complex phase diagrams which frequently arise in a magnetic field. The
same may be said of the lattice strain, and of the elastic constants (Bates
et al. 1988). Since the magnetoelastic effects in the rare earths are so
pronounced, external pressure or uniaxial stress can have a profound
effect on the stability of different magnetic states, as the few examples
which have been examined have demonstrated. The light rare earths
remain a largely unexplored terrain. The more that is learned about the
magnetic structures of Nd, the wider loom the areas which remain to be
investigated. Our understanding of γ -Ce is still at a rudimentary stage,
nor is the magnetic structure of Sm by any means completely resolved.
Although its crystal structure is complicated, and neutron experiments
require isotopically enriched samples to circumvent the large absorption
in the natural state, there is no fundamental obstacle to attaining a
more detailed description of the configurations of the moments under
different conditions than we have at present. The form factor is partic-
ularly interesting and unusual, and the theoretical understanding of its
variation with is still incompete. A dhcp phase can also be stabilized

in Sm; a comparison of its magnetic properties with those of the more
common allotrope would further elucidate the relation between the crys-
tal structure and the magnetic interactions. The magnetic structures of
films and superlattices constitute a field which has only existed for a
few years, and is in the process of rapid expansion. There appear to be
unlimited possibilities for fabricating new systems, and for discovering
new forms of ordering.
The mean-field theory, in conjunction with the standard model,
has proved to be ideally suited for explaining the general features of
the magnetic structures in terms of the interactions. Furthermore, de-
tailed self-consistent calculations provide an accurate description of the
arrangement of the moments determined by neutron diffraction and
macroscopic measurements, under specified conditions of temperature
and field, and also give a good account of the variation of the macro-
scopic anisotropy and magnetostriction parameters with the magnetiza-
tion. Although the crystal-field parameters are adjusted to fit, for ex-
ample, the low-temperature magnetic properties, they are not normally
thereafter varied in the calculation. The indirect-exchange interaction

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