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sometimes argued that, since the standard model with suitably chosen
interaction parameters accounts so well for the overwhelming majority
of the magnetic properties, the field is essentially closed and no problems
of fundamental interest remain, apart from the question of determining
the parameters from first principles, and of possible failures of the model.
This statement is however no more or less true or relevant than the as-
sertion that the Schrodinger equation, or perhaps the Dirac equation,
solves all of the basic problems of chemistry. The interesting and fun-
damental physics is to be found in the manner in which the interactions
express themselves under different circumstances. The same model ap-
plied to Pr and Tb gives very different, if equally striking, behaviour. A
suciently careful examination of any rare earth system will inevitably
produce surprises, puzzles, and ultimately deeper understanding. In the
last few years, this process has been clearly exemplified by the structures
and excitations of Ho, which were formerly believed to be essentially un-
derstood, and not particularly interesting. We are therefore confident
that, when the time comes to present another review of this subject, it
will contain physical principles and phenomena which are scarcely hinted
at in this topic.
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