ELEMENTS OF RARE EARTH MAGNETISM - Rare Earth Magnetism: Structures and Excitations - page 7

ing the conduction electrons, and given extensive information on their
interaction with the 4 f electrons.
The electronic structure of Ce has been of continued interest. Jo-
hansson (1974) elaborated the suggestion of Gustafson, McNutt, and
Roellig (1969) that α -Ce is a 4 f-band metal ,andGlotzel (1978) and
others have further explored this model by band structure calculations.
Single crystals of α -Ce suitable for dHvA experiments are extremely
dicult to prepare, but Johanson, Crabtree, Edelstein, and McMasters
(1981) have studied the related compound CeSn 3 , observing the 4 f char-
acter of the electrons at the Fermi surface. Photoemission experiments
by Wieliczka, Weaver, Lynch, and Olson (1982) and Martensson, Reihl,
and Parks (1982) proved highly informative in exploring the electronic
structure of Ce. This work reflects the intense interest in the 1980s in
the problem of non-integral 4 f occupancy, which gives rise to a variety
of phenomena subsumed under the description mixed-valent behaviour,
the most striking of which is the huge electronic heat capacity and as-
sociated effective masses measured in heavy-fermion materials . The dis-
covery of superconductivity in CeCu 2 Si 2 by Steglich, Aarts, Bredl, Lieke,
Meschede, Franz, and Schafer (1979) stimulated a major effort in study-
ing lanthanide and actinide heavy-fermion systems, and underlined the
significance of the earlier observation of superconductivity in Ce under
pressure by Probst and Wittig (1975).
The properties of itinerant 4 f electrons have predominantly been
studied through rare earth compounds. Indeed the main thrust of the
rare earth research programme has recently been towards understand-
ing compounds and alloys , which are generally beyond the scope of this
topic, but which may nevertheless be largely understood in terms of the
principles which we shall present. However, as will be discussed in later
sections, there still remain a number of problems in the elements which
await and occasionally obtain a solution. For example, the essential fea-
tures of the classic puzzle of the magnetic structure of Nd have been
clarified by McEwen, Forgan, Stanley, Bouillot, and Fort (1985). Gibbs,
Moncton, D'Amico, Bohr, and Grier (1985) have re-examined the con-
figurations of the moments in Ho and other heavy rare earths, using
a combination of synchrotron radiation , which shows promise for very
high-resolution structural studies, and neutron diffraction. They utilized
the concept of spin slips to explain their results, and hence refocused
attention on commensurable magnetic structures , which had originally
been studied by Koehler, Cable, Wilkinson, and Wollan (1966). Initial
studies of the excitations of such structures were performed by Larsen,
Jensen, and Mackintosh (1987), who thereby explained the long-standing
mystery of the stability of the cone structure in Ho at low temperatures.
Other unexplained features of the neutron diffraction patterns from Ho