ELEMENTS OF RARE EARTH MAGNETISM - Rare Earth Magnetism: Structures and Excitations - page 6

in Pr should be singlets, and in such singlet ground-state systems no
magnetic ordering should occur unless the exchange exceeds a critical
value. Johansson, Lebech, Nielsen, Bjerrum Møller, and Mackintosh
(1970) could indeed detect no signs of magnetic ordering at 4.2 K in
monocrystalline Pr. Shortly afterwards, the crystal-field excitations, or
magnetic excitons , were observed by Rainford and Houmann (1971) and,
on the basis of these results, Rainford (1972) proposed a crystal-field
level scheme which is very close to that accepted today.
The achievements of the classical period were summarized in the
compendium on the Magnetic Properties of Rare Earth Metals ,edited
by R.J. Elliott, which was published in 1972 and, in a sense, also signalled
the end of this period. In the modern era, the principles which had been
established by the early 1970s have been applied to attaining a deeper
and more complete understanding of the elements, even though the pri-
mary interest has increasingly turned towards rare earth compounds
and alloys. For example, the magnetic interactions in the exchange-
dominated system Tb were studied in exhaustive detail with inelastic
neutron scattering by Jensen, Houmann, and Bjerrum Møller (1975).
The crystal-field dominated system Pr was subjected to a similarly care-
ful investigation by Houmann, Rainford, Jensen, and Mackintosh (1979)
and, from his analysis of these results, Jensen (1976a) concluded that Pr
could be induced to order antiferromagnetically either by the application
of a modest stress or, through the hyperfine interaction , as first proposed
by Murao (1971), by cooling to about 40 mK. The former effect was ob-
served by McEwen, Stirling, and Vettier (1978) while magnetic ordering
at very low temperatures had been inferred from heat-capacity measure-
ments by Lindelof, Miller, and Pickett (1975). However, the controversy
surrounding this phenomenon was only finally settled by the unambigu-
ous observation of magnetic ordering by neutron diffraction (Bjerrum
Møller, Jensen, Wulff, Mackintosh, McMasters, and Gschneidner 1982).
The effects of the crystal field alone were studied by Touborg and Høg
(1974), by dissolving small amounts of the magnetic rare earths in Sc, Y,
and Lu and determining the crystal-field level scheme through suscep-
tibility measurements, in conjunction with inelastic neutron scattering
(Rathmann and Touborg 1977).
Efforts to increase the purity of rare earth samples were rewarded
by the observation of the de Haas-van Alphen (dHvA) effect in Gd by
Young, Jordan, and Jones (1973) and the subsequent detailed elucida-
tion of its Fermi surface, which could be satisfactorily accounted for
by band structures calculated with the inclusion of the exchange split-
ting between up- and down-spin levels. More recently, the careful study
of the dHvA effect in paramagnetic Pr by Wulff, Lonzarich, Fort, and
Skriver (1988) has confirmed the success of the band model in describ-