ELEMENTS OF RARE EARTH MAGNETISM - Rare Earth Magnetism: Structures and Excitations - page 5

the spin-wave dispersion relations in a variety of heavy-rare-earth mag-
netic structures by Cooper, Elliott, Nettel, and Suhl (1962). The first
observations of spin waves by inelastic neutron scattering were made
at Risø by Bjerrum Møller and Houmann (1966), who obtained rather
complete dispersion relations for Tb at 90 K. During the following years,
Bjerrum Møller and his colleagues performed a series of experiments
which revealed many novel phenomena, including the temperature- and
field-dependence of the magnon energies , allowing the deduction of the
exchange and its anisotropy, and crystal-field and magnetoelastic param-
eters. Magnons in the incommensurable helical phase, including phason
excitations at long wavelengths, were also observed, as was the inter-
action of magnons with each other, with the conduction electrons, and
with phonons, including coupling through a new mechanism involving
the spin-orbit interaction of the conduction electrons, explained by Liu
(1972a).
Callen and Callen (1963) further developed the theory of magneto-
striction , putting it in the form used by Rhyne and Legvold (1965a) to
interpret their pioneering measurements on single crystals. Callen and
Callen (1965) also generalized the treatment of the temperature depen-
dence of crystal-field and magnetoelastic parameters. Cooper (1968a,b)
considered in detail the role of the magnetoelastic effects in the helical-
ferromagnetic transition, and included them in calculations of the spin-
wave energies. Turov and Shavrov (1965) had earlier proposed that,
since the magneto-strain cannot follow the precession of the moments
in a spin wave, the energy gap should not vanish when the hexagonal
anisotropy is cancelled by an external magnetic field. This frozen lattice
effect was observed by Nielsen, Bjerrum Møller, Lindg ard, and Mack-
intosh (1970). In the late 1960s, the availability of separated isotopes
allowed spin-wave measurements at Oak Ridge on a number of the heavy
rare earths which, because of neutron absorption in the natural state,
could otherwise only be studied with great diculty. Of particular in-
terest were experiments on the isotropic ferromagnet Gd, in which the
magnetic form factor was studied by Moon and Koehler (1971) and the
spin waves by Koehler, Child, Nicklow, Smith, Moon, and Cable (1970),
and the clear evidence for a large exchange anisotropy in the conical
phase of Er (Nicklow, Wakabayashi, Wilkinson, and Reed 1971a).
With the increasing understanding of the magnetic behaviour of
the heavy rare earths, it was natural that attention began to turn to
the lighter metals. Moon, Cable, and Koehler (1964) began what was
destined to become a long-lasting study by a number of groups of the
magnetic structure of Nd, and Cable, Moon, Koehler, and Wollan (1964)
found indications of antiferromagnetic ordering in polycrystalline Pr.
Bleaney (1963) had earlier shown that the crystal-field ground states