ELEMENTS OF RARE EARTH MAGNETISM - Rare Earth Magnetism: Structures and Excitations - page 3

tion band, the so-called promotional model . Extensive measurements
were carried out on polycrystalline samples of all the stable lanthanides
through the 1950s, and summarized by Spedding, Legvold, Daane, and
Jennings (1957) at the close of this early period of rare earth research.
Of particular significance, in the light of later developments, was the ob-
servation of extra magnetic neutron-diffraction peaks in polycrystalline
Er by Koehler and Wollan (1955).
The disparate theoretical components which were later brought to-
gether to form the standard model of rare earth magnetism were also
formulated in the 1950s. Zener (1951) suggested that localized moments
could be coupled together by an indirect exchange through the medium
of the conduction electrons, and Ruderman and Kittel (1954) calcu-
lated this coupling quantitatively for nuclear moments embedded in a
free-electron gas. Kasuya (1956) and Yosida (1957) extended the treat-
ment of this RKKY interaction to localized electronic moments. Stevens
(1952) invented his method of operator equivalents , which was of deci-
sive importance for a satisfactory treatment of the crystal fields. Mason
(1954) formulated a theory of magnetoelastic effects , while Zener (1954)
showed how to calculate the temperature dependence of the magnetic
anisotropy.
The classical period of rare earth magnetism was heralded by the
publication of the magnetization measurements on monocrystalline Dy
by Behrendt, Legvold, and Spedding (1957). The fabrication of single
crystals of all the heavy rare earths followed successively, and their bulk
magnetic properties were studied at Iowa State by Legvold and his stu-
dents. They were also made available to Koehler and his colleagues at
Oak Ridge for neutron-diffraction measurements, which revealed what
he later described as 'a panoply of exotic spin configurations'. By the
time of the First Rare Earth Conference at Lake Arrowhead, Califor-
nia in October 1960, both the magnetic susceptibilities and structures
had been extensively investigated. The papers of Legvold (1961) and
Koehler, Wollan, Wilkinson, and Cable (1961) summarized the remark-
able progress which had been made by that time.
Theoretical developments lagged little behind. Almost simultane-
ously with the observation of the helical structure in Dy, Enz (1960)
showed that the magnetization curves implied such a structure, and
pointed out the importance of magnetoelastic effects in inducing the
transition to the ferromagnetic phase. Niira (1960) successfully inter-
preted the magnetization of Dy in the ferromagnetic phase by calculating
the spin-wave spectrum of an anisotropic magnet, showing that a finite
energy is required to create a long-wavelength excitation. This energy
gap gives rise to an exponential decrease of the magnetization at low
temperatures. Elliott (1961) considered the magnetic structures of the