MAGNETIC STRUCTURES - Rare Earth Magnetism: Structures and Excitations - page 123

moment of 7.0 µ B . At 32 K, there is a first-order transition to a com-
mensurable state, with a seven-layer repeat distance, which has a fer-
romagnetic component (Brun et al. 1970). At the lowest temperatures,
this has developed into a ferrimagnetic square-wave structure, compris-
ing a repeating pattern of four layers of positive moments followed by
three of negative moments. These structures, the susceptibility curves
of Fig. 2.1, and the excitation spectrum have been used to determine
the parameters of a model for Tm with the usual basic ingredients of
isotropic exchange, crystal fields, and dipolar interactions (McEwen et
al. 1991). As shown in Fig. 2.7, the observed squaring-up process is very
well accounted for by mean-field calculations based on this model. The
principal discrepancy with experiment is in the magnitude of the field
along the c -axis which is required to form a ferromagnetic structure,
where the calculation gives a value about 50% above the observed 28
kOe. This may indicate that the form of ( q ) in Tm which, as illus-

trated in Fig. 1.17, has the largest peak in the whole heavy rare earth
series, changes substantially at this first-order transition.
The magnetic structures of the light rare earths have not generally
been described in the same detail as those of the hcp metals, with the
exception of Nd, which has been intensively studied for several decades.

J

Fig. 2.7. The calculated harmonics of the c -axismomentinTmasa
function of temperature, compared with the results of neutron diffraction
measurements, and the ferromagnetic moment (7 Q ).