CRYSTAL-FIELD EXCITATIONS IN THE LIGHT RARE EARTHS - Rare Earth Magnetism: Structures and Excitations - page 356

Fig. 7.9. The effective moments, deduced from the intensities of the
narrower peaks in scans of the type illustrated in Fig. 7.8, in Pr and
Pr 97 . 5 Nd 2 . 5 . Only below about 50 mK and 3.5 K respectively do these
moments correspond to a long-range magnetically ordered state.

where R ( T ) is the critical parameter of eqn. (7.1.6). This expression
gives T N

1. A rapid increase of T N at
small concentrations of Nd ions in Pr was indeed observed by Lebech
et al. (1975). As illustrated in Fig. 7.9, the study of a single crystal of
Pr 97 . 5 Nd 2 . 5 by Bjerrum Møller et al. (1982) revealed a number of infor-
mative details. The temperature dependence of the scattered intensity
follows qualitatively the behaviour observed in pure Pr. The quasielastic
peak appears around 10 K, a strong satellite which is broader than the
experimental resolution emerges from it around 6 K, and a diffraction
peak, signifying true long-range order, develops below about 3.5 K. As
in Pr, the quasielastic peak continues to grow below T N .Therisein
the magnetization below about 0.2 K is ascribed to the polarization of
the nuclei and their hyperfine interaction with the 4 f moments. Inelas-
tic neutron-scattering experiments by Wulff et al. (1983) gave results
consistent with a crystal-field model in which the Nd ions have a pre-
dominantly

12 . 5 cT N ( c =1),for c

3 1

|±

2 > ground state, and excited

|±

2 > and predominantly

5

|±

2 > states at about 0.3 meV and 1.2 meV respectively.
The application of an external uniaxial pressure along the a -axis
in the basal plane lifts the degeneracy of the |± 1 > first excited-state
and may therefore induce magnetic ordering, as predicted by Jensen
(1976a) and observed by McEwen et al. (1978). The magnetoelastic
phenomena described in Section 7.3.1, particularly the magnitude of the
field-induced interaction between the magnetic excitations and the trans-
verse phonons, may be used for estimating the coupling parameter B γ 2