Environmental Engineering Reference
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Fig. 7.8.
Neutron-diffraction scans in Pr. The solid lines show the
sum of two Gaussian functions fitted to the data. Only below 50 mK is
the width of the narrower of these equal to the experimental resolution,
indicating true long-range magnetic order.
to correspond to those of a normal degenerate system within 10% of
its critical temperature. However, the satellite in Fig. 7.8 is much more
intense than such fluctuations could normally give rise to. An alternative
possibility, which has been analysed theoretically by Murao (1984), is
that much of the intensity of the satellite above
T
N
is due to an ordering
of the moments close to the surface of the crystal, which gives rise to
a Bragg peak of non-zero width. The crystalline electric field acting on
the surface ions is different from that determining the bulk properties,
and the magnetic response of these ions will therefore also be different.
For instance, the lowering of the symmetry near the surface splits the
degeneracy of the
1
>
-states, thereby enhancing one of the basal-plane
components of the susceptibility tensor.
The occurrence of the other peak in the scans shown in Fig. 7.8,
known as the
central
or
quasielastic peak
, has been a long-standing mys-
tery. It is much broader than the satellite and constitutes a ring of
scattering around Γ in the basal-ΓMK-plane, with a radius which is
slightly smaller than that of the contour of energy mimima found in the
excitation spectrum, illustrated in Fig. 7.1. The integrated quasielastic-
scattering intensity from this ring is therefore rather large, and around 1
K it is found to correspond to a moment of the order of 0.1
µ
B
per hexag-
onal ion. In a polycrystalline sample, this ring of scattering cannot be
distinguished from scattering from a single point in
|±
-space, which pre-
sumably explains why diffraction studies of polycrystalline Pr indicate
that it is antiferromagnetic at 4 K (Cable
et al
. 1964).
κ
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