Neutron Scattering

Y c
3
c
Cc
E
Figure 16 Magnon dispersion in RbMnF3 (from reference [4]) .
the antiferromagnetic order in the insulating parent compounds .
13 .3 .3 Crystal field excitations for rare earth ions
In the rare earth series one fieds unpaired electrons in the 4f-shell, which are strongly
localized and therefore screened from the surrounding ions .
In consequence the total
momentum J remains a good quantum number .
For a free ion the the ground state
would be (2J + 1) times degenerate .
In a crystal this degeneraty is partially lifted due to
the - weak - Coulomb-fields of the surrounding ionic charges .
The transitions between
the single levels may be observed by inelastic neutron scattering .
(This is valid for the
crystal field splittings in transition metals too, but due to the larger overlap of the d-
orbitals the excited levels in these compounds are usually to high in energy .)
earth ions are sufficiently diluted, interactions amongst them may be neglected .
If the rare
The level
frequencies show then no dispersion and may be studied with time of flight methods on a
polycristalline sample .
In figure 17 we show the observed spectrum compared to the crystal field scheme for
Pr13 3 . The levels of Pr 3+ with a J of 4 may split into not more than 9 levels . The local
symmetry of the site occupied by the ion in the lattice determines which levels may exist
with which multiplicity.
However, the symmetry cannot predict the sequence of the levels,
for this purpose one needs a quantitative information for the surrounding fields .
The single
levels are designated according to thé irreducible représentations of thé local symmetry
13-24