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Symmetry constraints on magnetic parameters
As with other crystallographic parameters such as site multiplicities, anisotropic
temperature factors, etc, CCSL takes care of the constraints imposed by symmetry on
the magnetic parameters. Such constraints may apply to the propagation vector, the
magnetic moment directions and the magnetic moments themselves. The propagation vector
must be invariant under the symmetry operations of the reciprocal lattice, if it is
not, either the cell symmetry is reduced, leading to configuration domains, or the
structure is a multi-q one. Propagation vectors corresponding to special points in the
Brillouin zone are fixed, those on symmetry axes or planes are constrained so as to
continue to lie on them. The magnetic moments on atoms which are at special positions
are constrained to have the point group symmetries of those positions. For instance,
the magnetic moment of an atom which lies on an axis of order higher than 2 is
constrained to be parallel to the axis. For an atom on a mirror plane the moment is
parallel to the plane if the reflection operation is combined with time-reversal, and
perpendicular if it is not. Atoms on centres of symmetry which are combined with
time-reversal can carry no moment. The subroutine
MAGCON determines the
symmetry constraints on the magnetic moments and prints them on the listing file. If
the value of the magnetic moment is determined to be fixed by symmetry it must be
zero.
Next: Magnetic Structure Factors
Up: MAGNETIC STRUCTURES AND STRUCTURE
Previous: Types of Magnetic Order
P.J. Brown - Institut Laue Langevin, Grenoble, FRANCE. e-mail brown@ill.fr