SYMMETRY OF MAGNETICALLY ORDERED QUASI-CRYSTALS

The notion of magnetic symmetry is reexamined in light of the recent o bservation of long-range magnetic order in icosahedral quasicrystals [ Charrier et al., Phys. Rev. Lett. 78, 4637 (1997)]. The relation betwe en the symmetry of a magnetically ordered (periodic or quasiperiodic) crystal, given in terms of a ''spin space group,'' and its neutron dif fraction diagram is established. In doing so, an outline of a symmetry classification scheme for magnetically ordered quasiperiodic crystals , is provided. Predictions are given for the expected diffraction patt erns of magnetically ordered icosahedral crystals, provided their symm etry is well described by icosahedral spin space groups.