COMMENSURATE AND MODULATED MAGNETIC PHASES IN ORTHORHOMBIC AC(60) (A=K, RB)

Competing magnetically ordered structures in polymerized orthorhombic AC(60) are studied. A mean-field theory for the equilibrium phases is developed using an Ising model and a classical Heisenberg model to des cribe the competition between interchain and intrachain magnetic order in the solid. In the Ising model, the limiting commensurate one-dimen sional (1D) and three-dimensional phases are separated by a commensura te three-sublattice state and by two sectors containing higher-order c ommensurate modulated phases. For the Heisenberg model the quasi-one-d imensional phase is never the equilibrium state; instead the 3D commen surate phases exhibit a transition to a continuum of coplanar spiral m agnetic phases.