Lattice symmetries and regular magnetic orders in classical frustrated antiferromagnets
Résumé
We consider some classical and frustrated lattice spin models with global O(3) spin symmetry. No general analytical method to find a ground state exists when the spin dependence of the Hamiltonian is more than quadratic (i.e., beyond the Heisenberg model) and/or when the lattice has more than one site per unit cell. To deal with these cases, we introduce a family of variational spin configurations, dubbed "regular magnetic orders" (RMO's), which respect all the lattice symmetries modulo global O(3) spin transformations (rotations and/or spin flips). The construction of these states is made explicit through a group-theoretical approach, and all the RMO's on the square, triangular, honeycomb, and kagome lattices are listed. Their equal-time structure factors and powder averages are shown for comparison with experiments. Well known Néel states with one, two, or three sublattices on various lattices are RMO's, but the RMO's also encompass exotic nonplanar states with cubic, tetrahedral, or cuboctahedral geometry of the T=0 order parameter. Regardless of the details of the Hamiltonian (with the same symmetry group), a large fraction of these RMO's are energetically stationary with respect to small deviations of the spins. In fact, these RMO's appear as exact ground states in large domains of parameter space of simple models that we have considered. As examples, we display the variational phase diagrams of the J1-J2-J3 Heisenberg model on all the previous lattices as well as that of the J1-J2-K ring-exchange model on square and triangular lattices.