Exact magnetic ground states and competing interlayer exchanges in layered kagome magnets

Abstract

Magnetic ground states in layered kagome magnets are investigated using a Heisenberg model with six interlayer exchange interactions. For isotropic two-sublattice models with identical diagonal Fourier exchange functions, a coplanar single-wave-vector ground state is shown to exist for arbitrary exchange ranges. Extending the three-exchange model of Rosenfeld and Mushnikov reveals that longer-range interactions can change the preferred magnetic order from spiral to ferromagnetic and back to spiral, rather than merely modifying the spiral period. The analysis includes ground-state phase diagrams, harmonic stability, magnetic-field response, and sensitivity to individual exchange interactions. The propagation vector, internal layer angle, saturation field, and planar susceptibility are also found to be insufficient to determine all six exchange constants uniquely. These results provide a basis for predicting noncollinear magnetic order from first-principles exchange interactions.