Anharmonic Magnon Excitations in Noncollinear and Charge-Ordered \(RbFe^{2+}Fe^{3+}F_6\)

\(RbFe^{2+}Fe^{3+}F_6\) is an example of a charge ordered antiferromagnet where iron sites, with differing valences, are structurally separated into two interpenetrating sublattices. The low temperature magnetically ordered \(Fe^{2+}\) (S=2) and \(Fe^{3+}\) (S=5/2) moments form a noncollinear orthogonal structure with the \(Fe^{3+}\) site displaying a reduced static ordered moment. Neutron spectroscopy on single crystals finds two distinct spin wave branches with a dominant coupling along the \(Fe^{3+}\) chain axis (b axis). High resolution spectroscopic measurements find an intense energy and momentum broadened magnetic band of scattering bracketing a momentum-energy region where two magnon processes are kinematically allowed. These anharmonic excitations are enhanced in this noncollinear magnet owing to the orthogonal spin arrangement.