Magnon decay in charge-ordered RbFe²⁺Fe³⁺F₆

RbFe²⁺Fe³⁺F₆ is a charge-ordered antiferromagnet, belonging to the family of mixed-metal fluorides AM²⁺M³⁺F₆. This compound has a structure related to the well-known pyrochlores A₂B₂X₆X'. RbFe²⁺Fe³⁺F₆ displays a charge ordering on the iron sites, originating from their two different valences. Although it is usually presented as a pyrochlore system, its structure can also be described as interpenetrating chains of corner-shared Fe³⁺F₆ and Fe²⁺F₆ octahedra, running along the b and a axes respectively.
Neutron diffraction and magnetization measurements show that an antiferromagnetic order sets in below T_N = 16 K, where the spins associated to Fe³⁺ (S = 5/2) point along the a axis, perpendicularly to the spins associated to Fe²⁺ (S = 2), which point along the b axis. Inelastic neutron scattering on single crystals show two well separated spin wave bands due to the two charge-ordered chains, illustrating inter-chain superexchange coupling. High resolution measurements show a broad continuum located at an energy scale of twice the lower spin wave mode. This continuum is interpreted as magnon decay, in agreement with theoretical views on non-collinear two dimensional antiferromagnets.
RbFe²⁺Fe³⁺F₆ displays coupled charge and spin order through multi magnon processes.