Excitonic transverse and amplitude fluctuations in noncollinear and charge-ordered RbFe2+Fe3+F6

RbFe(2+)Fe(3+)F(6 )is an example of an antiferromagnet with charge ordering of the octahedrally coordinated Fe2+ and Fe3+ ions. As well as different spin values, Fe2+ (S = 2) and Fe3+ (S = 5/2 ) possess differing orbital ground states with Fe2+ having an orbital degeneracy with an effective orbital angular momentum of l = 1. The resulting low-temperature magnetic structure is noncollinear with the spins aligned perpendicular to nearest neighbors [S. W. Kim et al. Chem. Sci. 3, 741 (2012)]. The combination of an orbital degeneracy and noncollinear spin arrangements introduces the possibility for unusual types of excitations such as amplitude modes of the order parameter. In this paper we investigate this by applying a multilevel analysis to model neutron spectroscopy data [M. Songvilay et al. Phys. Rev. Lett. 121, 087201 (2018)]. In particular, we discuss the possible origins of the momentum and energy broadened continuum scattering observed in terms of amplitude fluctuations allowed through the presence of an orbital degree of freedom on the Fe(2+ )site. We extend previous spin-orbit exciton models based on a collinear spin structure to understand the measured low-energy excitations and also to predict and discuss possible amplitude mode scattering in RbFe2+Fe3+F6.