Magnetism in orbitally unquenched chainar compounds. II. The ferromagnetic case: RbFeCl3

Measurements of quadrupole splitting by the Mössbauer effect and of single-crystal magnetic susceptibility are reported for the ferromagnetic chainar crystal RbFe${\mathrm{Cl}}_3$. By use of crystal-field theory and the correlated-effective-field statistical approximation these results are analyzed in terms of a model with only three adjustable parameters, a spin-orbit coupling constant, a trigonal crystal-field parameter, and an intrachain Heisenberg exchange $J$ between real spins. A quantitative analysis paralleling that used in a previous paper for isomorphic RbFe${\mathrm{Br}}_3$ is less successful in the present case, discrepancies arising at low temperatures which are at least partly due to small interchain exchange. The latter, which for RbFe${\mathrm{Cl}}_3$ perturbs a quasiferro magnetic system in an antiferro magnetic fashion, produces qualitative effects on magnetic response at sufficiently low temperatures. In addition, we suggest possible evidence for the importance of a non-Heisenberg contribution to exchange, the form and origin of which are discussed.