Linear Chains with a Ferromagnetic Interaction in RbFeCl3

The temperature dependence of the electric field gradient of RbFe${\mathrm{Cl}}_3$ has been measured in the range 4.2-300°K ($T_N=2.55\mathrm{}°$K). These results are analyzed together with the parallel and perpendicular susceptibilities observed by Achiwa. The Hamiltonian describing the system contains a single-ion crystal field term and an anisotropic bilinear interaction between nearest-neighbor ${\mathrm{Fe}}^{2+}$ pairs along the $c$ axis. A best fit is obtained assuming a ferromagnetic pair interaction $J_{\parallel }=5\mathrm{}\pm 1$ and $J_{\perp }=11\mathrm{}\pm 2$ ${\mathrm{cm}}^{-1\mathrm{}}$, with the spins lying in the basal plane. At low temperatures ($T<\mathrm{}{T}_N$) the correlation length along the chains (parallel to the $c$ axis) is long enough for a one-dimensional spin-wave theory to be applicable. This assumes that the magnetic properties are predominantly determined by excitations propagating along the chains. Using this approach the magnetic moment per ${\mathrm{Fe}}^{2+}$ ion at 0 °K is calculated to be $2.2{\mu }_B$, as compared with $2{\mu }_B$ observed by Davidson et al. by neutron scattering.