Raman scattering from the 1D antiferromagnets RbCoCl₃and RbNiCl₃

Abstract

The Raman spectra of hexagonal RbCoCl3 and RbNiCl3 have been studied as functions of temperature. No light scattering from magnons was observed in RnBiCl3 in contrast to the high intensities found in RbCoCl3. The magnetic properties of RbCoCl3 are similar to those of its structural isomorphs CsCoCl3 and CsCoBr3. The temperature dependence of the scattering revealed three magnetic phases: a quasi-1D phase for T<TNI approximately=28K; a partially ordered 3D phase for TNI<T<TNII approximately=11K; and a differently ordered 3D phase for T<TNII. From the magnon scattering the authors find the intrachain exchange J approximately=50 cm-1, with epsilon approximately=0.1, and the effective interchain exchange J' approximately=1.5 cm-1. The intensity of the exciton at 308 cm-1 was found to be spin-dependent, involving the interchain exchange with J'=0.9+or-0.1 cm-1. Major features in the electronic Raman spectrum of RbCoCl3 were fairly well explained by an effective Hamiltonian model that included the intrachain exchange within a molecular-field approximation. However, other secondary features in the spectrum could not be accounted for. Magnetic scattering from Co2+ ions neighbouring divalent metal impurities in the crystal was observed at frequencies approximately J. The A1g phonon in RbNiCl3 and RbCoCl3 was found to have an anomalous intensity in alpha xy polarisation. Strong coupling was found to exist between the E1g phonon and magnon in RbCoCl3.