Spin, exchange, and anisotropy in the covalent-chain antiferromagnet TlFeS2.

Abstract

Single crystals of the minimally dimerized chain antiferromagnet ${\mathrm{TlFeS}}_{2}$ have been investigated by thermal neutron diffraction and bulk susceptibility measurements. Collinear order perpendicular to the chain axis appears below a N\'eel temperature ${\mathit{T}}_{\mathit{N}}$=196(1) K, confirming earlier powder-derived data after correction for an ambiguity in the moment direction. Analysis of the spin-wave dispersion in terms of a classical Heisenberg model, scrH =-2${\mathit{tsum}}_{\mathrm{〈}\mathit{i}\mathit{j}\mathrm{〉}}$${\mathit{J}}_{\mathit{i}\mathit{j}}$${\mathbf{S}}_{\mathit{i}}$\ensuremath{\cdot}${\mathbf{S}}_{\mathit{j}}$ ${\mathit{Dts}\mathit{m}}_{\mathit{i}}$(${\mathit{S}}_{\mathit{i}}^{\mathrm{\ensuremath{\parallel}}}$${)}^{2}$, with a spin S=0.93 corresponding to the powder-diffraction ordered moment yields an intrachain exchange constant J=-55(3) meV and interchain-intrachain exchange ratios of order 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$. The magnetic anisotropy D/\ensuremath{\Vert}2J\ensuremath{\Vert}=2.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ is very small, demonstrating spin-orbit cancellation in a half-filled 3d shell in spite of a substantial polarization from in-chain Fe-Fe binding being manifest in an anisotropic Van Vleck susceptibility; its unusual easy-axis symmetry normal to the chains, deduced from coincident in-plane and out-of-plane magnon modes, is considered accidental. A combined analysis of spin-wave, susceptibility, and paramagnetic correlation data corroborates a strongly covalency-reduced moment where a local spin close to S=3/2 gives rise to conventional quantum fluctuation and renormalization effects, albeit with no obvious indication of the singlet formation predicted for dimerized local-spin chains; this model implies J\ensuremath{\approxeq}-29 meV. The low-temperature perpendicular susceptibility may indicate a competitive second-neighbor interaction (\ensuremath{\approxeq}12%) in the chains.