Structure, magnetic susceptibility, and specific heat of the spin-orbital-liquid candidate FeSc2S4 : Influence of Fe off-stoichiometry

Abstract

We report structure, susceptibility, and specific heat studies of stoichiometric and off-stoichiometric poly- and single crystals of the $A$-site spinel compound $\mathrm{FeS}{\mathrm{c}}_{2}{\mathrm{S}}_{4}$. In stoichiometric samples, no long-range magnetic order is found down to 1.8 K. The magnetic susceptibility of these samples is field independent in the temperature range 10--400 K and does not show irreversible effects at low temperatures. In contrast, the magnetic susceptibility of samples with iron excess shows substantial field dependence at high temperatures and manifests a pronounced magnetic irreversibility at low temperatures with a difference between zero-field cooled (ZFC) and field cooled (FC) susceptibilities and a maximum at 10 K, reminiscent of a magnetic transition. Single-crystal x-ray diffraction of the stoichiometric samples revealed a single phase spinel structure without site inversion. In single crystalline samples with Fe excess, in addition to the main spinel phase, a second ordered single-crystal phase was detected with the diffraction pattern of a vacancy-ordered superstructure of iron sulfide, close to the 5C polytype $\mathrm{F}{\mathrm{e}}_{9}{\mathrm{S}}_{10}$. Specific heat studies reveal a broad anomaly, which evolves below 20 K in both stoichiometric and off-stoichiometric crystals. We show that the low-temperature specific heat can be well described by considering the low-lying spin-orbital electronic levels of $\mathrm{F}{\mathrm{e}}^{2+}$ ions. Our results demonstrate significant influence of excess Fe ions on intrinsic magnetic behavior of $\mathrm{FeS}{\mathrm{c}}_{2}{\mathrm{S}}_{4}$ and provide support for the spin-orbital liquid scenario proposed in earlier studies for the stoichiometric compound.