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

TL;DR

This study investigates how iron off-stoichiometry affects the structural, magnetic, and thermal properties of FeSc2S4, supporting its classification as a spin-orbital liquid and revealing the emergence of magnetic irreversibility with excess Fe.

Contribution

It provides detailed analysis of off-stoichiometric FeSc2S4, showing the influence of Fe excess on magnetic behavior and structural phases, advancing understanding of spin-orbital liquids.

Findings

Stoichiometric FeSc2S4 shows no magnetic order down to 1.8 K.

Fe excess induces magnetic irreversibility and a magnetic transition around 10 K.

Low-temperature specific heat aligns with low-lying spin-orbital levels of Fe2+ ions.

Abstract

We report structural, susceptibility and specific heat studies of stoichiometric and off-stoichiometric poly- and single crystals of the A-site spinel compound FeSc2S4. 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 ZFC and 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 besides 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 Fe9S10. 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 Fe2+ ions. Our results demonstrate significant influence of excess Fe ions on intrinsic magnetic behavior of FeSc2S4 and provide support for the spin-orbital liquid scenario proposed in earlier studies for the stoichiometric compound.