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