Evidence for a temperature-induced spin-state transition of Co3+ in La2-xSrxCoO4

The magnetic susceptibility of mixed-valent La2-xSrxCoO4 single crystals in a doping range of 0.5≤x ≤0.8 and in a temperature range up to 1000 K is studied with the focus on the spin-state of the Co ions. The materials bear close resemblance to LaCoO3, a compound well-known for a spin-state transition of its Co3+ ions. In La2-xSrxCoO4, the magnetism below room temperature can be described by paramagnetic Co2+ in the high-spin state and Co3+ in the non-magnetic low-spin state. Above room temperature, however, an increase in susceptibility compared to the behavior expected from Co2+ is seen. We attribute this rise in moment to a thermally-induced spin-state transition of Co3+, and analyze the experimental data by comparison to full-multiplet calculations for the thermal population of a higher-spin state of Co3+. We find the energy gap between the spin states being in the order of 2000 K, which hardly changes with doping.