Spin and orbital states in single-layered La2-xCaxCoO4 studied by doping- and temperature-dependent near-edge x-ray absorption fine structure

The doping-dependent valence, orbital, and spin-state configurations of single-layered La2-xCaxCoO4 (x = 0, 0.5, 1, and 1.5) were investigated with temperature-dependent near-edge x-ray absorption fine structure at the Co L-2,L-3 and O K edges. The spectra show that in La2CoO4, the superexchange between neighboring Co2+ HS states is responsible for the strong antiferromagnetism. With increasing hole doping, the superexchange interactions between Co2+ HS ions are rapidly reduced by interlaced nonmagnetic Co3+ LS. For La1.5Ca0.5CoO4, the low Neel temperature of the samples together with the 50% Co2+ HS and 50% Co3+ LS configuration suggests a checkerboard arrangement of these ions. The spin blockade resulting from this arrangement naturally explains the high resistivity of La1.5Ca0.5CoO4. Upon further doping, Co2+ HS ions are replaced by Co3+ HS, and for LaCaCoO4 a mixture of Co3+ LS and Co3+ HS occurs. Superexchange via configuration fluctuation processes between these two species seems to induce long-range ferromagnetism, while the superexchange between adjacent Co3+ HS neighbors may lead to a competing antiferromagnetic exchange. For a doping content beyond x = 1, Co4+ HS is introduced to the system at the expense of Co3+ LS, and a t(2g) double exchange between Co3+ HS and Co4+ HS is established, which further enhances ferromagnetic interactions and reduces resistivity. No indications for a Co3+ IS state are found throughout the La2-xCaxCoO4 doping series.