Spin and orbital states in single-layered La2−xCaxCoO4studied by doping- and temperature-dependent near-edge x-ray absorption fine structure

Abstract

The doping-dependent valence, orbital, and spin-state configurations of single-layered La${}_{2\ensuremath{-}x}$Ca${}_{x}$CoO${}_{4}$ ($x=0$, 0.5, 1, and 1.5) were investigated with temperature-dependent near-edge x-ray absorption fine structure at the Co ${L}_{2,3}$ and O $K$ edges. The spectra show that in La${}_{2}$CoO${}_{4}$, the superexchange between neighboring Co${}^{2+}$ HS states is responsible for the strong antiferromagnetism. With increasing hole doping, the superexchange interactions between Co${}^{2+}$ HS ions are rapidly reduced by interlaced nonmagnetic Co${}^{3+}$ LS. For La${}_{1.5}$Ca${}_{0.5}$CoO${}_{4}$, the low N\'eel temperature of the samples together with the 50% Co${}^{2+}$ HS and 50% Co${}^{3+}$ LS configuration suggests a checkerboard arrangement of these ions. The spin blockade resulting from this arrangement naturally explains the high resistivity of La${}_{1.5}$Ca${}_{0.5}$CoO${}_{4}$. Upon further doping, Co${}^{2+}$ HS ions are replaced by Co${}^{3+}$ HS, and for LaCaCoO${}_{4}$ a mixture of Co${}^{3+}$ LS and Co${}^{3+}$ HS occurs. Superexchange via configuration fluctuation processes between these two species seems to induce long-range ferromagnetism, while the superexchange between adjacent Co${}^{3+}$ HS neighbors may lead to a competing antiferromagnetic exchange. For a doping content beyond $x=1$, Co${}^{4+}$ HS is introduced to the system at the expense of Co${}^{3+}$ LS, and a ${t}_{2g}$ double exchange between Co${}^{3+}$ HS and Co${}^{4+}$ HS is established, which further enhances ferromagnetic interactions and reduces resistivity. No indications for a Co${}^{3+}$ IS state are found throughout the La${}_{2\ensuremath{-}x}$Ca${}_{x}$CoO${}_{4}$ doping series.