Thermal effects in the x-ray spectra of La1-xCaxMnO3

Complementary x-ray emission (XES) and x-ray-absorption (XAS) measurements of La1-xCaxMnO3 as a function of temperature have been performed. For 0.5≤x≤0.8 we find changes in the XAS and XES spectra consistent with a small increase in the average Mn valence accompanying the passage to the low-temperature state. It is suggested that this small Mn-configuration shift could be due to the competition between the energies associated with ferromagnetic (FM) and charge/orbital (CO) correlations. In the region 0.0≤x≤0.4 we observe thermal spectral changes quite different from those found in the x≥0.5 materials. Modest thermal changes in the Mn XAS pre-edge (K-edge), for 0.0≤x≤0.4, were observed to be coupled to the robustness of the FM-metallic ground state. The clearest pre-edge feature variation appears to be related to Mn-eg-majority-spin state changes. In contrast the thermal XES spectral changes appear similar in character in the antiferromagnetic (AF)-insulating x=0.0 and FM-metallic x=0.3 materials. An enhancement of the effective local moment via a coupling to the internal exchange field, in the magnetically ordered states, is proposed to explain these XES results. The x=1.0 end member (CaMnO3) was found to exhibit significant temperature dependence of the absorption and emission spectra. These XAS and XES results, for CaMnO3, are discussed in terms of thermal perturbations on the degree of covalency of this material