Mn K-edge XANES study of the La1−xCaxMnO3 colossal magnetoresistive manganites

Abstract

We report Mn K-edge x-ray absorption near edge structure (XANES) calculations of the ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3$ manganites considering three pair correlations, according to the three fundamental degrees of freedom governing their unusual electronic properties, namely, the electronic structure of the unoccupied states probed by the Mn K-edge profile versus (i) local atomic distortions; (ii) local magnetic ordering; and (iii) the charge-transfer nature of the Mn-O bonds. The calculations are accompanied by Mn K-edge XANES measurements in the temperature range 30–300 K. The main features of the absorption edge can be qualitatively reproduced in terms of single-electron multiple-scattering calculations for an 87-atom cluster. Lattice polaronic distortions in ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ are simulated assuming a strongly distorted orthorhombic structure above and an almost undistorted rhombohedral structure below $T_c.$ The results roughly reproduce the energy “shift” across $T_c$ observed experimentally. Mn K-edge spin-polarized XANES spectra of the $x=0,$ 0.3, and 1 samples are presented. An energy splitting between the majority- and minority-spin spectra of 0.5–1.1 eV contributes to the total XANES broadening below the Néel (Curie) temperature. A small feature $B_3$ standing approximately 6 eV above the main absorption peak is beyond the scope of single-electron calculations; it is assigned to a shake-up transition. To illustrate, the calculated Mn K edge is obtained as the convolution product of the single-electron XANES and the spectrum of many-body excitations in the Mn-O electronic states upon the sudden switching on of the Mn $1s$ core hole. We investigate the charge-transfer (CT) versus. Mott-Hubbard-type ground state using the $U_d,$ Δ, and the $T_{\mathrm{dp}}$ parameters determined by previous Mn $2p$ x-ray photoemission spectroscopy measurements and find that ${\mathrm{LaMnO}}_3$ should be viewed as a CT-type insulator with a substantial O $2p$ component in the ground state. In light of these results the controversial issue of Mn valence states in the manganites is critically reexamined. We argue that the disproportionation may be understood as a mixture of the CT many-body electronic configurations coupled with spin and lattice degrees of freedom.