High-resolution Mn K-edge x-ray emission and absorption spectroscopy study of the electronic and local structure of the three different phases in Nd_(0.5)Sr_(0.5)MnO_3

摘要

Nd_(0.5)Sr_(0.5)MnO_3 is particularly representative of mixed-valent manganites since their three characteristic macroscopic phases (charge-ordered insulator, ferromagnetic-metallic, and paramagnetic insulator) appear at different temperatures. We here report a complete x-ray emission and absorption spectroscopy (XES-XAS) study of Nd_(0.5)Sr_(0.5)MnO_3 as a function of temperature to investigate the electronic and local structure changes of the Mn atom in these three phases. Compared with the differences in the XES-XAS spectra between Nd_(0.5)Sr_(0.5)MnO_3 and the single-valent reference compounds NdMnO_3 (Mn~(3+)) and Sr/CaMnO_3 (Mn~(4+)), only modest changes have been obtained across the magnetoelectrical transitions. The XES spectra, including both the Mn Kα and Kβ emission lines, have mainly shown a subtle decrease in the local spin density accompanying the passage to the ferromagnetic-metallic phase. For the same phase, the small intensity variations in the pre-edge region of the high-resolution XAS spectra reflect an increase of the p-d mixing. The analysis of these XAS spectra imply a charge segregation between the two different Mn sites far from one electron, being in intermediate valences Mn~(+3.5±δ/2)(δ ＜ 0.2 e-) for all the phases. Our results indicate that the spin, charge, and geometrical structure of the Mn atom hardly change among the three macroscopic phases demonstrating the strong competition between the ferromagnetic conductor and the charge-ordered insulator behaviors in the manganites.