Valence electronic states related to Mn~(2+) at La_(0.7)Sr_(0.3)MnO_3 surfaces characterized by resonant photoemission

摘要

Nonferromagnetic Mn~(2+) ions can be readily formed at the surface of half metallic La_(0.7)Sr_(0.3)MnO_3 manganite as demonstrated by deoxygenating surface treatments. The 3d~5 contribution of these Mn~(2+) ions to the valence-band electronic structure has been characterized using Mn(2p) to 3d resonant photoemission measurements. The Mn~(2+) related 3d electrons were found to be stabilized by about 2 eV with respect to the mixed-valence 3d states, indicating their strong localization. Active participation of Mn~(2+) states in both spin and charge conductivity processes is therefore excluded. A two-channel picture, including independent Mn~(3+)/Mn~(4+) and Mn~(2+) channels, emerges from detailed data analysis. Reversible Mn~(2+) formation and straightforward oxygen annealing effects point to a direct bonding between Mn~(2+) and oxygen vacancies that are most probably created at preexisting structural defects. The t_(2g) and e_g states of the mixed valence Mn~(3+)/Mn~(4+) ions remain unaffected as the Mn~(2+) content increases, indicating a robust Mn~(3+)/Mn~(4+) channel independent of structural defects.