Positron Annihilation Studies on La_(1-x)Sr_xMnO_3 Manganites

摘要

Positron lifetime and coincidence Doppler broadening spectra of La_(1-x)Sr_xMnO_3 and LaMnO_3 manganites were measured. The effects of Sr contents on the microdefects of La_(1-x)Sr_xMnO_3 were investigated by means of positron lifetime results. The signals of 3d electrons of La_(1-x)Sr_xMnO_3 were extracted from the coincidence Doppler broadening spectra. The results show that the electron density of La_(1-x)Sr_xMnO_3 is larger than that of LaMnO_3. The open volume of the defect in La_(1-x)Sr_xMnO_3 increases with the Sr contents. The signals of 3d electrons of ferromagnetic La_(1-x)Sr_xMnO_3 is larger than that of antiferromagnetic LaMnO_3, and it reaches the maximum at x=0.33, that is, the signals of 3d electrons of the La_(0.67)Sr_(0.33)MnO_3 is the highest in all testing samples, which corresponds to a lowest resistivity and a higher Curie temperature of the manganite. The influence of 3d electrons on the ferromagnetic double-exchange coupling and anti-ferromagnetic coupling of La_(1-x)Sr_xMnO_3 were discussed.