X-ray photoemission spectra and the electro-transport properties studies on the layered manganites Sm_2-2xSr_1+2xMn₂O₇

Layered perovskite manganite ceramics with a nominal chemistry Sm_2-2xSr_1+2xMn₂O₇ (x = 0.2, 0.4, 0.5) were prepared using sol-gel method. X-ray photoemission spectroscopy (XPS) was used to investigate their electronic structures. The manganese ions are highly mixed states of Mn³⁺ and Mn⁴⁺. It is found that the Mn³⁺/Mn⁴⁺ ratio decreases with the Sr doping content increasing. The O 1s photoemission spectra show that there are lattice oxygen and chemical absorbed oxygen in all the samples. Meanwhile, the conductivity and magnetoresistance (MR) of Sm_2-2xSr_1+2xMn₂O₇ (x = 0.2, 0.4, 0.5) were studied. The sample of x = 0.2 shows metal insulator transition (MIT) at 115 K. The MIT peak temperature (T_p) shifts towards higher temperature with increasing applied magnetic field. All the samples can be well fitted to the variable-range hopping (VRH) model. The maximum value of MR (%) [ρ(0) - ρ(H)]/ρ(0) × 100 for x = 0.2 is 79.05% (105 K, 3 T).