Magnetic and magnetotransport properties of La1-xSrxMn0.5Co0.5O3 perovskites

摘要

La1-xSrxMn0.5Co0.5O3 (x = 0.75) perovskites have been studied as a function of temperature by neutron powder diffraction (NPD), magnetization and magnetoresistance measurements. The NPD data show that x = 0.15 and 0.5 compounds are stoichiometric, so the Sr2+ doping transforms Co2+ ions into the Co3+ ones, whereas manganese ions remain in the 4+ oxidation state as in the parent ferromagnetic compound LaCo0.52+Mn0.54+O3. The magnetization data show a decrease in the Curie temperature from 215 K for the compound with x = 0 down to 147 K for the compound with x = 0.05. The compounds with x 0.15 show an increase in T-C up to 260 K (x = 0.75) in spite of a gradual decrease of the spontaneous magnetization. The stoichiometric compound x = 0.5 demonstrates a sharp ferromagnet-paramagnet transition with T-C = 250 K. However, there is no visible coherent magnetic contribution to the NPD patterns. All compounds are semiconductors and exhibit large magnetoresistance gradually increasing with decrease of temperature. The magnetic data have been interpreted assuming that the Co3+ ions are in high spin state, however, there is a fraction of cobalt ions in low spin state. It is suggested that the superexchange interaction between Co3+ ions in the high spin state and Mn4+ ions is ferromagnetic and that the ferromagnetism of the compounds with x 0.5 and high T-C is associated with positive exchange interactions between Co3+ being in high spin state and Mn4+ ions distributed within the short range regions. Based on the NPD results and magnetization data the magnetic phase diagram has been constructed.