Unusual charge disproportionation and associated magnetic behaviour in nanocrystalline LaMn0.5Co0.5O3

Evidence for an unusual double charge disproportionation, Mn4+ + Co2+ --> Mn3+ + Co3+ --> Mn4+ + Co2+, is observed in nanocrystalline LaMn0.5Co0.5O3 synthesized by a low-temperature method and annealed in the temperature range 200-1300 degreesC. The Curie temperature of nanocrystalline LaMn0.5Co0.5O3 is increased from 150 to 230 K when annealed in the temperature range 200-700 degreesC and is further decreased back to 150 K when annealed in the temperature range 700-1300 degreesC. This increase and decrease in T-c is found to be associated with the conversion of the spin states of Mn and Co from Mn4+ and Co2+ to Mn3+ and low-spin Co3+, and again back to Mn4+ and Co2+ as evidenced from x-ray photoelectron spectroscopic studies.