Magnetic and magnetotransport properties of Co-doped manganites with perovskite structure

A systematic study of the doping of the Mn-sites by cobalt in three series of manganites - La0.76Ba0.24(Mn(1-x)Cox)O-3 single crystals, La2/3Ba1/3(Mn1 -xCox)O-3 and La(Mn1-xCox)O-3 ceramics has been performed. It was found tha t La(Mn1-xCox)O-3 annealed at 800 degrees C in the range 0.4 less than or e qual to x less than or equal to 0.9 is a mixture of ferromagnetic domains w ith ordered Mn and Co ions and ionically disordered spin-glass domains. In the quenched samples the fraction of spin-glass-type component increases st rongly. The La2/3B1/3(Mn1-xCox)O-3 solid solutions exhibit also an evidence for phase separation in the range 0.5 less than or equal to x less than or equal to 0.8. All the La(Mn1-xCox)O-3 samples show an insulating behavior, however, magnetoresistance reduces strongly when the cobalt content rises to x = 0.5. The La0.76Ba0.24(Mn1-xCox)O-3 single crystals show first-order phase transition below their Curie points associated with a change of groun d state of the Co2+ ions. The magnetic phase diagrams are depicted. The res ults are discussed in terms of positive Mn3+-O-Mn4+, Mn3+-O-Mn3+, Mn4+-O-Co 2+ and negative Mn4+-O-Mn4+, Co2+-O-Co2+, Co2+-O-Mn3+ superexchange interac tions as well as Co2+ and Mn4+ ionic ordering. (C) 2000 Elsevier Science B. V. All rights reserved.