New Magnetic Transitions in the Ordered Oxygen-Deficient Perovskite LnBaCo₂O_5.50+δ

The investigation of the ordered oxygen-deficient perovskites LnBaCo 2 O 5.50+δ with Ln = Eu and Sm shows that a small deviation from the “O 5.50 ” stoichiometry influences dramatically the magnetic properties of this structural type. Three ferromagnetic (FM) states can be obtained, depending on the oxygen content, δ. For a cobalt valency close to +3 (δ ∼ 0) a ferromagnetic ordering FM1 takes place around 250 K, previously observed for Ln = Gd, Tb cobaltites. For δ < −0.1 corresponding to the mixed valency Co 2+ /Co 3+ a new ferromagnetic state FM2 is observed which can be interpreted either in the frame of phase separation scenario or as a ferrimagnetic state, Co 2+ and Co 3+ species sitting in different sites. For the mixed valency Co 3+ /Co 4+ (δ > +0.10) a new ferromagnetic state FM3 appears, involving Co 3+ −O−Co 4+ superexchange interactions.