Switching from unusual to usual ferromagnetism in “112” LnBaCo2O5.50±δ: By calcium doping

The effect of calcium doping on the magnetic and transport properties of the intermediate size lanthanide cobaltites of the type Ln1−xCaxBaCo2O5.50±δ (Ln=Y, Gd, Eu and Sm) has been investigated for 0⩽x⩽0.2. The substitution of Ln by calcium induces a large expansion of the ferromagnetic state in the whole temperature range below TC. The unusual trend of the decrease in TC with the increase in Ln size in the undoped parent oxides becomes opposite in the calcium doped samples. Such an unusual behavior of the ferromagnetic TC in the parent compounds is explained on the basis of thermally activated hole-mediated ferromagnetic coupling between the high-spin cobalt ions. The ferromagnetic state of the Ca-doped samples originates from the Co3+–O–Co4+ superexchange interaction, where Co4+ emerges from the disproportionation mechanism of the cobalt Co3+ into Co2+ and Co4+. However, the Ca-doping does not significantly affect the metal–insulator transition, which is associated with the structural change and not related to the spin state transition.