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 Ln(1-x)Ca(x)BaCo(2)O(5.50 +/-delta) (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 T-C. The unusual trend of the decrease in T-C 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 T-C 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. (C) 2008 Elsevier B. V. All rights reserved.