Expansion of ferromagnetism by calcium doping in the ordered oxygen deficient perovskite EuBaCo2O5.50 ± δ

The effect of calcium doping on the magnetic and transport properties of Eu1-xCaxBaCo2O5.50 +/-delta has been investigated for x <= 0.2. The substitution of calcium for europium does not change the cobalt valency, which remains +3, due to a small oxygen deficiency with respect to the ideal 'O-5.5' composition. This substitution induces a large expansion of the ferromagnetic state in the whole temperature range from 10 to 320 K, T-C increasing with the calcium content. Moreover, we observe a substantial decrease of the effective moment in the paramagnetic phase as x increases. In contrast, the Ca-doping does not significantly affect the metal-insulator transition. However, the resistivity and magnetoresistance decrease drastically for a small doping rate. These results are explained by a new scenario where we consider the presence of canted antiferromagnetic domains and a disproportionation mechanism of the cobalt Co3+ into Co2+ and Co4+.