Abstract
The effect of calcium doping on the magnetic and transport properties ofEu1−xCaxBaCo2O5.50 ± δ has beeninvestigated 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‘O5.5’ composition. This substitution induces a large expansion of theferromagnetic state in the whole temperature range from 10 to 320 K,TC increasing with the calcium content. Moreover, we observe a substantialdecrease of the effective moment in the paramagnetic phase asx increases. In contrast, the Ca-doping does not significantly affect the metal–insulatortransition. However, the resistivity and magnetoresistance decrease drastically for a smalldoping rate. These results are explained by a new scenario where we consider the presenceof canted antiferromagnetic domains and a disproportionation mechanism of the cobaltCo3+ intoCo2+ and Co4+.
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