Primordial role of cobalt valence in the magnetotransport properties of oxygen deficient perovskites Sr1-xLnxCoO3-δ

Oxygen deficient perovskite cobaltites Sr(1-chi)Ln(chi)CoO(3-delta) (X = 0. 10, 0.20) were studied, varying the nature of Ln from La to Ho. In these materials, whose oxygen deficiency and consequently cobalt valence, V-Co, vary continuously with the size of Ln(3+) it is shown that V-Co plays a primordial role in the magnetotransport properties. There indeed exists a crossover around V-Co = +3.43, below which the oxides are weakly ferromagnetic and semiconductors at low temperature, with a large magnitude of the negative magnetoresistance, up to -60% at 4.2 K, whereas above this valence the oxides are metallic ferromagnets. Moreover, the magnetoresistance increases importantly as V-Co decreases in the first region, whereas Tc increases with V-Co in the second one.