Magnetoresistance in the oxygen deficient LnBaCo2O5.4 (Ln=Eu, Gd) phases

Abstract

New “112” phases, ${\mathrm{LnBaCo}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{5.4}},$ with an ordered oxygen deficient perovskite structure, derived from the ${\mathrm{YBaFeCuO}}_{\mathrm{5}}$ -type were studied for $\mathrm{Ln=Eu},$ Gd. The appearance of giant negative magnetoresistance in this structural type is demonstrated. Resistance ratio $R_0{\mathrm{/R}}_{\mathrm{7\hspace{0.5em}T}}$ reaches at least 10 at 10 K, i.e., is significantly larger than those observed in the other cobalt perovskites, such as ${\mathrm{La}}_{\text{1−x}}{\mathrm{Sr}}_x{\mathrm{CoO}}_{\mathrm{3}}.$ These properties are linked to an original magnetic behavior of these materials that exhibit two types of transition—antiferromagnetic to ferromagnetic, and ferromagnetic to paramagnetic—as $T$ increases. This magnetic behavior may be related to a possible I.S. and L.S. spin ordering of trivalent cobalt in pyramidal and octahedral coordinations, respectively.