Structural, magnetic and electric characterization of Ni-doped YBaCo$_4$O$_7$

TORRE, L. M.; AURELIO, G.; SÁNCHEZ, R. D.; BAUDELET, F.

Buenos Aires

Workshop; X Latinamerican Workshop on Magnetism and Magnetic Materials; 2013

In recent years, the cobaltite ``114´´, named after the formula LnBaCo$_4$O$_{7+delta}$ (Ln: rare earth, Y or Ca), has received considerable interest due to two attractive phenomena: i) it exhibits magnetic frustration showing an almost unique geometrical arrangement of Co$^{2+}$ and Co$^{3+}$ cations in tetrahedral networks and ii) its remarkable kinetics of absorption/desorption of oxygen, highly attractive for several technological applications as for example, oxygen storage. The hexagonal/trigonal symmetry which characterizes these cobaltites can be transformed to orthorhombic by two mechanisms: an increase in the Ln size and/or an increase in the oxygen content ($deltageq$1). The parameter $delta$, determined by the synthesis conditions, affects not only the crystal structure but also the physical properties, as it controls the ratio Co$^{3+}$/Co$^{2+}$. A further ingredient to explore the magnetism of this compound regards the doping of the Co site with other magnetic cations, which is a topical issue in current literature. In this work we present a study of Ni$^{2+}$ doping in the Y-114 cobaltite for samples with a controlled oxygen content $delta$=0. Results will be presented of thermogravimetry measurements, X-ray diffraction, X-ray absorption spectroscopy (XANES) and magnetization and resistivity measurements. We will show strong evidence that Ni$^{2+}$ is incorporated into the structure at the Y$^{3+}$ site, instead of the Co site as claimed in the literature. The observed changes in the crystal structure, phase transition temperatures, magnetism and transport will be reinterpreted in terms of this new finding.