Magnetic response of polycrystalline YBaCo 4 O 7+δ synthesized through the physical and chemical route: The role of phase inhomogeneities

Abstract Polycrystalline YBaCo4O7+δ samples were obtained through a standard solid state reaction, and their structural, morphological, electrical, and magnetic properties are carefully studied. The X-ray powder diffraction (XRD) patterns showed reflections of a pure hexagonal structure (space group P63mc) with lattice parameters being very close to those reported in the literature. Although XRD analysis showed that the main phase present is 114, the presence of secondary phases could not be ruled out based solely on the XRD characterization. Indeed, sensitive SQUID magnetic measurements showed that the samples were affected by very small quantities of the 112 phase (YBaCo2O5.5), which typically manifests itself through a conspicuous increase in the magnetization at~300 K. The results achieved corroborated the predictions concerning the difficulty of stabilizing the 114 phase when synthesized via the standard solid-state reaction. With this in mind, we next attempted to obtain the compound with improved phase purity. In so doing, the YBaCo4O7+δ compound was synthesized through a wet chemistry method based on a citrates route. The XRD patterns recorded for these samples revealed well-defined peaks corresponding to a pure hexagonal structure. More interestingly, SQUID measurements show no sign of features in the M(T) curve at temperatures as low as~80 K. This result was consistent with the magnetic behavior observed in YBaCo4O7+δ single-crystals. At temperatures below~80 K, a clear feature was observed which seemed to correlate with a transition into an antiferromagnetic state. Isothermal magnetization recorded at 70 K showed that field-induced effects manifested themselves through the appearance of a ferromagnetic-like component. This ferromagnetic component may arise from spin canting of the underlying antiferromagnetic state or through field-induced structural transition (at least at local scale). Although a definitive interpretation of the in-field behavior from magnetization data alone is difficult because of the unknown role of the yttrium ion, the results achieved suggest that the magnetic behavior observed in members of the R-114 family is not necessarily linked to the moment of the rare-earth ion, as in case of YBaCo4O7+δ, since the yttrium ion is not magnetic. Beyond this important finding, the experimental results reported in the present paper demonstrate that the tested chemical route is suitable for synthesizing complex, single-phase oxides, such as the YBaCo4O7+δ cobaltate. The success in synthesizing high-purity YBaCo4O7+δ allows one to subtract parasitic effects from the intrinsic magnetic behavior of this challenging system.