It was shown recently LnBaCo₂O_5+delta (Ln-112, Ln being rare earth ion, 0 < = delta < = 1) layered cobaltites present giant magnetoresistivity (GMR) phenomena accompanied by fascinated features with structural, metal-insulator, and magnetic field induced transitions, as well as charge and orbital ordering phenomena on the verge of Co³) spin-state transitions. Early work focused on studies sintered polycrystalline and powder samples. High quality single crystals are essential to understand nature of GMR phenomena in the layered cobaltites, which is differing cardinal from the nature of colossal magnetoresistance in the wellknown rare earth manganites. The original approach to flux growth was developed to provide large and high quality single crystals of the oxygen deficient layered cobaltates. Fortunately, we found co-crystallization of the Ln-112 phase and new cobaltite phases in wide ranges of flux melt composition and temperature for the most of Ln₂O₃ - BaO - CoO systems. The data on primary crystallization fields of the phases, which appear in parts of corresponding Gibbs triangles at temperature in the range 1373 - 1523K allow us to presume a tentative (T-x) quasi binary cut of the system phase diagram. Here we also report on first growth of the Ln-112 (Ln = Pr, Eu, Gd, Tb, Dy) single crystals as large as 6 x 6 x 1.5 mm³ and confirm GMR property of the compounds by meausurements of the crystals in external fields up to 30T. Highly anisotropic negative GMR effect (up to 80% at 4T, 40K) was registered in Gd-112 single crystal. Data of neutron powder diffraction of Tb-112 and magnetic susceptibility of weekly twinned Gd-112 single crystal unambiguosly shown the layered cobaltites are easy axis magnets with spontaneous magnetization alignment along a or b-axis of the structure. Metamagnetic behavior of cobalt sublattice in the basal plane of Eu-112 single crystal was observed to serve as a base for constructing of generic H-T magnetic phase diagram.

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1. Layered cobaltites: single crystal growth, structure, transport and magnetic properties
2. Layered cobaltites: synthesis, oxygen nonstoichiometry, transport and magnetic properties