Transport and magnetic properties of GdBaCo2O5+x single crystals:: A cobalt oxide with square-lattice CoO2 planes over a wide range of electron and hole doping -: art. no. 134414

Single crystals of the layered perovskite GdBaCo2O5+x (GBCO) have been grown by the floating-zone method, and their transport, magnetic, and structural properties have been studied in detail over a wide range of oxygen contents 0 <= x <= 0.77. The obtained data are used to establish a rich phase diagram centered at the "parent" compound GdBaCo2O5.5-an insulator with Co ions in the 3+ state. An attractive feature of GdBaCo2O5+x is that it allows a precise and continuous doping of CoO2 planes with either electrons or holes, spanning a wide range from the charge-ordered insulator at 50% electron doping (x=0) to the undoped band insulator (x=0.5), and further towards the heavily hole-doped metallic state. This continuous doping is clearly manifested in the behavior of thermoelectric power which exhibits a spectacular divergence with approaching x=0.5, where it reaches large absolute values (+/- 800 mu V/K) and abruptly changes its sign. At low temperatures, the homogeneous distribution of doped carriers in GBCO becomes unstable, as is often the case with strongly correlated systems, and both the magnetic and transport properties point to an intriguing nanoscopic phase separation into two insulating phases (for electron-doped region) or an insulating and a metallic phase (for hole-doped region). We also find that throughout the composition range the magnetic behavior in GBCO is governed by a delicate balance between ferromagnetic (FM) and antiferromagnetic (AF) interactions, which can be easily affected by temperature, doping, or magnetic field, bringing about FM-AF transitions and a giant magnetoresistance (MR) phenomenon. What distinguishes GBCO from the colossal-MR manganites is an exceptionally strong uniaxial anisotropy of the Co spins, which dramatically simplifies the possible spin arrangements. This spin anisotropy together with the possibility of continuous ambipolar doping turn GdBaCo2O5+x into a model system for studying the competing magnetic interactions, nanoscopic phase separation, and accompanying magnetoresistance phenomena.