Novel physics and functions in the layered cobalt oxides from thermoelectricity to ferromagnetism

Three interesting and useful layered Co oxides are briefly reviewed. NaxCoO2 is a good thermoelectric material, in which large entropy on Co4+ plays an important role. Unconventional dielectric response in Bi2Sr2CoO6+d is explained in terms of dielectric mixture formed by charge segregation. A new A-site ordered Co oxide Sr3YCo4O10.5 is found to be a room-temperature ferromagnet, in which an orbital order of magnetic Co3+ ions may drive the ferromagnetism.