Novel high Tc half-metallic phases and metal-insulator transitions in 3d-5d double perovskites

Double perovskite materials have come into the limelight in recent years owing to the half-metallicity of some of its members, and a large magnetoresistance (MR). The MR however, requires a large polarization at room temperatures and hence a high Tc. We propose a method of obtaining half-metallic phases with high Tc, by doping an unlikely candidate, an insulator. The parent compound Sr_{2}CrOsO_{6} is a 3d-5d ferrimagnetic insulator with a very high Tc. Upon slight electron doping with La, or hole doping with Na, it is found to become a half-metal retaining a high Tc. A novel reentrant Metal-Insulator transition is obtained as a function of filling as one goes from the Na-doped (low filling) side to the La-doped (high filling) side. We analyze this phase transition, and the Tc behaviour using a model Hamiltonian which we had proposed earlier for the parent compound, and find that the carriers change sign for La (electron) and Na (hole) doping, apart from a sharp rise in polarization. Using the abovementioned model Hamiltonian, we had earlier explained the high Tc insulating behaviour of Sr2CrOsO6, and why it has the highest Tc amongst all 3d-5d double perovskites. This series of compounds presents an interesting comparison, and counterpoint, with the La-doped Sr_{2}FeMoO_{6} series which we had proposed earlier, where a novel and rare Antiferromagnetic Metal phase has been observed both analytically, numerically, as well as experimentally. Possibility of enigmatic new phases and materials will be discussed. more details» copy to my calendar»