Study On Superconductivity And Magnetism Of Na_xCoO₂ System

To research the superconductivity and magnetism of Na_xCoO₂ is the aim of this thesis. During the superconducting studying, result is unrepeatable, which indicated that the sample was instability in room temperature. The work of preparing the polycrystalline Na_0.7CoO₂ is successfully acheived, and which is simple and easy to do.Duing magnetism studying, to prepared the polycrystalline Na_0.7CoO₂ samples doping with K, Al, Ti, Mn four elements are also successfully achieved. The methods of doping are vary from Na seat to Co seat, and from sole element doping to two elements doping. The magnetism of Na_0.7CoO₂ system have been studied in a relatively comprehensive way. From 5K to 300K, the change of susceptibility was mostly cared. During studying, spin-glass formation and antiferromagnetic transition in Na_0.7Co_1-xAl_xO₂ have been found, and below room temperature most doped samples had very complex magnetism. Most samples have twice magnetic transitions and three magnetic states. The Al and Ti ions ,not K and Mn ions, influence the magnetism of the system essentially. A spin-glass state in low temperature, and a antiferromagnetic state in a higher temperature had been found in the samples being doped with Al ion, which indicated the paramagnetism of Na_0.7CoO₂ was changed. Twice antiferromagnetic transitions have been found in the samples being doped with Ti ion. No essential effect on the magnetism of Na_0.7CoO₂ which being doped with the K ion, only found that the susceptibility change with doping quantity at 300K. Being doped with Al and Ti ions simultaneous, samples have an abnormity at low temperature and antiferromagnetic transitions. Which indicates only Ti ion affect the magnetic of Na_0.7CoO₂, and Al ion has smalle effect on the magnetism. Being doped with Al and Ti ions simultaneous, have no essential affect on the magnetism of Na_0.7CoO₂. The resistance test result of Na_0.7Co_.75Al_0.25O₂ sample indicate that the non-magnetic Al ion strongly influence the conductance of Na_0.7CoO₂ above 75K, and the resistivity has a typical characteristic of metallic-semiconductor transition.