Abstract:
Polycrysttdline samples of NaiCoOa were prepared using the "Rapid heat-up" method. One
set of samples was annealed in flowing O2, while the other set in flowing Argon. X-Ray
diffraction measurements indicated a stable phase of Nao.7Co02 mixed with C03O4 for all
the samples even though they differed in concentration of Na. Argon annealed samples were
insulators, whereas the ones annealed in O2 were metallic. Most of the measurements were
performed on the sample Nao.7Co02, because it is the host compound for the superconductor
sample Nao.35Co02-H20. Magnetization measurement showed that the magnetic moment
decreased with increasing sodium concentration. This is due to the existence of C03O4 in
samples with Na^ 0.7. As sodium concentration decreases, the magnetic moment increases
due to the increasing concentration of C03O4 and its large magnetic moment. Magnetization
measurements showed that the magnetic moment of Nao.7Co02 is field-dependent in low
fields eind field-independent in fields higher than 100 G. Resistivity changes with temperature
(dp/dT) increased with increasing Na concentration. Also resistivity measurements
were performed under different hydrostatic pressures on Nao.7Co02. Two transitions were
observed; one at a temperature Ti ~20 K and the other at T2 ^280 K, the transition at
Ti has a magnetic origin and the one at T2 is a structiural transition. It was noticed that
pressure aJfects resistivity of the sample. At higher pressures resistivity changes faster with
temperature. Magnetoresistance measurement showed a small change in the resistivity, especially
at lower temperatures. A novel layered superconductor Nao.35Co02H20 was prepared
using de-intercalation of Na from the host compound Nao.7Co02. FVom the temperature dependence of the magnetization, the superconducting transition temperature and lower
critictil field have been estimated as Tc=4.12 K and Hci=66 G, respectively.
