Magnetic studies on superconducting oxides

Abstract

In the first part of this Thesis the structural and superconducting properties of some Yttrium-Barium-Copper-Oxides (YBCO) superconducting materials have been examined. These materials, which were discovered only about three years ago, have the important property that they become superconducting at temperatures below about 86K. Because this transition temperature (T(_c)) is well above the temperature (77K) of liquid nitrogen- a readily available refrigerant- there is very considerable interest in potential technological applications. The nominal composition of YBCO material is YBa(_2)Cu(_3)O(_7) and for superconductivity the value of x must be near 7-x = 7.0. X-ray diffraction methods have been used to determine the lattice parameters and values of x for a range of oxides prepared from different starting materials by different routes. These structural studies were extended by Scanning Electron Microscopy which gave information about the grain size and microstructure of the materials. A major part of the work considered the development of an inductance probe technique for examining the superconducting properties. This technique was based on the Meissner effect and involved measuring the inductance of a small coil which was filled with superconducting material as a function of temperature and magnetic field. From these measurements several superconducting properties have been determined. These included the transition temperature (T(_c)), the percentage of superconducting material present in a given sample, the first critical field (H(_c1), and hysteresis. The critical field is the value at which an external magnetic field penetrates into the superconductor to a depth known as penetration depth (A) which corresponds to a non-superconducting layer. The values of H(_c1) were all found to be about 15 gauss, rather smaller than those previously reported in the Uterature. The variation of A with temperature was also examined and shown to be in excellent agreement with theory based on a Type II model. In an attempt to correlate these superconducting properties with the structural studies it was shown that the most promising superconductors were those in which the grain size was relatively large (greater than lOpm) and in which the grains were closely packed together. In the second part of the Thesis some non-superconducting oxides were examined by magnetic resonance methods. In the first group of experiments the epr linewidths of the M= + l/2 ↔ -1/2 transition of the Cr(^3+) spectrum of Cr/MgO single crystals and powders have been measured at 9GHz before and after annealing. The nominal chromium concentrations ranged from 800 to 15,000 ppm. Specimens were annealed at 500 C in oxygen for up to 150 hours. Annealing produced reductions in peak-to-peak linewidth (∆H(_pp)) which occurred in two stages. Each followed an exponential [∆ H pp] (_t) = [∆ H pp] (_0)exp(^-at)and the initial (a(_1)) and final (a(_2_) decay rates were determined from the slopes of Ln[∆ H pp] (_t) versus time plots. For single crystals all the values of a(_1) were about 0.04 h(^-l); the smaller values of a(_2) were concentration dependent ranging from 0.9 x 10(^-3) to 1.25 X lO(^-3)h(^-1) for the two sources of strain. The rapid decay rate is associated with cationic vacancy strain relief and the slower decay with removal of lattice distortion brought about when the Mg(^2+) ions are replaced by Cr(^3+) dopant ions. Similar effects were observed with powders but in these both types of lattice strain were relieved at a slower rate in the powder than in the corresponding single crystal, e.g. for 3600 ppm Cr, a(_1) = 0.016h(^-1) and a(_2) = 1.0 x 10(^-3)h(^-1) In the second group of experiments the mechanisms of magnesiochromite formation were studied and the effect of this on epr linewidth was examined. The results of both groups of experiments were related to the more general problem of the dipolar mechanisms responsible for the variation of epr linewidth with concentration in the Cr/MgO system.