Experimental

Five samples of Bi-2212, originating from three different growth methods, were selected for use in this study. The first was the Oxford flux grown crystal, the x-ray characterisation for which was presented in Chapter 3. Two other crystals which will be identified as A1 and A2 were grown by the travelling solvent floating zone method (TSFZ) at Warwick. Both A1 and A2 were from the same batch, grown from a starting composition of Bi-Sr-Ca-Cu=2-2-1-2. The x-ray characterisation of one of these, A1, has already been presented in Chapter 3. The final two crystals were also TSFZ grown by Warwick, but were grown from a batch with a starting composition of Bi-Sr-Ca-Cu=2.0-1.64-1.16-2.0, they will be identified as B1 and B2. Knowledge from previous studies of TSFZ grown crystals has already been published by Balakrishnan [104]. It is known that the 2212 stoichiometric growths produce crystals which are close to optimum doping in their as-grown state with T$_c$ close to its maximum. The off-stoichiometric composition produces on the whole larger single crystals which are, however, overdoped and with a lower T$_c$. In this case, the A1 and A2 samples had similar dimensions 6x3x0.1mm while the B1 and B2 samples were $\approx$1-2mm longer.

Of the five samples under study, three were used for the determination of differences in composition and included the A2, B2 and Oxford crystals. The determination of the composition was made by electron probe microanalysis (EPMA) by A.J.S. Chowdhury at Oxford University. The EPMA method has only recently been established as an important tool for the characterisation of oxides, a full review of the technique is given by Mackenzie [153]. Improvements in the method's accuracy have now made possible the determination of oxygen content to an accuracy comparable to that of thermogravimetry, providing a new, more expedient means which is ideally suited to the study of single crystals.

The remaining two samples, A1 and B1, were selected to undergo ex situ high temperature annealing treatments. The annealing strategy is based upon the pre-existing knowledge about the characteristics of the two growth types and was designed to bring about an opposite effect upon T$_c$ in each case. The A1 sample was annealed for 24 hours in an oxygen atmosphere at 720$^o$C with the intention of increasing the oxygen content of this crystal and hence lowering T$_c$. The crystal B1, however, was annealed in a nitrogen atmosphere at 720$^o$C for only 10 minutes. Both samples were quenched to room temperature after annealing. The anneal times were chosen based upon previous experience of heat treatments on such crystals. It has been found that only a very short anneal period is necessary to effect an increase in T$_c$ when the as-grown value of T$_c$ is lower than optimum, while much longer anneal times have been found to be necessary to produce any lowering of T$_c$ by oxygen incorporation.

The superconducting transitions were characterised for all five crystals in their as-grown state as well as post-anneal for A1 and B1. The measurements were made using AC-susceptibility with H$_{AC}$ parallel to the c axis. The critical temperature T$_c$ was defined as the onset temperature of diamagnetism. These measurements were carried out by J. Gardener at Warwick University for crystals A1 and B1, and by A. J. S. Chowdhury at Oxford University for the remainder.

X-ray characterisation of both A1 and B1 samples was made prior to and after the annealing treatments, and to the Oxford crystal for comparison. The x-ray measurements were carried out as described in the earlier chapter, using graphite for both monochromator and analyser crystals. It was consistently found by the x-ray measurements that the samples were generally made up of at least two or three large misaligned single crystals, which means the crystals under study have dimensions somewhat less than those given for the sample. However, the triple crystal geometry is such that the scattering from any additional crystallites, even when only slightly misaligned from the one under study, can be easily discriminated against.