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Resistivity and magnetic susceptibility of superconducting Bi0.25-y Sr0.25-y Ca2y Cu0.5O x (y=0.1, 0.125, 0.15) have been measured and compared with the results of X-ray powder diffraction. The volume fraction of the low-T c phase (T c∼75 K) decreases as the Ca concentration is increased and superconductivity attributed to the high-T c phase (T c>100 K) is observed. Resistivity of Bi0.125Sr0.125Ca0.25Cu0.5O x shows a sharp transition above 100 K and becomes zero at 97 K.

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The effects of the substrate birefringence on the read back signal envelope of the magneto-optical disk are studied by a numerical model and model experiments. The envelope modulation of the RF signal amplitude is confirmed to be reduced by several percents by the adjustment of the conditions in the dynamic test. However. a low frequency noise is caused by local variation of the birefringence of the substrate. It is suggested that the conventional birefringence model for a typical plastic substrate might not be sufficient and a new model is proposed. INTRODUCT IONInjection molded polycarbonate (PC) substrate is one of the most suitable substrates for the magneto-optical disk for commercial use.Discussions have been held on the effects of the substrate birefringence on the Carrier to Noise Ratio (CNR) [1.2.3J. Now it is well known that the birefringence of the ordinary PC substrate has little effect on the CNR of the magneto-optical disk when using a differential detection system.The envelope of the readback signal in the PC substrate disk was usually found to have a much greater modulation than that of disks with a glass substrate. The modulation was divided into two categories in accordance w1th its source. (1) the modulation of the RF signal amplitude. and (2) the modulation of low frequency noise. The former corresponds to the local variation at the carrier level and so was not removed by a high pass filter. The latter corresponds to the meandering of the signal envelope which contributed to the jitter and was regarded as a local variation in DC bias. effectively removed by a high pass filter.We measured the distribution of birefringence on several PC disks and the signal envelope of those disks.We then investigated these modulations through numerical models and model experiments. The Jones matrix method was applied to a simplified optical system. in which the substrate birefr'ingence at a point was represented by a single. biaxial. refractive ellipsoid [4J. 2.EXPERIMENTALThe magneto-optical disks covered with a thin film of TbFeCo were prepared with several disk substrates by a sputtering method. The envelopes of these disks were measured by a dynamic tester with a PIN-differential detection system. The conditions for readout were as follows: read power = 0.8mW. carrier frequency = 0.5 or 1.0 MHz. and a constant linear velocity of 4m/s. Static Model experiments were performed for the reflection and transmission cases.

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