IRREVERSIBILITY LINE OF 1-XCAXSRBACU2.9CO0.1O6-LESS-THAN-X-LESS-THAN-0.25) - A COMPARATIVE-STUDY OF DC MAGNETIZATION AND COMPLEX AC SUSCEPTIBILITY(Z (0)

We present detailed measurements on the complex susceptibility, chi'' (1500 Hz) and field cooled (FC) and zero field cooled (ZFC) DC magneti zation, M, of Y1-xCaSrBaCu2.9Co0.1O6+z (0 less-than-or-equal-to x less -than-or-equal-to 0.25) as a function of temperature and DC field, H ( 0 to 800 Oe). We confirm earlier published data (on YBaCuO) that T(c) increases as a function of x(Ca). Further, Ca enhances the irreversibi lity line derived both from the chi'' data (ILAC) and the M-H (ILDC) d ata. The data are analyzed using the relation H = K(1-T(irr)/T(c))n. H ere T(irr) represents the peak T(p) of the chi'' data in the case of A C susceptibility measurements, whereas T represents the temperature w here the FC and the ZFC curves meet. The value of K is found to increa se as a function of x(Ca) which is taken to indicate enhanced flux pin ning due to Ca. However, the ILAC does not coincide with the ILDC. Som e possible causes are discussed.