Upper critical field of four-CuO2-layer TlBa2Ca3Cu4O11+delta single crystals

Measurements of magnetization in the reversible region were performed as a function of temperature on TlBa2Ca3Cu4O11+delta single crystals in magnetic fields up to 5.5 T with the field direction applied along the crystalline c-axis. A linear temperature dependence was observed between 70 and 116 K f or the reversible magnetization and the slope, (dM)/(dT), decreased with in creasing field strength. Near T-c (128 K), the magnetization showed a round ing. Both the field-dependent dM/dT - values in the linear region and the r ounding behavior of the magnetization near T, indicated that the upper crit ical field, H-c2, in the TlBa2Ca3Cu4O11+delta system might be very large. T herefore, the reversible magnetization was measured in the low field range, i.e. H much less than H-c2 and the traditional method used to obtain the H -c2 value based on the Abrikosov formula near H-c2 was not applicable. By u sing the model of Hao et al. [Z. Hao, J.R. Clem, M.W. McElfresh, L. Civale, A.P. Malozemoff, F. Holtzberg, Phys. Rev. B 43 (1991)2844], the reversible magnetization data were analyzed to determine the Ginzburg-Landau paramete r K and the slope of the upper critical field at T-c, (dH(c2)/dT)\(Tc). It was found that kappa = 109, and H-c2(0) = 160 T in the TlBa2Ca3Cu4O11+delta crystals with the field parallel to the c-axis. (C) 1999 Elsevier Science Ltd. All rights reserved.