Measurement of the irreversibility boundary of superconducting YBa2Cu3O7 single crystals

Abstract

Results from dc-magnetization M(T) measurements on ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ single crystals (with the magnetic field H parallel to the c axis) near the irreversibility boundary in the H-T plane suggest a temperature dependence of the irreversibility boundary ${\mathit{H}}^{\mathrm{*}}$(T)≊${\mathit{H}}_0^{\mathrm{*}}$(1-T/${\mathit{T}}_{\mathit{c}}$ ${)}^{3/2}$. It is shown that the measurement of M(T,H) at temperatures T below the irreversibility temperature ${\mathit{T}}^{\mathrm{*}}$(H) is seriously affected by a spatial inhomogeneity of the applied field H, if the sample has to be moved through a detection-coil system. An alternative procedure for the ${\mathit{H}}^{\mathrm{*}}$(T) detection by dc magnetometry without moving the sample is suggested.