Magnetic measurements of the upper critical field of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals

Dc magnetization measurements in fields up to 5 T on single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ reveal a well defined onset of diamagnetism allowing a unique magnetic determination of the upper critical field and its anisotropy. The upper-critical-field slopes are -1.9 and -10.5 T/K corresponding to coherence lengths of 16 and 3 Å for fields perpendicular and parallel to the CuO planes. At the magnetically determined nucleation temperatures no distinct features are found in resistive transition curves.