Onset of the fishtail peak in an untwinned YBa2Cu3O7-delta crystal

Isothermal global magnetic measurements in a detwinned YBa2Cu3O7-delta sing le crystal reveal a jump in the slope of the increasing part of the virgin magnetization curves. This feature is broadened or even disappears at low t emperatures (T<15 K). At high temperatures (T>75 K) and below the H-sp(T) c urve in the obtained vortex-matter phase diagram, it is also hard to discer n. The magnetic field at which this anomaly occurs (H-3) decreases as the t emperature increases. The observed feature may be attributed to a recently proposed disorder-induced transition (or crossover) from a relatively order ed vortex lattice to a highly disordered vortex solid.