Vortex solid-solid transition in a Bi1.6Pb0.4Sr2CaCu2O8+delta crystal

The magnetic vortex phase diagram of Bi1.6Pb0.4Sr2CaCu2O8 + delta was measu red using a Hall sensor array. Pb doping was shown to drastically change th e magnetic phase diagram including a significant increase in the irreversib ility field and a temperature dependence of the onset field of the second m agnetization peak. Anisotropy was determined using a recently proposed scal ing expression, which relates the anisotropy to the irreversibility line. B ased on the obtained anisotropy an excellent fit of the experimental B-on(T ) data to the model of disorder-induced transition was found for three basi c regimes: L-c<s<L-0 for T<48 K, s<L-c<L-0 for 48 <T<66 K, and s<L-0<L-c fo r T>66 K, where s, L-0, and L-c are, respectively, the interlayer spacing, the characteristic size of the longitudinal fluctuations in a cage, and the size of a coherently pinned vortex segment.