Vortex solid-solid transition in aBi1.6Pb0.4Sr2CaCu2O8+δcrystal

The magnetic vortex phase diagram of ${\mathrm{Bi}}_{1.6}{\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ was measured using a Hall sensor array. Pb doping was shown to drastically change the magnetic phase diagram including a significant increase in the irreversibility field and a temperature dependence of the onset field of the second magnetization peak. Anisotropy was determined using a recently proposed scaling expression, which relates the anisotropy to the irreversibility line. Based on the obtained anisotropy an excellent fit of the experimental $B_{\mathrm{on}}\mathrm{}\left(T\right)\mathrm{}$ data to the model of disorder-induced transition was found for three basic regimes: $L_c<s<\mathrm{}{L}_0$ for $T<\mathrm{}48\mathrm{}$ K, $s<\mathrm{}{L}_c<L_0$ for $48<T<\mathrm{}66\mathrm{}$ K, and $s<\mathrm{}{L}_0<L_c$ for $T>\mathrm{}66\mathrm{}$ 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.