Competing pinning mechanisms in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{\mathit{y}}$ single crystals by magnetic and defect structural studies

A correlation between magnetic hysteresis [measured with a vibrating-sample magnetometer (VSM)] and defect structures (studied by transmission-electron microscopy) is reported for a number of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{y}}$ single crystals in the as-grown state and after annealing in oxygen or vacuum above 400 °C. Different regimes of flux pinning can be distinguished from VSM measurements, which we associated with point defects (oxygen vacancies) and planar dislocation networks. Dislocation networks observed in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathit{y}}$ single crystals provide effective pinning centers for decoupled two-dimensional (2D) pancake vortices, especially when the characteristic length scale of the network matches that of the 2D pancake vortex spacing. Anomalies in the magnetic properties are discussed in terms of the competition between pinning from point defects and dislocation networks, together with a transition from a 3D flux lattice to a 2D pancake vortex regime.