Angle-dependent reversible and irreversible magnetic torque in single-crystalline Y2Ba4Cu8O16

A systematic study of the angle-dependent reversible and irreversible magnetic torque in single-crystalline Y2Ba4Cu8O16 is presented. The high purity of the crystals allows us to show some intrinsic pinning properties of vortices due to the layered crystal structure. The irreversible component of the torque, which is unusually small, exhibits a peculiar angular dependence: It is minimal as the magnetic field B is applied along the ab plane and displays a pronounced maximum at finite angles, reminiscent of the ''fishtail'' effect. The unusual shape of the irreversible torque is attributed to the pinning of the vortex core, which becomes discontinuous below the two- to three-dimensional (2D-3D) crossover temperature. Another property shown by the angle-dependent torque is the lock-in of the vortex lines between the CuO2 layers for B parallel to the ab plane. Applying the anisotropic 3D London model to fit the reversible torque data, we derive the in-plane London penetration depth lambda(ab)=143 nm, the coherence length xi(ab)=1.9 nm, and the effective mass anisotropy ratio gamma=12.3 for Y2Ba4Cu8O16.