The transitions from the ordered Bragg glass into the disordered vortex states at low and high fields are investigated by magnetization measurements varying the angle between magnetic field and crystal lattice orientations in a distinct manner. A single crystalline sphere is used to obtain an uniform demagnetization and not to introduce vortex disorder from sample edges. No angular dependence was observed when the pinning energy dominates. However, if the elastic energy of the vortex lattice is superior, the formation of the Bragg glass depends strongly on the relative orientation of both lattices. If the sphere is rotated around the [100] direction of the crystal with the field being perpendicular to it, a fourfold symmetry, both of the transition fields and of the strength of the vortex metastability, is found. The recent observation by Yethiraj et al. [Phys. Rev. Lett. 82, 5112 (1999)], that the vortex lattice reflects the symmetry of the crystal lattice also in ${\mathrm{V}}_3\mathrm{Si}$ is in good agreement with our investigations. Comparative measurements made on the sphere and on a crystal with edges revealed the influence of disorder on the Bragg glass induced by the sample shape.

• Received 30 September 2002

DOI:https://doi.org/10.1103/PhysRevB.67.064507