ANISOTROPIC VORTEX-FLUX ROTATION IN GRAIN-ORIENTED YBA2CU3O7

Magnetization-vector measurements were made at 4.2 K on a grain-oriented YBa2Cu3O7 sample rotated in the fixed external field H(e) that was initially applied in the a-b plane or along the c axis during cooling from above T(c). The penetrating vortex-flux component of the sample magnetization was determined as a vector (M(p)) over a complete rotational cycle, with diamagnetic shielding and demagnetization effects taken into account. At low H(e), M(p) turns rigidly with the rotating sample, and this simple behavior persists up to H(e) = 1 kOe when applied along c during cooling. However, for H(e) = 1 kOe applied along a-b during cooling, M(p) reverses its direction in the a-b plane periodically, while its magnitude varies roughly as the projection of H(e) on a-b. At higher H(e) (greater-than-or-equal-to 3 kOe), the rotational behavior of M(p) is the same for both field-cooling directions. One part of M(p) continues to show a periodic vortex motion in and out of the a-b plane, while another part stays fixed in direction relative to H(e) and thus turns frictionally relative to the rotating sample. The observed properties testify to a pronounced anisotropy of the vortex pinning forces, but many of their detailed features, including those that reveal a vortex cross-flux effect, are not yet clearly understood.