Field-induced anisotropy in high-Tc superconductors.

We have studied the angular dependence of the magnetization of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ by rotating the sample relative to the applied magnetic field. At low temperatures and low fields the irreversible part of the field-cooled magnetization ${M}_{\mathrm{irr}}$ rotates with the sample as a rigid entity. The rigidity is broken and part of ${M}_{\mathrm{irr}}$ disappears above an angle ${\ensuremath{\varphi}}^{\mathrm{*}}$ which decreases with increasing either temperature or field. This behavior resembles that of spin glasses and differs qualitatively from the behavior found for Nb, a type-II superconductor.