Field-induced anisotropy in high-Tcsuperconductors

Abstract

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 ${\phi }^{\mathrm{*}}$ 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.