We observed the appearance of a paramagnetic moment in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals after cooling in a strong field (3–7 T). It is found that the effect depends on the cooling rate and sample size, indicating that the paramagnetic moment can be induced by compression of the magnetic flux in the course of rapid cooling. After rapid cooling, the temperature dependence of the magnetization during the field-cooled warming process exhibits a very articulated negative dip between irreversibility temperature ${\mathrm{T}}_{\mathrm{irr}}$ and ${\mathrm{T}}_{\mathrm{c}}$. The diamagnetic dip may result from escaping the compressed flux. Comparing the dip widths for a number of crystals with different irreversibility lines ${\mathrm{H}}_{\mathrm{irr}}$(T) we found that a remarkable ``fishtail'' effect appears in the temperature region below the dip. This suggests that the field-induced pinning observed by the fishtail effect is linked with the paramagnetic effect that appears after rapid cooling in the high field.

DOI:https://doi.org/10.1103/PhysRevB.55.8557