During systematic low-field magnetization measurements on high-quality single crystals of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ we discovered several crystals with a paramagnetic contribution to the field-cooled magnetization. This magnetization saturates at very low fields. It is attributed to spontaneous current loops caused by π junctions which are aligned in small external fields (Wohlleben effect). The effect is only seen with the field parallel to the c axis, demonstrating that the spontaneous currents are confined to the ${\mathrm{CuO}}_2$ planes.

• Received 7 March 1994

DOI:https://doi.org/10.1103/PhysRevB.49.13283