Magnetic properties of Y-Ba-Cu-O superconductors

The magnetic properties of polycrystalline and single-crystal samples of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ are compared. Transition temperatures are in the range of 85 to 89 K. The Meissner effect is below 30% of the diamagnetic shielding in all cases. Based on the high-temperature susceptibility it is suggested that an itinerant antiferromagnetic $3d$ band of the ${\mathrm{Cu}}^{2+}$ is possible. Anisotropy occurs in the single crystal for applied fields above 200 Oe. There is a difference in the lower critical field of a factor of 20 where $H_{c1\mathrm{}}$ is about 4000 Oe for $H$ parallel to the orthorhombic $c$ axis and 200 Oe for $H$ perpendicular to the $c$ axis of the crystal.