Characteristics of YBaCuO magnetic shields

Magnetic shielding properties of YBaCuO tubes have been studied at temperatures of 77 K and 4 K. Shielding effectiveness was determined by measuring the magnetic field inside the tube in the presence of external magnetic fields. A high degree of shielding was achieved up to a critical external field determined by the critical current density of the material. Typically, this critical field is 23 Oe at 77 K extending up to 105 Oe at 4 K. Tubes made with material containing 10% silver exhibited much stronger flux pinning behavior when the external magnetic field started to penetrate the tubes. The tubes studied were effective in shielding a RF-biased SQUID (superconducting quantum interference device) for operation at 77 K.