Magnetization method for long high-T_c bulk superconductors used for magnetic separation

We have developed a new water-treatment system consisting of a membrane separator and a magnetic separator that uses long (>200 mm) high-temperature bulk superconductors (SCs) as permanent magnets. For magnetizing the long bulk SCs, we developed a new magnetization method. Namely, a field is applied, the bulk SCs are cooled below their transition temperature (T_c), and during cooling, the bulk SCs are inserted into and withdrawn from the applied field repeatedly. As a result, the bulk SCs experience high and low fields under a gradually cooled ambient, and the trapped field gradually increases. The benefit of this method is that it can magnetize large bulk SCs by using a small field magnet. We fabricated an embedded-SC bulk magnet more than 200-mm long, and magnetized it by split-solenoid superconducting magnets with a diameter of approximately 100 mm. Field measurements showed that the developed method can magnetize a wider area over the bulk SCs than that possible with a conventional field-cooling method.