Magnetization and magnetic suspension of YBa2Cu3Ox-AgO ceramic superconductors

Abstract The magnetic force which accounts for the newly-discovered suspension of a superconductor below a permanent magnet is determined by the magnetization of the superconductor and the magnetic-field gradient. Magnetization measurements were carried out on a series of YBa 2 Cu 3 O x -AgO ceramic superconductors, with T c ≈93 K. The samples were from the set of samples in which the magnetic-suspension phenomenon was first discovered. Magnetization data were taken at 4.2 and 77 K in magnetic fields up to 180 kOe. Hysteresis loops at low fields, up to 1.2 kOe, were also studied at 4.2, 77 and 87 to 88 K. The magnetization and hysteresis in most of the samples are among the largest observed to date in ceramic high- T c superconductors. In most of our samples, the remanent moment at 4.2 K is about 80 emu/g, and about 3 emu/g at 77 K. The large magnetization and hysteresis indicate the presence of strong pinning forces. The strong hysteresis at 77 K results in an appreciable positive magnetization, parallel to the field, when the field H is decreased from a finite value (above≈0.5 kOe). This positive magnetization increases with decreasing H . The positive magnetization can be produced by bringing a permanent magnet close to the superconductor, and then withdrawing it slowly. This leads to an attractive magnetic force between the superconductor and the permanent magnet. Calculations, based on a realistic model, show that at 77 K this magnetic attraction can be sufficiently strong to balance the gravitational force. As a result, the superconductor can be suspended below a permanent magnet. The expected damped oscillatory motion near the suspension point, following the application of a vertical impulse to the superconductor, is discussed. This motion is more complicated than that near the bottom of a conventional potential well. Some remaining problems associated with the magnetic-suspension phenomenon are outlined.