No abstract

Due to their ability to trap large magnetic inductions, superconducting bulk materials can be used as powerful permanent magnets. The permanent magnetization of such materials, however, can be significantly affected by the application of several cycles of a transverse variable magnetic field. In this work, we study, at T = 77 K, the long term influence of transverse ac magnetic fields of small amplitudes (i.e. much smaller than the full penetration field) on the axial magnetization of a bulk single grain superconducting GdBCO pellet over a wide range of low frequencies (1 mHz -20 Hz). Thermocouples are placed against the pellet surface to probe possible self-heating of the material during the experiments. A high sensitivity cryogenic Hall probe is placed close to the surface to record the local magnetic induction normal to the surface. The results show first that, for a given number of applied triangular transverse cycles, higher values of dBapp/dt induce smaller magnetization decays. An important feature of practical interest is that, after a very large number of cycles which cause the loss of a substantial amount of magnetization (depending on the amplitude and the frequency of the field), the rate of the magnetization decay goes back to its initial value, corresponding to the relaxation of the superconducting currents due to flux creep only. In the amplitude and frequency range investigated, the thermocouples measurements and a 2D magneto-thermal modelling show no evidence of sufficient self-heating to affect the magnetization so that the effect of the transverse magnetic field cycles on the trapped magnetic moment is only attributed to a redistribution of superconducting currents in the volume of the sample and not to a thermal effect.

We study the AC losses in an infinitely long cylinder made of a superconducting core surrounded by a non-magnetic metallic sheath and subjected to an axial magnetic field. The losses are computed by assuming the Bean-Kim model for the superconductor and Ohmic dissipation for the metal. The time varying magnetic flux crossing the superconductor induces eddy currents in the metal sheath and, due to the nonlinear response of the superconducting material, generates harmonics in the metal current density. In turn, these currents generate distorted magnetic fields acting back on the superconductor. This coupling mechanism is sensitive to the magnetic constitutive law of the superconductor and affects both the waveform of the fields and the total losses. In this paper, we study the importance of the harmonics in the metal on the total losses, as well as their sensitivity to a field dependent critical current density following Kim's law.