Experimental study of GdBCO pellets subjected to small transverse alternating magnetic fields at very low frequency: Evidence of saturation of the axial magnetization decay

Due to their ability to trap large magnetic inductions, bulk (RE)BCO materials (where RE stands for rare-earth ion) can be used as powerful permanent magnets. Previous works have shown that the magnetization of a bulk superconductor can be destroyed by the presence of an ac magnetic field applied transversally to the remnant magnetization. Up to now, the exact process of this magnetization decay is not completely understood, i.e. it is not clear if the decay is only due to a redistribution of the superconducting currents in the volume of the pellet or if a part of this decay can be due to self-heating of the material subjected to the ac excitation. In this work, we carry out, in liquid nitrogen, measurements for which the ac field is applied during several hours. We investigate the range of relatively small ac field amplitudes (in comparison to the penetration field of the pellet) and in a wide range of low frequencies (1mHz–20Hz). The waveform of the parasitic ac field is either sinusoidal or triangular, the latter corresponding to a constant variation of the applied induction, dBapp/dt. The studied sample is a cylindrical bulk single grain GdBCO pellet whose dimensions are 10mm in height and 15mm in diameter. Thermocouples are placed on 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 measure the local magnetic induction normal to the surface (and parallel to the transverse ac field).
The results show first that, for a given number of applied triangular transverse cycles, lower values of dBapp/dt induce larger magnetization decays.. An important feature of practical interest is that, after a very large number of cycles (>10000) which cause the removal of a given 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. The thermocouples show no evidence of self-heating in that range of amplitude and frequency of the applied field.

Acknowledgements

We thank the FRS-FNRS, the Communauté Française de Belgique for cryofluid, travel and equipment grants, under reference ARC 11/16-03.