Measurement of trapped flux distribution in a high-T_c superconductor using a flux-gate magnetometer

The distribution of the trapped flux was measured using a miniaturized flux-gate magnetometer with resolution as high as 0.46 mG at 77 K for a BiSrCaCuO high-T_c superconductor cooled down to 77 K in the presence of ac magnetic field (0.5-50 Hz) of 0.6 G in peak-to-peak and in-plane thermal gradient. The amplitude of the trapped flux decreased with increasing the frequency of ac magnetic field. The amplitude was as small as 2 mG for a 50 Hz ac field, and 43 mG for a 0.5 Hz ac field. The amplitude also became small when the cooling rate was made slow. A symbiotic combination of the high-T _c superconducting magnetic shield and the ferromagnetic one with magnetic shaking is proposed, in which merits of both can be enhanced.