Study on a symbiotic combination of the high-Tc superconducting magnetic shield and the ferromagnetic one with magnetic shaking

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The amplitude of the trapped flux in high-Tc superconductors is strongly affected by the frequency of the magnetic field applied during cooling and the cooling rate. The amplitude is small when the frequency is high or the cooling rate is low.For example,dc magnetic field was trapped in a BiSrCaCuO high-Tc superconducting bulk plate almost as it was;however,ac magnetic field of 10Hz was trapped 1/43 times as small as the amplitude applied to the sample.This nature of the trapped flux allows a new symbiotic combination of ferromagnetic shield with magnetic shaking and high-Tc superconducting magnetic shield.In this combination,the earth's field causing a trapped-flux problem can be attenuated to as small as 0.1mG by the ferromagnetic shield with magnetic shaking whereas any care to suppress the leakage of the shaking field is not needed,making the ferromagnetic shield very thin and lightweight.A miniaturized fluxgate magnetometer(resolution as high as 0.46mG at 77K)is useful to measure the distribution of the trapped flux in high-Tc superconductors,in which,by encapsulating the tip of the core with a piece of copper,the specimen can be avoided to be exposed to high-frequency exciting field of the order of 1G whereas it is transparent to the trapped flux.Amorphous magnetic material(Metglas 2705M)with highly rectangular B-H loop exhibits large enhancement of shielding performance by magnetic shaking which is 20-30 times better than Permally.