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

高温超导体中俘获磁通的幅度受冷却时的磁场频率和冷却速度的影响很大。当频率较高或冷却速率较低时，其幅值较小。例如，直流磁场捕获在BiSrCaCuO高T_c超导块材中，几乎是原样；然而，10赫兹的交流磁场被捕获的幅度是施加到样品上的幅度的1/43。捕获的磁通的这一性质允许铁磁屏蔽与高T_c超导磁屏蔽的新的共生组合。在这种组合中，引起捕获磁通问题的地磁场可以通过磁振动的铁磁屏蔽衰减到小到0.1毫克，而不需要注意抑制抖动磁场的泄漏，小型化磁通门磁力仪(77K时分辨率高达0.46 mG)可用于测量高温超导体中俘获磁通的分布，通过在磁芯尖端包裹一块铜，样品可以避免暴露在1G量级的高频激励磁场中，而对俘获磁通是透明的。具有高度矩形B-H环的非晶态磁性材料(Metglas 2705M)通过磁振荡可以大大提高屏蔽性能，是Permally的20-30倍。

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.