Development of high Tc SQUID magnetometer system for biological application

In this research，we developed high Tc SQUID magnetometer system for biological application。First，we developed high performance SQUID utilizing bicrystal junctions with30-degree misorientation angles。With30-degree bicrystal junction，we can routinely obtain the voltage modulation depth V&gt；20µV and the flux noise S I D2ΦIII D11/2 IED1&lt；12µΦD2o IIE D2/Hz I D11/2ID1at T=77K.Transport and noise properties of the bicrystal junctions are also clarified.Next，the so-called direct-coupled magnetometer is fabricated.When we use a conventional pickup coil with wide line-width，we observed high 1/f noise。It is shown that the 1/f noise comes from the hopping of the flux trapped in the pickup coil.In order to avoid the flux trapping，we develop a new type of the pickup coil。The pickup coil consists of 4 parallel loop with narrow line-width and involves the so-called flux dam.With this pickup coil，we can significantly reduce the 1/f noise。The flux noise is reduced to S I D2ΦD 2 I D 11/2 I D 1=23µΦI D 2 o I D 2/Hz D 11/2 I D 1 at f=1 Hz.This value is only 2.3 times higher than the white noise of S I D2ΦD 2 I D 11/2 I D 1=10µΦI D 1 o I D 1/Hz I D 11/2 I D 1.In the present magnetometer，the size of the pickup coil is 6毫米by 3 mm，and the effective area of the magnetometer is A I D 2 eff D 2=0.12毫米I D 12 I D 1.This means that the magnetic noise of the magnetometer is S I D 2 B I D 2 I D 11/2 I D 1=S I D2ΦD 2 I D 11/2 I D 1/A I D 2 eff D 2=460fT/Hz I D 11/2 I D 1 at f=1 Hz，and S I D 2 B I D 2 I D 11/2 I D 1=175fT/Hz I D 11/2 I D 1 in the white noise region。If we use a large pickup coil with 10 mm by 10 mm size，the effective area will be A I D 2 eff D 2=0.4 mm I D 12 I D 1.In this case，we can expect S I D 2 B I D 2 I D 11/2 I D 1=138 fT/Hz I D 11/2 I D 1 at f=1 Hz，and S I D 2 B I D 2 I D 11/2 I D 1=52 fT/Hz I D 11/2 I D 1 in the white noise region。This sensitivity will be enough for the application to the magnetocardiogram.

In this research, we developed high Tc SQUID magnetometer system for biological application. First, we developed high performance SQUID utilizing bicrystal junctions with 30-degree misorientation angles. with 30-degree bicrystal junction, we can routinely obtain the voltage modulation depth △V>20μV and the flux noise SィイD2ΦィエD2ィイD11/2ィエD1<12μΦィイD2oィエD2/HzィイD11/2ィエD1 at T=77 K. Transport and noise properties of the bicrystal junctions are also clarified.Next, the so-called direct-coupled magnetometer is fabricated. When we use a conventional pickup coil with wide line-width, we observed high 1/f noise. It is shown that the 1/f noise comes from the hopping of the flux trapped in the pickup coil. In order to avoid the flux trapping, we develop a new type of the pickup coil. The pickup coil consists of 4 parallel loop with narrow line-width and involves the so-called flux dam. With this pickup coil, we can significantly reduce the 1/f noise. The flux noise is reduced to SィイD2ΦィエD2ィイD11/2ィエD1=23 μΦィイD2oィエD2/HzィイD11/2ィエD1 at f=1 Hz. This value is only 2.3 times higher than the white noise of SィイD2ΦィエD2ィイD11/2ィエD1=10μΦィイD1oィエD1/HzィイD11/2ィエD1.In the present magnetometer, the size of the pickup coil is 6 mm by 3 mm, and the effective area of the magnetometer is AィイD2effィエD2=0.12 mmィイD12ィエD1. This means that the magnetic noise of the magnetometer is SィイD2BィエD2ィイD11/2ィエD1=SィイD2ΦィエD2ィイD11/2ィエD1/AィイD2effィエD2=460 fT/HzィイD11/2ィエD1 at f=1 Hz, and SィイD2BィエD2ィイD11/2ィエD1=175 fT/HzィイD11/2ィエD1 in the white noise region. If we use a large pickup coil with 10 mm by 10 mm size, the effective area will be AィイD2effィエD2=0.4 mmィイD12ィエD1. In this case, we can expect SィイD2BィエD2ィイD11/2ィエD1=138 fT/HzィイD11/2ィエD1 at f=1 Hz, and SィイD2BィエD2ィイD11/2ィエD1=52 fT/HzィイD11/2ィエD1 in the white noise region. This sensitivity will be enough for the application to the magnetocardiogram.