High sensitive magnetic field sensing system for measurement of bio-magnetic signal at room temperature

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A highly sensitive magnetic field sensor operating at room temperature shows promise for non-destructive, biomagnetic measurement (such as MCG or MEG). High frequency carrier type sensors have potential for high sensitivity, as in SQUID (Superconducting Quantum Interference Devices). In the present study we developed a very sensitive high-frequency carrier-type magnetic field sensor with a sub-pT resolution using a resonating transmission line-type sensor element since high impedance change can be obtained with such a sensor. The CoNbZr film was deposited by RF sputtering, and a low anisotropy field of 0.8 Oe was applied by annealing. The CoNbZr film thickness of 4 micro m was found to be the optimum value by which to obtain the highest impedance change (dZ/dH). Carrier current flows in the Cu meander pattern, not in CoNbZr film, so the sensor is different from conventional GMI senors in that point. A small AC magnetic field was applied to the sensor element and a very small AM signal was detected using a carrier suppressing circuit. The experiments were carried out in a magnetically shielded room. A resolution of 0.7pT per square root Hz was obtained at 501kHz. The detectable magnetic field was limited by the chaotic noise caused by nonlinear magnetic excitation. A resolution of about 2pT per squire root Hz was obtained at 1kHz and a resolution of about 80pT per square root Hz was obtained at 45Hz.