Nyquist magnetic noise analysis and experiments of low-frequency shielding soft magnetic materials

Sensitive magnetic measurements such as biomagnetism need to be carried out in a weak magnetic environment, which is generally achieved using magnetic shielding materials. The low-frequency (1-100 Hz) Nyquist magnetic noise of shielding materials is close to the frequency band of biomagnetic signals and is difficult to remove by filtering, becoming an important factor restricting the measurement accuracy. It is of great significance to study the low-frequency Nyquist magnetic noise of shielding materials. The current research on the magnetic noise of low-frequency shielding soft magnetic materials cannot accurately separate the noise components of different mechanisms, resulting in low calculation accuracy of the magnetic noise of the shielding body. In this paper, we established a material equivalent noise factor through the fluctuation dissipation theorem and proposed a material noise analysis method considering the external magnetic field and frequency dependence. The experimental and calculation results verified the effectiveness of this method. We analyzed and separated the noise of four typical low-frequency shielding materials, revealing the Nyquist magnetic noise components and characteristics of the four materials. The research results have guiding significance for noise processing in the field of biomagnetism.