Improved Measurement of the Low-Frequency Complex Permeability of Ferrite Annulus for Low-Noise Magnetic Shielding

The ferrite magnetic shield can provide a low-noise magnetic environment for various ultrahigh-sensitivity measurements. The low-frequency complex permeability, which determines the magnetic hysteresis noise and the shielding factor, is a key parameter of the ferrite shield and thus should be measured accurately. In this paper, the measurement error of conventional coil method is analyzed at low frequency, and an improved method is proposed which could eliminate the measurement error introduced by the wire resistance. Using this method, four ferrite samples made of PC40 and PC47 materials are measured. The measurement accuracy of the low-frequency complex permeability has improved by two orders of magnitude. On the basis of the measurement results, magnetic hysteresis noise and the shielding factor of ferrite shields are calculated. The calculated results of the noise analysis indicate that the PC40 material has lower magnetic hysteresis noise. This study can help select the ferrite materials and estimate the performance for low-noise magnetic shield design.