A low-noise ferrite magnetic shield

摘要

Ferrite materials provide magnetic shielding performance similar to commonly used high permeability metals but have lower intrinsic magnetic noise generated by thermal Johnson currents due to their high electrical resistivity. Measurements of magnetic fields inside a ferrite shield with a spin-exchange relaxation-free (SERF) atomic magnetometer reveal a noise level of 0.75 fT Hz^(-1/2), 25 times lower than would be expected in a comparable mu-metal shield. We also identify a 1/f component of magnetic noise due to magnetization fluctuations and derive general relationships for the Johnson current noise and magnetization noise in ferromagnetic shields in terms of their conductivity and complex magnetic permeability.