Suppression of magnetic field residual and gradient in ferrite shield based on thermal demagnetization for atomic magnetometer

Mn-Zn ferrites exhibit high permeability and low magnetic noise and are typically used in atomic sensors as the inner layer of a magnetic shielding structure. To provide good magnetic shielding performance, ferrites must to be demagnetized regularly. However, due to the higher coercivity of ferrites, electric demagnetization methods are ineffective, limiting the applications in high-precision instruments. Therefore, a thermal demagnetization method was proposed for ferrites in this study. Double-layer counter-wound non-magnetic heating films with excellent temperature uniformity and the weakest possible magnetic field were designed. The parameters relevant for thermal demagnetization were adjusted to optimize the demagnetization effectiveness: demagnetization holding time, external magnetic fields, and the sequence of electric demagnetization of the outer permalloy layer and the thermal demagnetization of the inner ferrite layer. The results indicate that the thermal demagnetization reduces the residual magnetic field to below 0.2 nT with a gradient of 0.015 nT/cm (radial) and below 0.05 nT with a gradient of 0.013 nT/cm (axial) ± 15 mm from the center. Compared to the traditional electric demagnetization method, the residual magnetic field and its gradient of thermal demagnetization are reduced by more than 5 times and 10 times, respectively. This article strongly advocates for the application of ferrites in magnetic shields for measurement instruments, especially in atomic magnetometers and in cutting-edge physics research.