Fe‐Based Nanocrystalline Magnetic Shielding Cylinder with Subfemtotesla‐Level Magnetic Noise

ABSTRACT The precision measurement of extremely weak magnetic fields using spin‐exchange relaxation‐free atomic magnetometers demands magnetic shielding system with high shielding efficiency and low magnetic noise. Conventional magnetic shielding system (CMSS) based on µ ‐metal, however, often exhibits high magnetic noise. Herein, we propose a strategy to overcome this challenge, which is realized by using the Fe‐based nanocrystalline (Fe‐Nano) magnetic shielding cylinder (MSC) as the innermost magnetic shield of CMSS. The Fe‐based nanocrystals (Fe‐Nanos) prepared via the melt‐spinning method followed by a three‐step annealing process exhibit excellent soft magnetic properties with the permeability of 64 000, indicating superior magnetic shielding performance. The average loss factor of the Fe‐Nanos is only 2.4 × 10 −7 , which is significantly lower than that of µ ‐metal and indicates that Fe‐Nanos has lower intrinsic magnetization noise. The Fe‐Nano MSC with different thicknesses significantly suppresses magnetic noise when used as the innermost magnetic shield of CMSS. In particular, the ten‐layer Fe‐Nano MSC achieves an average magnetic noise of 0.84 fT Hz −1/2 in the frequency range of 20–100 Hz. This work provides a lightweight, easily processable, and cost‐effective strategy for the magnetic shielding system with ultra‐low magnetic noise, which will promote its large‐scale application in emerging fields such as biomagnetism and fundamental physics.