Fe₂O₃ composition optimization for MnZn ferrite and its magnetic noise evaluation for low-frequency magnetic shielding applications

A magnetic noise-free environment is essential for ultra-high sensitivity atomic magnetometry operating in a spin exchange relaxation-free regime. The critical limitation to achieving theoretical sensitivity arises from the magnetic noise of the innermost MnZn ferrite, which includes magnetization noise (related to ferrite hysteresis loss) and Johnson current noise (associated with ferrite resistivity). To address the issue, MnZn ferrites with high resistivity and permeability were designed and fabricated in this work. A series of MnZn ferrites with varying Fe₂O₃ content were synthesized using the solid-phase reaction method. The experimental samples were denoted as Mn_0.678-xZn_0.4Fe_1.922+xO₄ (x = 0–0.208, 0.026/step). All samples exhibited a uniform single spinel structure. As Fe₂O₃ content increased, the grain size grew, and the pores migrated from the intragranular regions to grain boundaries. Saturation magnetization and permeability of samples initially increased followed by a subsequent decrease, whereas coercivity demonstrated an inverse trend. The resistivity decreased from approximately 2.40 × 10⁵Ω·mm to 2.62 × 10²Ω·mm. The ferrite sample with x = 0.078 exhibited optimal electromagnetic performance, with a resistivity of 4.86 × 10² Ω·mm, permeability of 1536, and an A value of 6.058 × 10⁻⁸ (associated magnetization noise, A=tanδ/ωμ′, 100 Hz, 25°C). The optimized composition was employed to fabricate large-sized ferrite ceramic plates measuring 150 mm × 150 mm × 3 mm using the cold isostatic pressing process. A ferrite shielding chamber was subsequently constructed. The application of the ferrite shield reduced the magnetic noise of the experimental device from 73.7 fT/Hz^1/2 to 20 fT/Hz^1/2.