As biomagnetic sensing and molecular diagnostics increasingly demand the detection of extremely weak magnetic signals, conventional magnetic sensors remain constrained by intrinsic low-frequency noise, magnetic-domain fluctuations, and limited sensitivity. Researchers from Lanzhou University and the First Hospital of Lanzhou University have developed a low-noise orthogonal fluxgate (OFG) sensor that combines an amorphous–nanocrystalline CoP/Ag composite core with closed-loop signal conditioning. This integrated strategy substantially suppresses magnetic and electronic noise, enabling highly sensitive weak-field detection and biomolecular sensing. 

Why This Magnetic Sensor Matters

Orthogonal fluxgates are attractive for weak magnetic-field measurements because of their high sensitivity and room-temperature operation, but intrinsic 1/f noise, magnetic hysteresis, and Barkhausen fluctuations can obscure extremely weak signals. These limitations become particularly important for biomagnetic and magnetic-bead detection, where the magnetic response from a small number of labeled biomolecules can be extremely weak. The new sensor addresses this challenge by coordinating magnetic-core microstructure with circuit-level noise suppression. 

Innovative Design and Mechanism

The sensor employs a conductive Ag core coated with an amorphous CoP layer, followed by controlled DC annealing to introduce 10–20 nm nanocrystalline domains within the amorphous matrix. This dual-phase architecture combines the low magnetic-loss characteristics of an amorphous structure with improved magnetic ordering and permeability from nanocrystallization. Optimization of the electrodeposition current density and annealing conditions produces a uniform magnetic-domain structure and reduced dynamic hysteresis loss. Meanwhile, a closed-loop feedback circuit integrates phase-stable excitation, synchronous demodulation, and zero-drift compensation to further suppress electronic and low-frequency noise.  

Outstanding Performance

The optimized CoP/Ag OFG achieves an exceptionally low system-level equivalent magnetic noise of only 8 pT/√Hz at 1 Hz, together with a sensitivity of 153.8 kV T^-1 and a measured −3 dB bandwidth of 46 Hz. Compared with non-optimized cores, the optimized structure dramatically reduces low-frequency noise through improved permeability, more regular magnetic domains, and lower dynamic magnetic loss. The sensor can clearly resolve signals from superparamagnetic magnetic beads and provides a wide quantitative response for biomolecular detection.  

Applications and Future Outlook

Using alpha-fetoprotein (AFP) as a model biomarker in an immunomagnetic bead assay, the sensor delivers a linear response over an exceptionally broad concentration range of 50 fg mL^-1–100 ng mL^-1, with a detection limit as low as 50 fg mL^-1. The work establishes a clear structure–magnetism–noise relationship and demonstrates that low-noise OFGs can translate advanced magnetic-core engineering into practical biomolecular sensing. Beyond AFP detection, the platform shows promise for biomagnetic measurements, geomagnetic sensing, weak-field detection, and future clinical biosensing, with serum-based validation and anti-interference optimization identified as important next steps.