Ultralow Noise Orthogonal Fluxgates Enabling Weak Magnetic Field and Biomolecular Detection

Noninvasive detection of ultra-weak biomagnetic signals is crucial for modern biosensing, but conventional magnetic sensors are fundamentally limited by intrinsic noise. Orthogonal fluxgates (OFGs) enable high signal-to-noise-ratio detection of weak magnetic fields, yet their performance is still constrained by intrinsic 1/f noise and Barkhausen-related magnetic fluctuations. This study reports a low-noise orthogonal fluxgate in which a newly designed magnetic core and circuit mitigate existing noise limitations. The CoP/Ag composite core, featuring an amorphous-nanocrystalline dual phase, is associated with reduced low-frequency magnetic loss and improved noise performance. When integrated with a closed-loop feedback, the sensor achieves a noise floor of 8 pT/√Hz at 1 Hz. The sensor enables reliable detection of ultralow-concentration magnetic-bead signals. Alpha-fetoprotein (AFP) was used as a model biomarker in an immunomagnetic bead assay, yielding a linear response from 50 fg mL^-1 to 100 ng mL^-1 and a detection limit of 50 fg mL^-1, which compares favorably with representative reported AFP magnetic biosensors. OFGs demonstrate strong prospects in biosensing, geomagnetic measurements, and weak field detection.