Composite‐Field Annealing Enables Superior High‐Temperature Noise Stability in Fluxgate Sensors With Co‐Rich Amorphous Microwires

ABSTRACT Fluxgate sensors with Co‐rich amorphous microwires, widely used for their low noise in magnetic field measurements, encounter significant challenges in high‐temperature environments such as well drilling and geological exploration. Prolonged thermal aging degrades the performance of Co‐rich amorphous microwires, resulting in increased noise levels. To address these issues, this study innovatively proposed a composite‐field annealing (CFA)—a sequential treatment involving tensile stress annealing, followed by longitudinal and transverse magnetic field annealing processes—to enhance the thermal aging stability of Co‐rich amorphous microwires. A sensor incorporating CFA‐aged microwires achieved a significantly lower noise level of 18 pT Hz −1/2 at 1 Hz, compared to 130 pT Hz −1/2 in normally annealed (NA) counterparts. CFA microwires exhibited superior long‐term stability in permeability, with a lower rate of 34.2% of permeability variation over time at 175°C for 150 h. The enhanced stability was attributed to the stabilization of magnetic anisotropy for CFA microwires, as confirmed by higher activation energy calculated by local magnetic anisotropy energy during aging and a negligible domain wall movement rate under in situ high‐temperature Lorentz transmission electron microscopy (L‐TEM) experiments. This work introduces CFA as a critical technique for optimizing Co‐rich amorphous microwires, enabling the development of high‐stability and low‐noise fluxgate sensors for harsh environmental applications.