Field annealed closed-path fluxgate sensors made of metallic-glass ribbons

The possibilities to improve the performance of certain fluxgate sensor types are still not exhausted. Two types of closed-path sensors – ring core and racetrack – were checked to reveal risky combination of material and construction parameters, and means to optimize the combination were tested. One-step field annealing of Co-based metallic glass was chosen to acquire low-magnetostrictive material with anisotropy required to reduce noise by favoring magnetization rotation. Locally misaligned anisotropy promoted incoherent rotation and handicapped racetrack noise performance. Despite the risk of bending stress aggravated by small – 12 mm diameter, the ring cores fared better and reproducibly achieved noise 7 pT/√Hz @ 1 Hz when thoroughly fixed an annealed in an optimal sheath. As far as we know these are the lowest noise values achieved for this size of fluxgate sensors.

► Field annealing is a viable option for inducing transversal anisotropy to reduce sensor noise.
► Finished core-sheath assembly can be annealed lowering manufacturing stress as well as costs.
► Noise appears to be strongly correlated to characteristic hysteresis loop shape.
► Field annealed racetrack to ring core noise comparison shows clear preferences for ring cores.
► Achieved noise as low as 5 pT/√Hz @ 1 Hz is probably the lowest figure for 10 mm small fluxgate sensors.