Fluxgate sensors with cores made of amorphous microwire have low sensitivity due to the small cross-sectional area of the wire. Previous studies with multiwire cores have shown nonlinear increase of sensitivity with number of wires, which was explained by interaction between the wires. In this study, we show that the anomalous increase of sensitivity cannot be explained neither by exchange coupling nor by magnetostatic interaction, but by change in parametric amplification caused by change of the quality factor of the resonant circuit. Interactions between the wires may cause hysteresis effects; therefore, we recommend to keep the wires in distance. Optimization of the pick-up coil geometry may significantly increase the sensitivity and, thus, reduce the magnetic field noise. Using four wires in parallel, we reached a noise level of 120 pT/¿Hz@1 Hz for a 10-mm-long sensor excited by a 20-mA current.
