The limit of fluxgate sensitivity due to Barkhausen noise for single layer and bi-layer permalloy thin film cores

In its simplest form, the thin film fluxgate sensor measures the applied magnetic field by determining the difference in switching field as the cores change their direction of magnetisation. The ultimate sensitivity of such a configuration is determined by the intrinsic variation, or jitter, in the switching field value from cycle to cycle due to Barkhausen noise in the cores. Comparisons of the noise performance of different cores may be made by examining the associated jitter noise power spectra. The results show that under ambient conditions a single layer permalloy film of thickness 100 nm is less noisy than a bi-layer film of the same magnetic thickness. If a hard axis bias field, H_b, is applied, however, then as the bias field approaches the anisotropy field, H_k, the performance of the single layer material worsens and that of the bi-layer improves. For H_b>0.5 H_k, the bi-layer film shows the better performance. There is an associated decrease in the height of the switching pulse as the magnetisation vector rotates through a smaller angle when the bias field is applied, but provided the detection electronics are able to resolve the pulse above the electronic noise floor, a suitably designed fluxgate sensor will continue to operate