THE LIMIT OF FLUXGATE SENSITIVITY DUE TO BARKHAUSEN NOISE FOR SINGLE-LAYER AND BILAYER PERMALLOY THIN-FILM CORES

In its simplest form, the thin film fluxgate sensor measures the appli ed magnetic field by determining the difference in switching field as the cores change their direction of magnetisation. The ultimate sensit ivity of such a configuration is determined by the intrinsic variation , or jitter, in the switching field value from cycle to cycle due to B arkhausen noise in the cores. Comparisons of the noise performance of different cores may be made by examining the associated jitter noise p ower 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 t hat of the bi-layer improves. For H-b > 0.5 H-k, the bi-layer film sho ws the better performance. There is an associated decrease in the heig ht of the switching pulse as the magnetisation vector rotates through a smaller angle when the bias field is applied, but provided the detec tion electronics are able to resolve the pulse above the electronic no ise floor, a suitably designed fluxgate sensor will continue to operat e.