An Improved Composition of CoFeSiB Alloy for Orthogonal Fluxgates.

Mattia Butta,Jakub Pařez,Michal Janošek,Horia Chiriac,Alexander Valeriano Inchausti

doi:10.3390/s22062162

Abstract

Orthogonal fluxgates in fundamental mode based on cores have recorded very low noise in literature, especially if Joule annealing is performed on the core for a short period of time. However, for annealing time longer than 20–30 min, the noise of the sensor has a tendency to increase. In this work, we investigated this phenomenon, and we have found its origin in a monotonic increase of magnetostriction during the annealing process. We show that the wires with vanishing magnetostriction in their as-cast form exhibit positive magnetostriction after long-time annealing (more than 30 min), which increases the noise of the sensor. After researching the effect of the magnetostriction after annealing on the noise, we propose an alloy with a reduced amount of iron. Less iron leads to a larger as-cast negative magnetostriction, which becomes almost zero after long-time annealing (60 min), bringing further reduction of noise. We prove this effect on two wires from two different manufacturers, although with the same composition. The noise decrease with prolonged annealing is mainly observable in the low-frequency region: at 100 mHz, the noise of a single-wire sensor decreased from to when the annealing time was prolonged from 10 to 60 min.

Highlights

Academic Editors: Nicolò MarconatoCobalt-rich amorphous magnetic wires have been produced for decades using the rotating-water quenching method [1]
As the annealing time is increased, the circumferential anisotropy increases and the noise in the 1/ f region decreases. This is due to the fact that the 1/ f noise of an orthogonal fluxgate in fundamental mode is mainly due to the Barkhausen caused by rapid domain wall movements in the core
We showed that the cause for low-frequency noise increase in fundamental mode orthogonal fluxgate when the core is annealed for a long time is the increase of magnetostriction

Summary

Introduction

Cobalt-rich amorphous magnetic wires have been produced for decades using the rotating-water quenching method [1]. As the annealing time is increased, the circumferential anisotropy increases and the noise in the 1/ f region decreases This is due to the fact that the 1/ f noise of an orthogonal fluxgate in fundamental mode is mainly due to the Barkhausen caused by rapid domain wall movements in the core. Another possible reason for the increased noise after annealing the wires for a long time could be the change in sensitivity of the sensor. When the circumferential anisotropy of the wire increased, the sensitivity of the orthogonal fluxgates in fundamental mode decreased This is due to larger circumferential anisotropy; the projection of the magnetization in the axial direction of the wire (i.e., the sensing direction) was lower, leading to a smaller voltage induced in the pick-up coil wound around the wire [25]. This means that the reason for increased noise after long annealing is not the drop of sensitivity and it must originate somewhere else

Influence of Annealing on the Magnetostriction

Modified Composition

Additional Advantage of Long-Time Annealing

Findings

Conclusions