Dependence of the noise of an orthogonal fluxgate on the composition of its amorphous wire-core

Abstract

In this paper we study the effect of magnetostriction of Co-rich amorphous microwire to the noise of the orthogonal fluxgates based on such wires. The magnetostriction was modified by changing the relative amount of iron x with the respect of the total amount of cobalt and iron in the alloy. Specifically we changed x in the composition (Co1−xFex)75Si15B10 casting wires with the following values of x: 0.05, 0.055, 0.06, 0.062, 0.065, 0.07 and 0.08. We found out that the noise indeed depends on the composition of the wire: while it is minimum (2.5 pT/Hz) for x between 0.06 and 0.062 where the magnetostriction is vanishing (λs ≈ 10−7) it significantly increases to tens of pT/Hz for both positive and negative magnetostriction when λs becomes one order of magnitude bigger. We verified that once the composition returns a magnetostriction low enough (around x = 0.06) then the noise does not depend on mechanical stress anymore. In fact the noise of a sensor with x = 0.06 is the identical in its natural curved state and when bended straight; for vanishing magnetostriction the bending does not affect at all the noise, as on the contrary it happens with larger magnetostriction. This suggests that once the wire has composition with x = 0.06 the remaining noise is not caused by mechanical stress. Finally we show how to overcome the problem of offset arising after annealing if continuous annealing current is used. We explain how this could be due by the fact that magnetostriction changes with temperature and even a wire with vanishing magnetostriction at room temperature can became significantly magnetostrictive at annealing temperature. For this reason we propose a method of annealing consisting rising the temperature of the wire while the wire is kept in its natural curved state. In this way no mechanical stress is applied to the wire during the annealing process, when the temperature is high enough to make it magnetostrictive even if it is non-magnetostrictive at room temperature. We show how this method suppresses the offset and significantly reduces the noise.

Fabrication Advancements in Integrated Fluxgate Sensors: A Mini Review

“…The orthogonal fluxgate sensor fundamental mode of operation was explained in detail. Reproduced with permission [73][74][75][76][77][78][79] Copyright 2018, Elsevier.…”

An Improved Composition of CoFeSiB Alloy for Orthogonal Fluxgates

“…In a previous publication [ 26 ], we showed how the noise of an orthogonal fluxgate changes when we change the composition of the as-cast wire, modifying the quantity of cobalt and iron. We explained that the noise was minimum when the correct amount of cobalt and iron (94% of cobalt and 6% of iron of the total amount of cobalt plus iron) returned the minimum magnetostriction.…”

“…That could be achieved by lowering the content of iron in the alloy, with respect to the total amount of iron plus cobalt. The resulting as-cast magnetostriction is expected to be significantly negative [ 26 ], but as we perform annealing, the magnetostriction should gradually increase and eventually approach zero. This means that for long-time annealing we could combine both advantages: a larger circular anisotropy which reduces the Barkhausen noise on one hand and a vanishing magnetostriction on the other.…”

Recent Progress of Fluxgate Magnetic Sensors: Basic Research and Application

“…Butta et al investigated the relationship between the noise of an orthogonal fluxgate and the composition of the wire-core [ 20 ]. The noise of a fundamental mode orthogonal fluxgate is usually ascribed to Barkhausen noise [ 21 , 22 ].…”

“…Researchers have applied various methods such as optimizing the geometry to reduce the demagnetization and using multiple wires and the Barkhausen noise has thus been reduced to a very low level [ 23 ], but the total noise is still in a high level and the origin of the remaining noise is unclear. In [ 20 ], Butta et al proposed that the remaining noise may come from the magnetostriction of the wire. According to the definition of magnetostriction, on one hand, when an external magnetic field is applied on magnetic materials, there will be a deformation on the direction of the magnetic field.…”