**3.4 Glass insulation**

A more complex structure has been proposed by (Butta et al., 2009a) to overcome the problem of the current draining to the magnetic shell due to the skin effect. This is carried out by putting a glass layer between the copper core and the magnetic shell. The glass layer provides electrical insulation, helping thus to keep the excitation current flowing entirely within the copper core, regardless of the operating frequency. Even if the skin effect should occur in the copper core, given Ampere's law, this does not affect the magnetic field generated from the copper's diameter.

In order to manufacture a composite structure with glass insulation between the copper and the magnetic shell, glass coated copper wires are used as a base. Following this procedure, a

Orthogonal Fluxgates 29

In order to increase the sensitivity, multi-core sensors have been proposed wherein the core is composed of multiple magnetic wires closely packed, each of them excited by a current with equal amplitude and frequency. The wires are also not electrically in contact along their length. In case of amorphous wires, a thin glass coating (typically 2 µm) provides insulation between them. For composite Cu/Py wires a small nm layer of epoxy is added to

In (Li et al., 2006a) the sensitivity of a multi-wire core fluxgate with tuned output was measured for cores with a different number of wires and it was found to increase exponentially; for instance, a 16 wire core has sensitivity 65 times higher than the sensitivity of a single wire. Later on it was demonstrated (Li et al., 2006b) that such growth of sensitivity was not simply caused by the increase of ferromagnetic material composing the core. Let us consider a sensor having a single wire core and a sensor based on a two-wire core whose total cross-sectional area is comparable to the area of a single wire. In such a case, the sensitivity is higher for the two-wire core despite the cross sectional area being similar to the single sire core. It is shown that the increment of the sensitivity becomes linear if the wires are kept far enough (5 time the diameter). This suggests the cause of the exponential increment of sensitivity for multi-wire cores is the magnetic interaction between

An increment of sensitivity is, however, useless if the noise also increases. Further investigation (Jie et al., 2009) has proven that orthogonal fluxgates with a multi-wire core do not only have higher sensitivity but also lower noise. It is interesting to note that the noise is lowest for configurations where the wires are arranged in the most compact way, because the mutual interaction between the wires is stronger the closer they are. Therefore, multi-wire cores are convenient both in terms of sensitivity and in terms

Later (Ripka et al., 2009) suggested that the exponential increment of the sensitivity to the number of wires is due to the improvement of the quality factor of the tuning circuit. This was then confirmed in (Ripka et al., 2010) where the anomalous increase of sensitivity is explained to be due to changes of parametric amplification caused by changes in the quality

The total cross-sectional area is clearly higher for multi-core fluxgates and, therefore, the spatial resolution is worse than the single wire core. However, we should consider that the sensitivity increases exponentially, meaning that the sensitivity per unit of area is higher in multi-wire cores. In any case, if we consider a 16 wire core, the spatial resolution decreases by a factor of ~4, depending on the geometry of the configuration. This is still one order of

Another advantage of a multi-wire core is the mutual compensation of spurious voltages if wires are connected in an anti-serial configuration. As an example, two-wire core has 0.34

Finally, we must be careful about the interaction that may occur between the wires if closely packed. This might cause hysteresis in the response of the sensor for low field

the surface of the wire to assure insulation.

the wires.

of noise.

factor of the tuning circuit.

nT/√Hz noise at 1 Hz.

measurements (Ripka et al., 2010).

magnitude better than sensors based on bulk cores.

small nm thick gold layer is applied on the glass coating by means of sputtering. Finally, the electroplating of magnetic alloy is performed on the gold seed layer.

By using such structure the saturation current can be strongly reduced. In (Butta et al., 2009a), the saturation current is reduced by a factor of 3.
