**5. Multi-wire core**

One of the main drawbacks of orthogonal fluxgates based on magnetic wires is low sensitivity, mainly due to cross-sectional areas lower than traditional parallel fluxgates or orthogonal fluxgates based on bulk tubular cores.

small nm thick gold layer is applied on the glass coating by means of sputtering. Finally, the

By using such structure the saturation current can be strongly reduced. In (Butta et al.,

As already mentioned, the lack of an excitation coil is one of the main advantages of orthogonal fluxgates, because it strongly simplifies its structure, making high

The first attempt made in order to reduce the dimensions of an orthogonal fluxgate was carried out in (Zorlu et al., 2005) where a sensor is based on a wire composed of Au core (20 µm diameter) covered by a 10 µm thick FeNi electroplated layer. The total diameter of the wire is therefore 40 µm, and the length varies from 0.5 to 4 mm. The output voltage is picked-up by means of two planar coils fabricated on a Pyrex substrate by means of

The response of the sensor has a large linear range for excitation current, which can be as low as 50 mA (at 100 kHz), showing that the wire is saturated for such low current. If the current is further increased to 100 mA the linear range reaches ±250 µT, and sensitivity reaches 4.3 V/T. A higher current than the minimum current necessary to saturate the core is also useful against the perming effect. While perming shift after ± 50 µT shock field is 16

Orthogonal fluxgates based on a microwire, however, can hardly be manufactured at lower dimensions. The microfabrication of the sensor becomes more suitable for micro sensors, especially for mass production. In (Zorlu et al., 2006) a microfabricated orthogonal fluxgate is presented wherein the core is manufactured in three steps. First, a Permalloy bottom layer is electroplated on the Cr/Cu seed layer previously applied on the substrate, then the central copper core is electroplated in the middle and finally Permalloy is electroplated on the three open sides of the copper creating a closed loop of Permalloy around the copper. The resulting structure is composed of a rectangular shape core (8 µm x 2 µm) and a copper nucleus surrounded by a 4 µm Permalloy layer (the total dimensions of the structure is 16 µm x 10 µm). The length of the core is 1 mm. The dimension of the core was finely adjusted

Also in this case, the flux is picked-up using two planar coils formed in the substrate under the core (2 x 60 turns). The sensor has a large linear range (±200 µT) but rather low sensitivity, around 0.51 V/T for a 100 mA excitation current at 100 kHz. Thus, the resulting noise is higher than typical orthogonal fluxgates (95 nT/√Hz at 1 Hz). One of the problems of such configuration is that the planar coils cannot properly pick-up the flux as a concentric coil. Clearly, further investigation is necessary to understand whether a different configuration of the coil can significantly increase the sensitivity and then reduce the noise.

One of the main drawbacks of orthogonal fluxgates based on magnetic wires is low sensitivity, mainly due to cross-sectional areas lower than traditional parallel fluxgates or

µT for 50 mA excitation current, it drops down to 2 µT for 100 mA excitation current.

electroplating of magnetic alloy is performed on the gold seed layer.

2009a), the saturation current is reduced by a factor of 3.

**4. Micro orthogonal fluxgate** 

sputtering, photolithography and patterning.

thanks to the high precision of photolithography.

orthogonal fluxgates based on bulk tubular cores.

**5. Multi-wire core** 

miniaturization possible.

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 the wires.

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 of noise.

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 factor of the tuning circuit.

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 magnitude better than sensors based on bulk cores.

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 nT/√Hz noise at 1 Hz.

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 measurements (Ripka et al., 2010).

Orthogonal Fluxgates 31

contains mainly a second harmonic. On the contrary, in the fundamental mode orthogonal fluxgate the dc bias does not allow the magnetization to reverse polarity but only to oscillate with the same frequency f of H. Therefore, the output voltage induced in the pickup coil by time varying MZ (component of M in Z direction) will be sinusoidal at a

At this point, we should point out that this sensor must be, after due consideration, classified as a fluxgate sensor, despite some similarities with other sensors. The magnetic flux within the core is indeed still gated; the sensor works at best, returning a linear and a bipolar response when the excitation field is large enough to deeply saturate the core, as typically found in fluxgates. The only difference between traditional fluxgates without dc bias and fundamental mode orthogonal fluxgates is that the flux is gated only in one

So far we have not discussed the effect of anisotropy on the output voltage. The anisotropy contributes to determine the position of magnetization. For instance, if HZ=0 the resulting MZ is null only if =/2 (i.e. if anisotropy is circumferential). Contrarily, if the anisotropy is non circumferential (i.e. </2, as in Fig. 10) then MZ will be non-zero even for HZ=0 and M will lie between H and Ku (<</2). As a result, the output voltage due to time variation of MZ will be non-zero despite HZ=0. This means that the sensor's response will show an

Fig. 10. Non-circumferential anisotropy in a magnetic wire used as core for fundamental

t

H

Hac

Unfortunately, non-circumferential components of anisotropy are typically found both in amorphous wires and composite Cu/Py wires. The output offset is therefore always expected in fundamental mode orthogonal fluxgates. In order to suppress the offset, a technique is proposed in (Sasada, 2002b). Sasada's method is based on the fact that the sign of the characteristic is reversed if the dc bias becomes negative, while the offset is unchanged. For HZ=0 the magnetization M will oscillate around 0' for positive dc bias and around 0'' for negative dc bias (Fig. 11). The projection of M on the Z axis will be identical because 0''=0'+ and Hac makes M rotate in the opposite direction according to the

M

MZ

Z

Ku

fundamental frequency.

**6.2 Offset** 

polarity rather than in both polarities.

mode orthogonal fluxgates.

bias sign.

offset anytime the anisotropy is not circumferential.

H

Hdc
