**7. Coil-less fluxgates**

As previously mentioned, orthogonal fluxgates based on microwires gained popularity due to the absence of the excitation coil, which help to simplify the manufacturing process. To this extent, the wire-core needs only a pick-up coil, which can be easily wound around it with an automatic procedure. However, the presence of a coil, even if it is simply a pick-up coil, can make the sensor unsuitable for applications where high miniaturization is required. A possible solution to this problem is to use planar coils manufactured on a substrate under the fluxgate core as in (Zorlu et al., 2006) although this solution has a more complicated structure, which needs an extra step in the manufacturing process. It would be better to have a fluxgate without any pick-up coil at all. This can be achieved with coil-less fluxgates (Butta et al., 2008a).

### **7.1 Structure of the sensor**

In a coil-less fluxgate, torsion is applied to a composite microwire with a copper core covered by a ferromagnetic layer, while an ac excitation current flows through the wire (Fig.

Orthogonal Fluxgates 35

helical anisotropy is induced in the wire electroplating the Permalloy under torsion and releasing it at the end of the manufacturing process. The back-stress after such release is

In (Butta & Ripka, 2009b) a model for the working mechanism of the coil-less fluxgate is proposed, based on the effect of helical anisotropy on the magnetization of the magnetic wire, during the saturation process determined by the excitation current.Fig. 13 shows the circumferential BH loop (Ripka et al., 2008) of the magnetic wire with +80 µT, - 80 µT, and 0

Fig. 13. Circumferential BH of a magnetic wire with applied torsion for 0 µT and ±80 µT

The circumferential flux is obtained by the integration of the inductive part of the voltage across the wire's terminations Vwire. In turn, the inductive component of Vwire is obtained subtracting the resistive part of the voltage calculated as RwireIwire. The voltage measured on the terminations Vwire will then be the derivative of the circumferential flux; when the magnetization is reversed from positive to negative saturation and vice versa, the voltage

Let us consider a microwire with helical anisotropy as shown in Fig. 14, where is the angle axis of easy magnetization in regards to the axial direction of the wire Z. As observed in cases of traditional fluxgates, the magnetization is rotated by the excitation field H, which periodically saturates the wire in the opposite direction. However, the mechanism is now rotated by an angle . Therefore, the field responsible for the rotation of M is now the component of H perpendicular to the easy axis of magnetization, namely H. The dc axial field also has a component on the perpendicular axis, HZ, which acts as a dc offset to the ac H. This implies that the periodical process of saturation caused by the excitation field is shifted by the axial field through its component HZ. If we observe the circumferential BH loop using H as a reference, then we observe a shift of the loop under the effect of the axial

field applied in the axial direction. The loop is shifted by the external field.

peaks appear in Vwire in addition to the resistive voltage drop.

responsible for helical anisotropy.

µT of the external field applied to the axial direction.

12). If the excitation current is large enough to saturate the magnetic layer in both polarities and a magnetic field is applied in the axial direction, then even harmonics will arise in the voltage across the terminations of the wire. It was found that a second harmonic is proportional to the magnetic field applied in the axial direction; therefore this structure can be used as a magnetic sensor. Since the output voltage is obtained directly at the terminations of the wire no pick-up coil is required.

Fig. 12. Coil-less orthogonal fluxgate. The magnetic wire is twisted and the output is obtained at the wire's terminations.

It should be noted that this sensor must be classified, after due consideration, as an orthogonal fluxgate, even if the structure could recall that of magneto impedance (MI) sensors. Indeed, the sensor returns an output signal with linear characteristic only if full saturation of the wires is achieved in both polarities; if saturation is lost the signal vanishes. Moreover, the operative frequency for a coil-less fluxgate is around 10 kHz, whereas MI sensors are operated at MHz range. This means that the physical phenomena occurring within the wire are substantially different. In other words, MI sensors are mainly based on variation of skin effect in the magnetic wire due to a change of permeability caused by the external field (Knobel et al., 2003) whereas in coil-less fluxgates the external field causes linear shifting of a circumferential BH loop, giving rise to even harmonics. The difference between sensors becomes evident when considering their output characteristics. Coil-less fluxgates, have a second harmonic, which linearly depends on the external field with antisymmetrical characteristic. This allows one to discriminate between positive and negative fields. MI sensors have, on the other hand, impedance, which shows a non-linear symmetric characteristic. In order to be used in a magnetometer, MI sensors must be biased with a dc field (Malatek et al., 2005), so that the working point will move in the descendent branch of the characteristic (the output, however, will only be approximate to a linear function).

### **7.2 Working mechanism**

In (Butta et al., 2008a) it is shown how the sensitivity of a coil-less fluxgate depends on the twisting angle applied to the magnetic wire and how the sensitivity becomes negative if the wire was twisted in the opposite direction. No output signal was instead recorded for no twisting applied to the wire. Therefore, it was assumed that the working mechanism of the coil-less fluxgate took place due to helical anisotropy induced into the magnetic wire by mechanical twisting. This was later confirmed by observing coil-less fluxgate effect also on magnetic wires manufactured with built-in helical anisotropy. In (Butta et al., 2010b) a Permalloy layer is electroplated under the effect of a helical field, obtained as a combination of a longitudinal field imposed with a Helmholtz coil and a circular field generated by a dc current flowing in the wire. In (Atalay et al, 2011; Butta et al., 2010c; Kraus et. al, 2010)

12). If the excitation current is large enough to saturate the magnetic layer in both polarities and a magnetic field is applied in the axial direction, then even harmonics will arise in the voltage across the terminations of the wire. It was found that a second harmonic is proportional to the magnetic field applied in the axial direction; therefore this structure can be used as a magnetic sensor. Since the output voltage is obtained directly at the

Fig. 12. Coil-less orthogonal fluxgate. The magnetic wire is twisted and the output is

It should be noted that this sensor must be classified, after due consideration, as an orthogonal fluxgate, even if the structure could recall that of magneto impedance (MI) sensors. Indeed, the sensor returns an output signal with linear characteristic only if full saturation of the wires is achieved in both polarities; if saturation is lost the signal vanishes. Moreover, the operative frequency for a coil-less fluxgate is around 10 kHz, whereas MI sensors are operated at MHz range. This means that the physical phenomena occurring within the wire are substantially different. In other words, MI sensors are mainly based on variation of skin effect in the magnetic wire due to a change of permeability caused by the external field (Knobel et al., 2003) whereas in coil-less fluxgates the external field causes linear shifting of a circumferential BH loop, giving rise to even harmonics. The difference between sensors becomes evident when considering their output characteristics. Coil-less fluxgates, have a second harmonic, which linearly depends on the external field with antisymmetrical characteristic. This allows one to discriminate between positive and negative fields. MI sensors have, on the other hand, impedance, which shows a non-linear symmetric characteristic. In order to be used in a magnetometer, MI sensors must be biased with a dc field (Malatek et al., 2005), so that the working point will move in the descendent branch of the characteristic (the output, however, will only be approximate to a linear function).

Vwire

In (Butta et al., 2008a) it is shown how the sensitivity of a coil-less fluxgate depends on the twisting angle applied to the magnetic wire and how the sensitivity becomes negative if the wire was twisted in the opposite direction. No output signal was instead recorded for no twisting applied to the wire. Therefore, it was assumed that the working mechanism of the coil-less fluxgate took place due to helical anisotropy induced into the magnetic wire by mechanical twisting. This was later confirmed by observing coil-less fluxgate effect also on magnetic wires manufactured with built-in helical anisotropy. In (Butta et al., 2010b) a Permalloy layer is electroplated under the effect of a helical field, obtained as a combination of a longitudinal field imposed with a Helmholtz coil and a circular field generated by a dc current flowing in the wire. In (Atalay et al, 2011; Butta et al., 2010c; Kraus et. al, 2010)

terminations of the wire no pick-up coil is required.

Iwire

obtained at the wire's terminations.

**7.2 Working mechanism** 

helical anisotropy is induced in the wire electroplating the Permalloy under torsion and releasing it at the end of the manufacturing process. The back-stress after such release is responsible for helical anisotropy.

In (Butta & Ripka, 2009b) a model for the working mechanism of the coil-less fluxgate is proposed, based on the effect of helical anisotropy on the magnetization of the magnetic wire, during the saturation process determined by the excitation current.Fig. 13 shows the circumferential BH loop (Ripka et al., 2008) of the magnetic wire with +80 µT, - 80 µT, and 0 µT of the external field applied to the axial direction.

Fig. 13. Circumferential BH of a magnetic wire with applied torsion for 0 µT and ±80 µT field applied in the axial direction. The loop is shifted by the external field.

The circumferential flux is obtained by the integration of the inductive part of the voltage across the wire's terminations Vwire. In turn, the inductive component of Vwire is obtained subtracting the resistive part of the voltage calculated as RwireIwire. The voltage measured on the terminations Vwire will then be the derivative of the circumferential flux; when the magnetization is reversed from positive to negative saturation and vice versa, the voltage peaks appear in Vwire in addition to the resistive voltage drop.

Let us consider a microwire with helical anisotropy as shown in Fig. 14, where is the angle axis of easy magnetization in regards to the axial direction of the wire Z. As observed in cases of traditional fluxgates, the magnetization is rotated by the excitation field H, which periodically saturates the wire in the opposite direction. However, the mechanism is now rotated by an angle . Therefore, the field responsible for the rotation of M is now the component of H perpendicular to the easy axis of magnetization, namely H. The dc axial field also has a component on the perpendicular axis, HZ, which acts as a dc offset to the ac H. This implies that the periodical process of saturation caused by the excitation field is shifted by the axial field through its component HZ. If we observe the circumferential BH loop using H as a reference, then we observe a shift of the loop under the effect of the axial

Orthogonal Fluxgates 37

a coil-less fluxgate based on a composite copper wire with Co19Ni49.6Fe31.4 electroplated shell is proposed. The sensitivity in this case is about 120 V/T at 20 kHz. Further research on different materials will show if even higher sensitivity will be achievable with other alloys.

Fig. 15. Output characteristic of coil-less fluxgates for different amplitudes of excitation current Iwire. The higher Iwire becomes, the lower the sensitivity of the sensor will be.

excitation current.

**7.4 Linearity** 

Sensitivity can be also increased with higher angles of helical anisotropy but we should keep in mind that saturation current also increases, and this will require a higher

A drawback of coil-less fluxgates is that low sensitivity cannot be increased by using high gain amplification because the output voltage of the sensor includes large spurious voltage. This component of the voltage does not include a signal but contributes to enlarge its peak value, limiting the maximum amplification. The resistive part of the spurious voltage, due to the voltage drop on the wire's resistance can be easily removed by a classical resistive bridge. However, the inductive component of the voltage, given by the transition of the magnetization from one saturated state to the opposite, will be always present in the output. As previously explained, these peaks will be shifted by the external field to opposite directions, but they will continue to be present in the output. A technique proposed by (Butta et al., 2010a) is presented to remove the inductive peaks and obtain an output voltage that is null for no applied field and whose amplitude increases proportionally to it. The method is based on a double bridge with two sensing elements fed by current in opposite directions. In the output voltage, the positive peaks of the first wire will be compensated by the negative peaks of the second wire and vice versa. The sensitivity of the two wires must clearly point to opposite directions so that the sum of the voltage obtained with the opposite

A common technique used to improve linearity of magnetic sensors is to operate them in a closed loop mode, by generating a compensation field, which nullifies the measured field (Ripka, 2001). The pick-up coil is usually used for this purpose, because the compensating field must be generated at a low frequency, several orders of magnitude lower than the excitation frequency. Using the feedback mode, the working point of the sensor will always

current will be the sum of the two signals rather than their difference.

field as shown in Fig. 13. The sensitivity of the sensor increases together with the increasing anisotropy angle because the higher is the larger is HZ

Fig. 14. Working mechanism of coil-less fluxgates.
