**3.3 Composite wires**

Composite wires have been proposed to solve problems given by the unsaturated inner section of the wire (Ripka et al., 2005; Jie et al., 2006). The main idea involving composite wires is to have wires with non-magnetic cores surrounded by a soft magnetic shell. In this way, we avoid problems such as the hysteresis of the sensor's characteristic and the perming effect, which typically arise if the wire is not fully saturated.

Considering a core composed of a 20 µm diameter copper wire surrounded by a 2.5 µm thick Permalloy layer, the perming error (i.e. shift of offset after 10 mT shock field) is only 1 µT, for an excitation current as low as 20 mA. Moreover, it is shown that the perming error decreases for a higher excitation current, as typically found for bulk core fluxgates, because the core is more deeply saturated.

The most frequently used technique to produce composite wires consists of the electroplating of a magnetic alloy, for example Ni80Fe20 (Permalloy), on a copper microwire. The resistivity of copper (~17 nΩ·m) is lower than the resistivity of many magnetic alloys (for instance the resistivity of Permalloy is ~200 nΩ·m). For a typical wire composed of a 50 µm diameter core and surrounded by a 5 µm Permalloy shell, only 3.6% of the total current flows through the magnetic shell. If we operate the sensor with an excitation current low enough to make skin effect negligible, we can assume that the whole excitation current will flow through the copper core. Such simplified configuration is shown in Fig. 8 where the current density J is uniform within the copper core and zero in the magnetic shell. The circumferential magnetic field generated by the excitation current linearly rises until within the copper core (r=Rc) and then it decreases as 1/r for r>Rc (i.e. on the magnetic shell). In this case, the outer part of the magnetic layer is excited by a lower field, namely Hm. As far as the excitation current is high enough to make Hm>HS we can consider the wire completely saturated.

In this kind of structure, a larger magnetic layer requires a larger excitation current in order to avoid that the outer portion of the magnetic shell becomes unsaturated. Therefore, we

In this case the central part of the core will never contribute to the fluxgate effect, which will be given only by the outer shell. The inner part of the core usually shows a bistable behaviour, which means that its magnetization will switch direction upon the application of an axial field larger than the critical field. A fluxgate base on such wires will be affected by the perming effect (i.e. shift of the sensor's output characteristic after the application of a large magnetic

Fig. 7. Cross-section of a magnetic wire with bamboo structure, in case of negative (left) and

Composite wires have been proposed to solve problems given by the unsaturated inner section of the wire (Ripka et al., 2005; Jie et al., 2006). The main idea involving composite wires is to have wires with non-magnetic cores surrounded by a soft magnetic shell. In this way, we avoid problems such as the hysteresis of the sensor's characteristic and the perming

Considering a core composed of a 20 µm diameter copper wire surrounded by a 2.5 µm thick Permalloy layer, the perming error (i.e. shift of offset after 10 mT shock field) is only 1 µT, for an excitation current as low as 20 mA. Moreover, it is shown that the perming error decreases for a higher excitation current, as typically found for bulk core fluxgates, because

The most frequently used technique to produce composite wires consists of the electroplating of a magnetic alloy, for example Ni80Fe20 (Permalloy), on a copper microwire. The resistivity of copper (~17 nΩ·m) is lower than the resistivity of many magnetic alloys (for instance the resistivity of Permalloy is ~200 nΩ·m). For a typical wire composed of a 50 µm diameter core and surrounded by a 5 µm Permalloy shell, only 3.6% of the total current flows through the magnetic shell. If we operate the sensor with an excitation current low enough to make skin effect negligible, we can assume that the whole excitation current will flow through the copper core. Such simplified configuration is shown in Fig. 8 where the current density J is uniform within the copper core and zero in the magnetic shell. The circumferential magnetic field generated by the excitation current linearly rises until within the copper core (r=Rc) and then it decreases as 1/r for r>Rc (i.e. on the magnetic shell). In this case, the outer part of the magnetic layer is excited by a lower field, namely Hm. As far as the excitation current is high enough to make Hm>HS we can consider the wire

In this kind of structure, a larger magnetic layer requires a larger excitation current in order to avoid that the outer portion of the magnetic shell becomes unsaturated. Therefore, we

field) due to the switching of the magnetization in the central part of the wire.

positive (right) magnetostriction.

the core is more deeply saturated.

completely saturated.

effect, which typically arise if the wire is not fully saturated.

**3.3 Composite wires** 

must carefully weigh the advantages of lager sensitivity given by a thicker magnetic shell against the disadvantages caused by an increment of current required for the saturation.

Fig. 8. Composite wire with copper core and magnetic shell. The current flows entirely through the copper core so that the magnetic shell is fully saturated.

Skin effect, however, is not always negligible, especially when the sensor is operated at a high frequency in order to increase the sensitivity. In this case, the excitation current drains from the copper core to the magnetic shell, reducing the magnetic field in the magnetic shell. Depending on the actual current distribution, the magnetic field can strongly change. Numerical simulation is usually employed in order to predict the current distribution within composite wires (Sinnecker et al., 2002). The penetration depth strongly depends on the conductivity of both the conductive core and the magnetic shell as well as on the permeability of the latter. Therefore, a general value for a limit frequency to avoid draining the current to the magnetic shell cannot be given. Numerical simulation is suggested to predict current distribution within the wire.

Finally, designers of orthogonal fluxgates should carefully choose their operating frequency. On the one hand, a higher frequency increases the sensitivity, which contributes to the reduction of noise, whereas on the other hand, a higher frequency can cause parts of the wire not to be completely saturated, incrementing the noise (besides the hysteresis and perming effect). The excitation frequency should be chosen as a compromise between these two opposite effects.
