**3. Wire-core orthogonal fluxgates**

As previously mentioned, the availability of microwires suitable for the fluxgate cores gave new popularity to the orthogonal fluxgate principle.

Fig. 4 shows the structure of an orthogonal fluxgate based on a magnetic wire core. The excitation current Iex is injected to the magnetic wire and generates a circumferential field H while a pick-up coil is wound around the wire as usual.

Fig. 4. Orthogonal fluxgate based on a magnetic wire.

In this structure, the excitation coil is not required because the excitation field is generated by the current flowing through the wire. Therefore, the structure of the sensor is extremely simplified and the manufacturing of the sensor becomes easier. Even more importantly, the lack of an excitation coil makes it possible to significantly reduce the dimensions of the sensor. This plays strongly in favor of orthogonal fluxgates, because it makes them suitable for current applications where high miniaturization is required.

Fluxgates based on a microwire became popular also because during the last years the production techniques of magnetic wires have been subject of deep investigation. For example, in (Li et al., 2003) the effect of a magnetic field is shown during the electrodeposition of the NiFe film on a copper wire. By properly tuning the magnetic field's amplitude and direction it is possible to control the anisotropy direction (particularly useful for optimization of sensitivity and offset of the sensors) as well as to improve film uniformity, softness and grain size. Moreover, it has been shown that it is possible to strongly reduce the coercivity of electroplated Permalloy films as well as to increase their permeability by using pulse current instead of dc current for the electroplating process (Li et al., 2006).

Uniformity of the film is improved by a Cu seed layer sputtered on the Cu wire before electroplating because it minimizes the roughness of the surface, helping to reduce the coercivity. The effect of film thickness on the grain size, and finally on the coercivity, has also been studied in (Seet et al., 2006) where it is shown that grain size is lower for larger thickness. However, it is recommended to keep current density constant during the electroplating because if we use a constant current as the thickness increases, the current density decreases, and this is shown to increase the grain size.

Orthogonal Fluxgates 25

minimum field to saturate the material1, we observe that the inner part of the wire, for r<, where H<HS is not fully saturated. On the contrary, when we use a cylindrical core excited

Saturation is a vital requirement for the proper working of a fluxgate, wherein only the outer saturated shell will contribute to fluxgate mode whereas the inner unsaturated part of the core will not act as a fluxgate. Most important, having the central part of the core unsaturated causes hysteresis in the output characteristic of the fluxgate. Indeed, if we apply an axial magnetic field to the wire this will magnetize the central part of the core in its direction. Since that part of the core is not saturated, the magnetization cannot be restored by the excitation field through saturation in the circumferential direction. The centre of the

To this extent, it is very important to achieve the full saturation of the core to avoid the hysteretic behaviour of the sensor. Unfortunately, it is impossible to saturate the wire in its entire cross-section, since this would require an infinite current. Instead, we will always

Fig. 6. Magnetic wire with uniform current distribution. The magnetic field increases

Amorphous wires are often used as cores for orthogonal fluxgates. In this case, the wire has an inner cylinder with magnetization in the axial direction and a shell with radial or circumferential magnetization (Fig. 7) in case of positive or negative magnetostriction

1The saturation field is clearly not a brick wall border. The amount of saturated material asymptotically increases when the magnetic field grows. Therefore, we cannot define a clear border between the saturated and unsaturated state. However, we can define a condition when the core can be considered saturated from a practical point of view. That occurs when any increment of the magnetic field does not

linearly within the wire and only the outer shell where H>HS is saturated.

cause any significant change in the working mechanism of the fluxgate.

by a toroidal coil, then the whole core is equally saturated.

core will then naturally follow its hysteresis loop.

have an inner portion of the wire unsaturated.

respectively (Vázquez & Hernando, 1996).
