Mattia Butta

Crossfield Sensitivity in AMR Sensors

We discuss the origin of the crossfield sensitivity of AMR sensors, the way how this error may influence the performance of an AMR compass and methods for its correction. Finally, we confirm the simple formulas experimentally. Crossfield may cause compass error up to 2.6 deg., depending on the compass orientation. The most effective way to suppress the crossfield error is using magnetic feedback, however this is not always possible. We suggest a method of processing of the SET/RESET sensor outputs which is more efficient than the usual averaging.

Influence of Magnetostriction of NiFe Electroplated Film on the Noise of Fluxgate

One of the sources of fluxgate noise is the noise originating in the magnetic core. Several parameters, such as anisotropy and shape, are traditionally studied and optimized to reduce the noise. In this paper, we show how the magnetostriction influences the noise of the fluxgate. We electroplated NiFe thin-film over a copper layer using different current density. By energy-dispersive X-ray spectroscopy we show that the mutual percentage of Fe and Ni in the film depends on the current density. As a result the magnetostriction is changed from negative to positive values, with minimum magnetostriction found at around Fe19Ni81 composition. When we used these ring cores as base for a fluxgate we observed that the noise rapidly rises as the absolute value of magnetostriction increases, while the minimum noise is achieved at lowest value of magnetostriction. An increase of one order of magnitude of the magnetostriction causes and increment of almost two orders of magnitude in the noise. Therefore, we conclude that it is vital to employ alloys with the lowest possible magnetostriction as core for fluxgates.

Magnetic Microwires With Field-Induced Helical Anisotropy for Coil-Less Fluxgate

We present a new method for production of magnetic microwire with helical anisotropy. Coil-less fluxgate sensors are generally composed of a bimetallic wire excited by an alternating current; in order for the wire to work in coil-less fluxgate mode, the magnetic layer of the wire needs to have helical anisotropy. So far, we have achieved such anisotropy by mechanically twisting the wire. However, this method has some disadvantages for practical applications, mainly regarding the sensor stability. We propose a method that provides helical anisotropy by applying a helical field during the electrodeposition: this is achieved by the superposition of a longitudinal field generated by a Helmholtz coil and a circumferential field produced by a direct current flowing through the core of the wire during electrodeposition .

Bi-Metallic Magnetic Wire With Insulating Layer as Core for Orthogonal Fluxgate

In this paper, we examine the problems related to orthogonal fluxgates realized using magnetic microwires as core. Starting from a description of orthogonal fluxgates evolution, we give a theoretical analysis of the problems involving the full saturation of the wire, necessary condition to obtain proper working conditions. Bi-metallic wires (magnetic layer on copper wire, carrying the excitation current) have been proposed to achieve full saturation using lower current. In this paper, we present a further improvement: we realized microwires with insulation layer between the copper wire and the magnetic layer. The current flows only into the copper, regardless of the working frequency. Using insulation layer, we achieve 20 mA saturation current at 10 kHz, which is 3 times smaller than for similar wires without insulation layer.

Sensitivity and Noise of Wire-Core Transverse Fluxgate

Fluxgate sensors with cores made of amorphous microwire have low sensitivity due to the small cross-sectional area of the wire. Previous studies with multiwire cores have shown nonlinear increase of sensitivity with number of wires, which was explained by interaction between the wires. In this study, we show that the anomalous increase of sensitivity cannot be explained neither by exchange coupling nor by magnetostatic interaction, but by change in parametric amplification caused by change of the quality factor of the resonant circuit. Interactions between the wires may cause hysteresis effects; therefore, we recommend to keep the wires in distance. Optimization of the pick-up coil geometry may significantly increase the sensitivity and, thus, reduce the magnetic field noise. Using four wires in parallel, we reached a noise level of 120 pT/¿Hz@1 Hz for a 10-mm-long sensor excited by a 20-mA current.

M-H loop tracer based on digital signal processing for low frequency characterization of extremely thin magnetic wires

A high-sensitivity ac hysteresis loop tracer has been developed to measure the low frequency hysteresis loop of soft magnetic materials. It has been applied successfully to characterize straight pieces of amorphous glass-covered microwires with metallic nucleus down to 1.5 microm thick. Based on the electromagnetic induction law, the proposed design is extremely simple and exploits the capabilities of commercially available data acquisition cards together with digital signal processing in order to achieve high-sensitivity without the need of expensive analog equipment.

Two-Domain Model for Orthogonal Fluxgate

In this paper a new model for orthogonal fluxgate is presented. A first attempt to explain the working principle of the orthogonal fluxgates was done in the 1970s. We show that this model does not work well on recently developed orthogonal fluxgate sensors with thin-film core on microwire. A new more accurate two-domain model based both on domain wall motion and magnetization rotation is proposed. We show that the new model better explains the observed properties of thin-film orthogonal fluxgate.

Double Coil-Less Fluxgate in Bridge Configuration

In this paper, a new method for excitation of coil-less fluxgate is presented. The purpose of this method is to reduce the spurious component of the output voltage, allowing us to increase the amplification. The method is based on the employment of two coil-less fluxgates in a double bridge, which injects pulsing current in opposite direction in each wire. By taking the difference of the voltages on the two wires, we suppress the component of the voltages, which does not change under application of external measured field. The sensitive axes are in opposite direction, so the wire feels opposite field. As a result, we will obtain an output voltage with low peak value, including only the component of the voltage that changes when we apply external field. Finally, we propose an improved version of the double bridge to allow the employment of two sensing elements with difference characteristics. This is obtained by optimizing the suppression of the spurious voltages and, at the same time, setting independently chosen values of exciting current for each wire.

Linearity of Pulse Excited Coil-Less Fluxgate

In this paper, we study the open-loop linearity of pulse excited coil-less fluxgate. Narrow current pulses can be used instead of classical sinewave for the excitation of the coil-less fluxgate. This method has shown several advantages, mainly regarding reduction of power consumption, normally in order of tens of microwatts. The disadvantage is that traditional phase-sensitive detection cannot be used as it results in a very small sensitivity. Using pulse excitation, the output signal is obtained by integrating a part of the positive pulse and a part of the negative pulse, and then summing up the resulting voltages. This process was realized using two boxcar averagers SR 4153. The achieved open-loop linearity error is much higher than for sinewave excitation and it is not sufficient for precise applications. We show that the linearity strongly depends on the symmetry of the two integrating windows. In order to obtain precise timing, we realized an excitation board using PIC microcontrollers, which provides both the pulsing signals for excitation current and the gating signals for integration. Using this board and optimum position of the integrating window, we obtained a 0.6% maximum nonlinearity in plusmn100-muT range, which is sufficient for portable compass.

Electroplated FeNi ring cores for fluxgates with field induced radial anisotropy

Being able to control the anisotropy of a magnetic core plays an important role in the development of a fluxgate sensor. Our aim is to induce anisotropy orthogonal to the direction of excitation because it generates a stable, low-noise fluxgate, as cited in the literature. In this paper, we present an original method for electroplating a ring core for a fluxgate with built-in radial anisotropy by performing the electroplating in a radial field produced by a novel yoke. The results show that the resulting anisotropy is homogeneously radial and makes the magnetization rotate, avoiding domain wall movement for low excitation fields.