doc. Ing. Mattia Butta, Ph.D.

We show that the wires with vanishing magnetostriction in their as-cast form exhibit positive magnetostriction after long-time annealing (more than 30 min), which increases the noise of the sensor. After researching the effect of the magnetostriction after annealing on the noise, we propose an alloy with a reduced amount of iron.

In this paper we show an up-to-now unexplained source of offset drift affecting the fundamental mode orthogonal fluxgate. After a sudden change or removal of the magnetic field, we observe an offset transient which lasts units to hundreds of seconds. We exclude the thermal origin of such transient as well as the electric origin in the pick- up coil resonance circuit or sensing amplifiers. We prove that this transient has magnetic origin, since it depends on both the amplitude and the duration of the pulse of magnetic field applied to the sensor which can be expressed as magnetic energy. We conclude that operating the fundamental mode orthogonal fluxgate in a closed feedback-loop is useful by suppressing this transient behavior by keeping the core in a (almost) zero field, however when switching the magnetometer sensor on/off, this transient has to be taken into account.

Fluxgate sensors are vectorial magnetic field sensors suitable for the measurement of fields up to mT with maximum pT resolution. Due to their low weight, low power, and low cost, fluxgates are ideal sensors to monitor the Earth’s field and measure its spatial deviations. The sensitivity of the sensor directly depends on the material and geometry of the core, and on the geometric parameters of the coils. Fluxgate is still the most sensitive room-temperature vectorial magnetic field sensor [1]. This work is devoted to the study of parallel fluxgate with multiple wire cores. The achieved sensitivity for a 35 mm long sensor is 12 mV/µT and the minimum noise was 2350 pT/√Hz at 1 Hz for as-cast amorphous wires.

For long time amorphous microwires with (Co 94 Fe 6 ) 72.5 Si 12.5 B 15 composition have been extensively used for magnetic sensors, such as fluxgates but also magnetoimpedance, due to their vanishing magnetostriction. In fluxgate sensors we want to minimize the magnetostriction because it couples any mechanical stress (for instance due to thermal expansion) to the magnetic characteristic of the wire making the noise of the fluxgate sensor rise. In this paper we compared two magnetic wires, one with slightly negative and one with slightly positive magnetostriction in their as cast state and we anneal them measuring then how the magnetostriction and the noise evolves as we increase the annealing time.

Fluxgate sensors with a flat race-track core can provide significant advantages in terms of noise and manufacturing complexity. We present a comparison of parameters of several such sensors of a similar construction but with different dimensions that might be useful for a wide spectrum of applications, including state-of-the-art geomagnetic measurements. The effect of core demagnetization is discussed, and optical Kerr effect microscopy is used to present the results of the thermomagnetic treatment of these cores, which significantly improves overall sensor noise.

We have further lowered the white noise of an orthogonal fluxgate to about 0.3 pT/√Hz @ 8 Hz. So far, this is the lowest noise reported for a fluxgate magnetometer. The noise reduction was achieved by introducing a JFET input stage, embedded directly to the sensor head, allowing for high common-mode rejection and negligible loading of the resonant circuit. The origin of the noise was investigated by correlation measurements and we concluded that, at least in the white noise region, we observe the magnetic noise of the sensor, with about 0.1 pT/√Hz white noise contribution by the electronics. We were finally able to obtain sensor noise floor below 1 pT/√Hz @ 1 Hz also in a feedback-compensated closed-loop. Closed-loop operation allows for higher magnetometer stability and operation in Earth’s magnetic field without deteriorating its noise performance.

We present the development of a low-noise, fundamental-mode, orthogonal fluxgate magnetometer with four amorphous, annealed ferromagnetic wires. The 1-Hz noise obtained in open-loop and closed-loop is as low as 0.75 and 1.5 pTrms/√Hz, respectively, with white noise level about 0.6 pTrms/√Hz. This is to our knowledge the lowest figure published for a fluxgate magnetometer so far. We compared the instrument performance to a low-noise observatory magnetometer when doing geomagnetic measurements and show that it is fully suitable for measurements at mHz frequencies, e.g. magnetotellurics. The magnetometer performance enables room-temperature, unshielded magnetocardiography.

In this paper we study the effect of magnetostriction of Co-rich amorphous microwire to the noise of the orthogonal fluxgates based on such wires. The magnetostriction was modified by changing the relative amount of iron x with the respect of the total amount of cobalt and iron in the alloy. Specifically we changed x in the composition (Co1-xFex)75Si15B10 casting wires with the following values of x: 0.05, 0.055, 0.06, 0.062, 0.065, 0.07 and 0.08. We found out that the noise indeed depends on the composition of the wire: while it is minimum (2.5 pT/√Hz) for x between 0.06 and 0.062 where the magnetostriction is vanishing (lambda(s) approximate to 10(-7)) it significantly increases to tens of pT/√Hz for both positive and negative magnetostriction when lambda(s) becomes one order of magnitude bigger. We verified that once the composition returns a magnetostriction low enough (around x = 0.06) then the noise does not depend on mechanical stress anymore. In fact the noise of a sensor with x = 0.06 is the identical in its natural curved state and when bended straight; for vanishing magnetostriction the bending does not affect at all the noise, as on the contrary it happens with larger magnetostriction. This suggests that once the wire has composition with x = 0.06 the remaining noise is not caused by mechanical stress. Finally we show how to overcome the problem of offset arising after annealing if continuous annealing current is used. We explain how this could be due by the fact that magnetostriction changes with temperature and even a wire with vanishing magnetostriction at room temperature can became significantly magnetostrictive at annealing temperature. For this reason we propose a method of annealing consisting rising the temperature of the wire while the wire is kept in its natural curved state.

We present a design of a low-noise, fundamentalmode, orthogonal fluxgate magnetometer with amorphous wires. The noise obtained in open-loop was as low as 5, 2 and 0.8 pT/√ Hz at 0.1, 1 and 5 Hz, respectively. As the instrument is a self-contained device, powered with batteries, we were able to run a series of geomagnetic measurements and compare them to a low-noise observatory magnetometer. The geomagnetic measurements were performed at the SANSA Space Science facility in Hermanus, South Africa. The offset drift is still an issue, however we show that the instrument is fully suitable for measurements down to 10 mHz. With numeric compensation of the offset drift, sub-mHz measurements are possible. As the instrument is non-cryogenic and portable, more applications, such as magnetotellurics or an unshielded magnetocardiography with a gradiometric sensor, are foreseen.

It has been shown that annealing of amorphous magnetic wire used as a core for orthogonal fluxgates in the fundamental mode can significantly decrease the noise of the sensors in the 1/f region. This is due to an increase in the circumferential anisotropy due to the dc current flowing through the wire during annealing. This method, however, presents some drawbacks: first, it requires an infrared furnace and precise compensation of the magnetic field inside it. Second, it is very slow, because it requires the cooling of the whole furnace before removing the wire. Most importantly, while the 1/f noise decreases, the noise floor increases. In this paper, we present a method that allows the simultaneous reduction of 1/f noise and noise floor. This method is based on Joule annealing by means of a very large current in the wire. The current is periodically flipped with 0.25 Hz frequency in order to avoid an excessive increase in the offset. The annealing is performed in a four-layer shielding to avoid the presence of an external dc field. Annealing for 1 min, 1 wire-based sensor returned a considerable noise reduction both in the 1/f regions from 2.5 to 1.5 pT/root Hz) while the noise floor was unchanged at 650 fT/root Hz. For larger annealing time, however, the noise floor rose. We tried to compensate this problem by increasing the number of wires to four, but also in this case, we achieved the best noise reduction (from 1.7 to 0.75 pT/root Hz at 1 Hz and from 470 to 350 fT/root Hz noise floor) with 1 min annealing. By the use of thermocamera, we discovered that the problem of long-time annealing was that the sensor head support was warming up too much. Therefore, we repeated the experiment by annealing 21 min as a series of 1 min annealing followed by 3 min of no current for 21 times to let the sensor head totally cool to room temperature after every annealing period. In this way, we achieved the lowest noise of 630 fT/root Hz at 1 Hz and 400 fT/root Hz noise floor.

In this paper, we investigated the effects of different diameters versus the noise of the sensor. Our purpose is to understand that if there is a room for improvement of noise by producing amorphous wires with different diameters.

This contribution deals with challenges encountered in real-world geomagnetic measurements, and is focused on improving the performance of two variometer stations of Kelčany and Polom, which have been recently established in the Czech Republic.

In this paper, we study the possibility to exploit the noise correlation in a fundamental mode orthogonal fluxgate used as a gradiometer in order to maximize the noise suppression. We study the correlation of the noise along a single core fluxgate using two short coils at a variable distance, and we derive how it drops as the distance increases. Furthermore, we observed this phenomenon with the central point of the coils located at different parts of the wire, obtaining a similar behavior. We also explain that the response of such configuration of the gradiometer to the homogeneous field is zero even if the single coils are not symmetric with the respect of the center of the core if the pairs of coils are all symmetric.

In this paper we present a simple, cheap and effective method to obtain reduction of very low frequency noise in an orthogonal fluxgate in fundamental mode. This method consists in the application of a layer of silicone over the magnetic core of the sensor filling the whole space inside the pick-up coil. In this way we avoid fast variations of temperature to affect the offset of the sensor (which is very sensitive to temperature changes).

In this paper we present the effect of the thickness on the field-induced anisotropy of NiFe layers. We electroplated several ring-cores with thickness spanning from 2μm to 18μm changing the deposition time (and keeping unchanged the other parameters).

In this paper, we present an orthogonal fluxgate in fundamental mode with extremely stable offset and low noise. The typical solution to stabilizing the offset in this type of sensor is to flip the dc bias of its excitation current and subtract the output obtained with opposite polarities. In this case, on the contrary, we flip both the ac and dc components of the excitation current and sum the output voltages. This is performed using fast solid-state switches with low on-resistance. In this way, we achieved a large suppression of the dependence of the offset on temperature

In this paper we present a magnetic gradiometer based on a single core orthogonal fluxgate in fundamental mode. Contrarily to similar gradiometers based on detached sensor heads the use of a single core strongly reduces the offset instability of the output. The offset drift is caused by temperature variations and in a single ferromagnetic core the temperature is more uniform than in two detached cores. As a result the offset of both coils drift in a very similar way and when computing their difference to obtain the gradiometric signal the output is very stable. Moreover, we show that using a single core we have noise suppression due to correlation of the magnetic noise and this mechanism also leads to noise reduction of the gradiometric output. As a result we achieved 10 pT/m/VHz noise at 1 Hz.

In this paper we present a new type of magnetic gradiometer based on orthogonal fluxgate in fundamental mode. This type of sensor can be build using two different magnetic cores, each one with its own pick-up coil, or using single core with two pick-up coils wound on it.

Field-induced anisotropy has been proposed as an effective method to produce ferromagnetic rings with radial anisotropy for fluxgate cores. Compared with stress-induced anisotropy, this method easily allows generating a radial anisotropy in electroplated rings. However, it is still not clear what the source of such anisotropy is. It has been suggested that the observed anisotropy could be the effect of the magnetocrystalline anisotropy present in the electroplated film because of the non-ideal stoichiometry of the alloy. If true, this would make impossible to electroplate films with field-induced anisotropy and, simultaneously, low magnetostriction, because this condition occurs at Ni81Fe19 composition where the magnetocrystalline anisotropy vanishes. In order to verify this hypothesis, we electroplated several rings changing the composition of the alloy, with and without radial magnetic field applied. It turned out that the field-induced anisotropy was independent on the composition of the film to a large extend, and it did not show any vanishing trend as we approach Ni81Fe19 composition. This suggests that it is, in fact, possible to obtain field-induced anisotropy and simultaneously low magnetostriction. Finally, we show how such radial field-induced anisotropy reduced the noise of the fluxgate by almost one order of magnitude.

Saccharin is a common additive used in Watts-type baths for electroplating of NiFe alloys (especially in the Permalloy composition). The reason is that saccharin is helpful in reducing the internal stress of the resulting ferromagnetic film, which is most often unwanted in many applications of NiFe electroplated alloys. In this paper, we consider fluxgate sensors based on Permalloy rings. The rings were electroplates using the same bath composition except for saccharin concentration, which varied from 1.5 to 17 g/L being 6 g/L the almost universally used concentration. The pH was corrected to 2.8 for every bath by adding different quantities of KOH according to the different values of pH obtained with a different saccharin concentration. We did so to assure that for the same electroplating current density, we obtain the same potential and finally the same composition ratio of nickel and iron. The rings show a noise spectral density when used as a core of fluxgates, which dropped from 180 pT/Hz at 1 Hz for 1.5 g/L saccharin concentration to 60 pT/Hz when saccharin was increased to at least 15 g/L. This shows that the most common saccharin concentration (6 g/L) in fact is not the most best concentration when it comes to minimize the noise of the fluxgate, but larger concentration is required.

Samples of electrodeposited fluxgate sensors with varying magnetostriction coefficient were manufactured and their noise was studied at various temperatures. We show that the noise of the fluxgate is, under some conditions, dependent on temperature. We studied such behaviour of the noise for samples with low and high magnetostriction and we verified that the both types of sensors presented low and high temperature dependent noise. Then we investigated as a possible source of noise the thermal cooling rate applied to the samples after electroplating and we verified that large thermal shock could induce mechanical stress and noise in the sensor.

This book gathers, for the first time, an overview of nearly all of the magnetic sensors that exist today. The book is offering the readers a thorough and comprehensive knowledge from basics to state-of-the-art and is therefore suitable for both beginners and experts. From the more common and popular AMR magnetometers and up to the recently developed NV center magnetometers, each chapter is describing a specific type of sensor and providing all the information that is necessary to understand the magnetometer behavior including theoretical background, noise model, materials, electronics, design and fabrication techniques, etc.

Noise of orthogonal fluxgate in fundamental mode has been strongly reduced in the last years, making it a very competitive vectorial sensor of magnetic field at room temperature. The most important results have been achieved reducing the 1/f noise, which can reach about 1 pT/√Hz at 1 Hz. However, the noise floor is still an issue, which cannot be ignored anymore. The problem of the noise floor in fundamental mode orthogonal fluxgate became evident when sensors based on annealed wire-cores have been proposed. By annealing the wire with proper current flowing through it, the circumferential anisotropy is increased and therefore the 1/f noise reduced. However, the sensitivity is also reduced and that affects the noise floor, which is typically due to the white noise of analog demodulator. The challenge is to produce sensors with simultaneously low 1/f noise and low noise floor. For this reason we propose a transition to digital demodulation of the output voltage of the sensor. The voltage is digitized by high-speed high-resolution digitizer and the first harmonic extracted numerically. In this way we get rid of the noise of the analog demodulator. In this paper we prove that using this method we reduce the noise floor from 650 fT/√Hz to 400 fT/√Hz, a value which could never be achieved with analog demodulator, without increasing the 1/f noise. Moreover, we show how a digital demodulation allows us to efficiently compensate the noise in the output of the sensors due to excitation current. This result is achieved by simultaneous sampling of the excitation current and applying an algorithm based on its correlation to the output voltage of the sensor.

In this paper we show how an anisotropy perpendicular to the direction of excitation is favourable to obtain low noise in fluxgate sensors. We produced ring cores by electroplating NiFe thin film (6 μm) over a bended copper substrate. After the electroplating process the sample was released and the back-stress induced anisotropy in the magnetic film due to magnetostriction of the material. As a result we obtained rings with regions showing hard behavior and orthogonal regions showing soft behavior. Then we used such rings as core for fluxgate and we measure both noise and sensitivity for different orientation of the ring in the pick-up coil. When the pick-up coil covers the area with anisotropy perpendicular to the direction of excitation the sensitivity is lower due to lower permeability but finally the noise is lower. On the other hand, rotating the core by 90 degrees the anisotropy becomes parallel to the direction of excitation and thus the sensitivity increases. Nevertheless, the noise also increases, showing that such anisotropy parallel to direction of excitation is unfavorable even if it brings higher sensitivity.

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.

Magnetostriction Offset of Fluxgate Sensors: we examined the origoins of offset both theoretically an experimentally

Samples of electrodeposited fluxgate sensors with varying magnetostriction coefficient were manufactured and their noise was studied at various temperatures. We show that the noise component caused by magnetostriction is, under some conditions, dependent on temperature. The conditions affecting the temperature dependence of noise are also discussed. (C) 2015 article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ric-nd/4.0/) Peer-review under responsibility of the organizing committee of EUROSENSORS The Authors. Published by Elsevier Ltd. This is an open access

In this paper we studied multi-ring planar fluxgate with different number of rings. In particular we investigated the effect of the number of rings on the sensitivity and noise of the sensors. Multi-ring cores were expected to return lower noise due to mutual compensation of uncorrelated noise of every ring. Nevertheless, we observed an increase of noise for cores with higher number of rings. We believe this is due to non-uniform composition of the electroplated film, which make the inner rings magnetostrictive and therefore source of larger noise.

In this paper we study the influence of the roughness of the copper substrate on the magnetic properties of the FeNi film we electroplate onto it. The roughness and the thickness of the copper substrate are reduced by electropolishing in orthophosphoric acid: we show how to select the working point, which gives the highest smoothness, avoiding defects due to gas evolution. Finally we show how a smoother substrate contributes to reducing the coercivity of the magnetic film grown on it.

Offset and its long-term stability is a weak point of fluxgate sensors. Even the ultrastable sensors kept at no vibrations and stable temperature at magnetic observatories show offset drift. Such drift of the fluxgate triaxial sensor can only be partly corrected by the scalar resonance magnetometer. Periodical calibration of absolute reading should be made using nonmagnetic theodolite. In this paper, we study the origin of fluxgate offset. We distinguish the real magnetic sensor offset from the offset contributions originating in the false second harmonics signal that leaks to the sensor output from the distortion in the excitation signal, or which is borne as harmonic distortion when the signal processing electronics are subjected to the large first harmonic signal leaking from the excitation. We analyze the offset dependence on the angular position of the sensor core and its response to large field shocks. The experiments give an indication that only a part of the magnetic offset stems from a remanence of magnetically hard core regions. The residual part may be caused by a magnetostrictive signal, belonging to false signal contributions, but not considered in previous studies.

We electroplated NiFe thin-film over a copper layer using different current density. 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.

This papers proposes a method for annealing an amorphous wire to be used as core for orthogonal fluxgate in fundamental mode, avoiding growth of offset and hysteresis in its characteristic. This is obtained by identifying the working point where the effective longitudinal field applied to the wire is null. By using this method we both suppress the offset and reduce the noise of the magnetometer, which reaches 1 pT/√Hz at 1 Hz.

Magnetic anisotropy plays an important role in the behavior of a fluxgate core. It is typically desired to have an anisotropy orthogonal to the direction of the excitation field in order to move from one saturated state to the opposite saturated state without abrupt change of magnetization. In this paper we present a method for electroplating a Permalloy film in a shape of race track core with built-in anisotropy. The core is electroplated under bending and later released so that the resulting stress on the film generates the anisotropy. We show how the BH loop of the film changes as we increase the bending radius. Moreover, we demonstrate that electroplating under bending extends the frequency range before the inductance drops. As a result the fluxgates based on such cores can be used for higher frequencies (at the expense of lower sensitivity given by lower permeability).

Magnetic anisotropy of NiFe films electrodeposited on Cu wires was studied. The influence of current density, magnetic field and strain applied during deposition on the anisotropy constant and easy axis orientation was investigated using AC hysteresis loop tracer and magneto-impedance measurements. Magnetic field applied during the electrodeposition induces weak magnetic anisotropy with the easy axis parallel to the field and features typical for atomic pair ordering mechanism.

In this paper we present electroplated magnetic microwires with helical anisotropy given by torsion during electroplating. These wires have been used as core for coil-less fluxgate: linearity, noise and resolution have been measured, and they showed better performances than microwires with field induced helical anisotropy. Therefore, we conclude that electroplating under torsion is the most promising method for manufacturing microwires for coil-less fluxgates.

Magnetic sensors working in increased temperature range are required for many applications, such as: - down-hole orientation sensors for oil & gas exploration and production - angular position and speed measurement for turbines - engine sensors for automotive industry - electric current sensors and also for some planetary space missions (e.g. Mercury).

Temperature stability of fluxgate sensors is very important issue, because this kind of sensors has found applications in several field where temperature is not constant. Temperature determines dilatation of the material which can result in mechanical stress to the core. In this paper we compare orthogonal fluxgates in coil-less mode, obtained with bi-phase Copper/Permalloy microwires with and without glass layer between them. We show that a glass layer between Copper and Permalloy strongly decreases the dependence of both offset and sensitivity on the temperature. We explain this by the fact that glass has lower thermal expansion coefficient than copper, therefore it compensates its dilatation which is source of mechanical stress for the Permalloy. © 2010 FEI STU.

We developed a novel method which allows us to operate the coil-less fluxgate in a feedback-loop without the need of any external coil. The external field is nulled by adding a compensating current to the sinusoidal excitation current with proper sign. In this way, we can extend the linear range of the coil-less fluxgate, for high measured fields.

Crossfield effect often seriously limits the accuracy of magnetic sensors. However, this effect is only rarely discussed by sensor. manufacturers and users. In the case of bulk fluxgates, crossfield error in the Earth's field can be suppressed by proper design below 3 nT p-p for the race-track and rod fluxgates and also for carefully designed ring-core sensors. Crossfield for miniature fluxgate can be significantly higher mainly due to imperfections in the geometry- for PCB fluxgate we measured 60 nT error.

Double coil-less fluxgate in bridge configuration suppresses spurious signals on excitation frequency.

In this paper, a digital magnetometer based on printed circuit board fluxgate is presented. The fluxgate is pulse excited and the signal is extracted by gate integration. We investigate the possibility to perform integration on very narrow gates (typically 500 ns) by using digital techniques.

We fabricated microwires with pulsing current, and we studied their magnetic properties as well as their behavior when employed for orthogonal fluxgate. The noise at 1 Hz is minimum for duty cycle between 60% and 80%, whereas it increases for dc and lower duty cycles. However, wires electroplated with low duty cycle show much wider linear range than those realized with high duty cycle. Thus, we derive that high duty cycle must be preferred if the orthogonal fluxgate is used in feedback mode, whereas wires electroplated with lower duty cycle are preferred if the sensor is used in open loop.

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.

Sensitivity and noise of wire-core transverse fluxgate was measured as a function of the excitation amplitude and other parameters.

Temperature stability of fluxgate sensors is very important issue, because this kind of sensors find applications in several field where temperature is not constant. Temperature determines dilatation of the material which can result in mechanical stress to the core. In this paper we compare orthogonal fluxgates in coil-less mode, obtained with bi-phase Copper/Permalloy microwires with and without glass layer between them. We show that a glass layer between Copper and Permalloy strongly decreases the dependence of both offset and sensitivity on the temperature. We explain this by the fact that glass has lower thermal expansion coefficient than copper, therefore it compensates its dilatation which is source of mechanical stress for the Permalloy.

Two sources of cross-field error in racetrack fluxgate are identified: geometrical imperfection of the winding and nonhomogeneity of the sensor core.

In this paper a bi-metallic magnetic wire with insulating layer can be used as a core for orthogonal fluxgate.

Bi-metallic magnetic wire with insulating layer can be used as a core for orthogonal fluxgate.

Crossfield effect often seriously limits the accuracy of magnetic sensors. However, this effect is only rarely discussed by sensor manufacturers and users. In this paper, we present measurements on ring-core, racetrack and Vacquier type fluxgates and on one type of AMR sensor.

Crossfield Sensitivity in AMR Sensors can cause serious error. We show the origin of this error and suggest several possibilities how to corect it.

In this paper we present a precise and cheap system we developed to measure BH loops of magnetic microwires. We especially focused on low frequency BH loops because we are mostly interested in quasi-static magnetic properties. Due to low frequency, we had to face problems related to low signal to noise ratio. The extremely small cross-section of the microwires turned out in very small flux to be measured. We achieved our goal by realizing a BH tracer based on induction principle, which generates the current necessary to create H field, and it synchronously samples the induced voltage. Digitized voltages are then sent to PC, which performs numerical analysis and returns the BH loop. The system has proved to be cheap and it provided excellent measurement parameters.

We have concentrated on crossfield effect in flipped sensors and also on the role of anisotropy dispersion.

In this contribution we study the feasibility of other techniques than sine wave excitation for coil-less fluxgate. The main purpose is to show that sinusoidal current can be in fact replaced by pulsing current which returns the same saturation level and less power consumption. We also show a prototype we realized to perform pulse excitation of coil-less fluxgate, controlled by PIC microcontroller and interfaced to PC by USB bus.

Linearity of pulse excited coil-less fluxgate can be improved by roper design.

In this paper a linearity of pulse excited coil-less fluxgate is measured and discussed.

M-H loop tracer based on digital signal processing for low frequency characterization of extremely thin magnetic wires allows to measure magnetic properties of wires.

Theoretical model for coilless fluxgate is presented. The model consists of two domains with rotating magnetization. The volume of each domain is proportional to external field.

Crossfield Effect in AMR Sensors originates in the anisotropic character of these sensors

Characterisation of magnetic wires for fluxgate cores is important for the sensor design.

In this paper a novel kind of fluxgate is presented. The sensor is based on helical anisotropy of the ferromagnetic layer, electrodeposited on copper wire.

We measured the effect of crossfield Hx on the output voltage of AMR sensor. We show that Hx can significantly affect the magnetization and cause hysteresis when it reaches hundreds of A/m. For lower field region we observe disagreement with the commonly accepted theoretical model.

In this paper we present a model of coil-less fluxgate, which is based on simultaneous domain wall movement and magnetization rotation. Firstly we examine the case without twisting (classical orthogonal fluxgate), then we extend the derived result to the case with twisting (the final working state of our sensor).

Crossfield sensitivity is a non-linear effect inherent to AMR sensor. It is caused by the anisotropic character of the sensor. One can reduce the effect by changing the sensor geometry, but this also reduces the sensitivity and finally increases the sensor noise. It is important to understand this effect in order to use AMR sensors correctly. The basic model is directly derived from the single-domain energy equations. It works for sensors with barber poles and including the sensors forming Wheatstone bridge. Finalyy we will show that the effect can be 1) numerically corrected, 2) suppressed using flipping or 3) almost annihilated by feedback.

Coil-less fluxgate is a new kind of orthogonal fluxgate sensor. So far it has been excited by sine-wave current; 2nd harmonic extraction has been used to process the output voltage. This method was useful to prove the working mode of the sensor; however it shows serious drawbacks. In this paper we will show that the coil-less fluxgate can be excited by pulses of current, with proper height and width. The output voltage is obtained integrating a portion of the positive and negative pulse by two boxcar averagers, then summing up the resulting voltages.

Two-Domain Model For Orthogonal Fluxgate describes the function of the mentioned magnetic sensor

Algorithm for Noise Reduction in Output Signal of Race-track Core Fluxgate is based on digital signal processing.

How to measure orthogonal and circular hysteresis curves

In this paper the results of the research performed on the electrodeposited wires, as core for orthogonal fluxgates sensors, are presented. Methods for measurement of the BH loops and for the characterization of the second harmonic responses are presented. From the measured values we show that the minimum current necessary to avoid perming effect in the sensor output signal, is equal to the value requested to saturated the core in ircumferential direction. Dependence of this parameter on the the frequency of the excitation current is then shown. The sensitivity of the fluxgate using this wire as core is later analyzed and proved to be dependent on the amplitude of the excitation current. Based on these achieved parameters, hints for the choice of the working values for the excitation current are given.

Fluxgate effect in twisted magnetic wires can be used to measure magnetic fields.