Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799

Sensors based on soft magnetic materialsPanel discussionq

Pavel Ripka!, GaH bor VeH rtesy",*

!Czech Technical University, Faculty of Electrical Engineering, Technicka 2, 166 27 Praha 6, Czech Republic"Hungarian Academy of Sciences, Research Institute for Technical Physics and Materials Science, P.O. Box 49, H-1525 Budapest, Hungary

The economic realities of productivity, quality andreliability for the industrial societies of the 21th centuryare placing major demands on existing manufacturingtechnologies. To meet both present and anticipated re-quirements, new and improved methods are needed. Tobe e!ective, the measurement, electronics and controlcomponents, and sub-systems, in particular sensors andsensor systems, have to be developed parallely as a partof computer-controlled manufacturing systems. Softmagnetic materials serve as a basic material for a widescale of di!erent sensors. The aim of the present paneldiscussion was to give a survey of the latest technologyand of the most important features of the sensors, whichare based on soft magnetic materials, and to comparesystematically their parameters. Magnetic sensors whichare not based on soft magnetic materials (such asSQUIDs, NMR and semiconductor sensors) as well asreading heads were not covered by this panel The atten-tion was concentrated mainly on device parameters andon applications, not on the theoretical background.

P. Ripka gave the general introduction. He emphasi-zed the keywords of the panel discussion: application,speci"cations and requirements. He had some generalremarks: Sensors containing core, made of magnetic ma-terial seem to be always vectorial. It is possible tomeasure 1, 2, or even 3 components of the magnetic "eldusing a single core. We are going to speak about sensors,which do measure "elds from DC; AC magnetic sensors

*Corresponding author. Tel.:#36}1-169-5165; fax:#36-1-3959284.

E-mail address: vertesyg@mfa.kfki.hu (G VeH rtesy).qList of panelists: P. Ripka (discussion leader), Czech Tech-

nical University; L. Panina, Russian Academy of Sciences; M.Yamaguchi, Tohoku University; H. Hauser, Vienna Universityof Technology; D. Mapps, Plymouth University; J.M. Baran-diaran, Universidad del Pais Vasco; L. Lanotte, Universita diNapoli `Federico IIa.

(such as induction coils) are not a subject of this dis-cussion. We restricted our topic mainly to magnetic "eldsensors; although the broader meaning of the term &mag-netic sensor' includes devices which, using magnetic prin-ciple, measure non-magnetic variables such as positionand speed, force and torque, temperature or others.

Invited researchers gave a brief overview of di!erenttypes of sensors. Each contribution had a similar struc-ture. The general questions connected with each sensortype were:

f Which is the application "eld of the given sensor?f Are the sensors already being produced or are they just

laboratory prototypes?f What are their parameters?

Besides the parameters well known for other sensorsand instruments, magnetic sensors have peculiaritiesgiven by non-linear materials and complex e!ects used.The most important parameters and properties of mag-netic "eld sensors can be grouped into the followingcategories:

f Full-scale range, linearity, hysteresis, temperature co-e$cient of sensitivity

f Bias stability, o!set, o!set temperature coe$cient,long-term stability

f Perming (if the sensor is subjected to a strong magnetic"eld, the o!set can be changed)

f Noisef Resistance against environment (temperature, humid-

ity, vibrations, radiation)f Resistance against perpendicular "eld and "eld

gradientf Bandwidthf Power consumption, sizef Reliabilityf Cost

The possibilities of sensor miniaturization are of in-creasing importance. We often hear questions about the

0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 2 9 1 - 2

Table 1

Application types Application Requirements

Precise(high-amplitude resolution)

Location of ferromagnetic objectsNavigationNon-contact current meters

High linearityLow cross"eld e!ectStable o!set

Low-noise BiomagneticSpace research

Low-noise

Industrial Position, speedCar industryMagnetic marking

Low-cost resistance against environment

Miniature(high spatial resolution)

MarkingSurface mapping

Micropower

Fast Pulse "elds, reading High bandwidth

Table 2

Fluxgate sensor parameters Top parameter Standard

Range 10mT 200lTLinearity error 10ppm 100ppmTemperature coe!. of sensitivity 10ppm/3C 50ppm/3CO!set temperature coe$cient (0.05 nT/3C 0.2nT/3CPerming (1 nT o!set change after 10mT shock (5 nTNoise 5pT rms (0.05}10Hz) 100pT rmsLong-term stability of the o!set 2nT/yr 5nT/8 hoursBandwidth 10kHz 20HzOperating temperature range !60}#2003C !20}#703CPower consumption 1mW 100mWSize 2mm 30mmCross"eld error (1 nT for 50lT "eld 5nT

integration compatibility, relation between the core sizeand sensitivity and about the suitability of the usedtechnologies for the mass production.

The &application types' of magnetic "eld sensors can beassorted in the following way, where the main require-ments are also indicated (see Table 1):

Fluxgate sensors are historically the "rst type of high-resolution solid-state magnetic "eld sensors. They arestill widely used in applications, although they were farbeaten in production volume by magnetoresistors, espe-cially AMR. The parameters of #uxgates should serve asa reference for newer types of sensors. However, anycomparison should be made carefully, taking into consid-eration also the sensor application type. It is also trickyto compare the properties of the fresh new sensors witha traditional device which was constantly being im-proved for decades.

Table 2 summarizes &top' values achieved for special-ized types of #uxgates (but certainly not all of themsimultaneously) and &standard' values achieved in high-performance devices. The disadvantage of #uxgate sen-sors is that they are usually o!ered only as a part of

magnetometer; high-performance #uxgates are only rare-ly available as components. The recent improvementsand developments in #uxgate sensors are covered ina separate paper.

L. Panina gave an overview about giant magnetoim-pedance (GMI) sensors. Giant magnetoimpedance e!ect isa kind of high-frequency analogy of giant magnetic re-sitance (GMR), because it is basically a change in the ACimpedance under the application of a magnetic "eld.GMI is essentially a high-frequency e!ect: a kind of sidee!ect of the skin e!ect. The sensitivity is very high com-pared to GMR. The typical material of this type of sensoris amorphous wire with well-de"ned circumferential an-isotropy and corresponding circular domain structure. Inthis case the sensing "eld is applied along the wire Thesensitivity is almost zero if the "eld is perpendicular tothe wire axis. It is possible to create a directional sensorusing three wires perpendicular to each other. If a ribbonor a thin "lm is used instead of a wire, two-dimensionalsensor using a single core can be created. GMI sensorsare already used by industry. The response of thesensor strongly depends on excitation. Symmetrical

796 P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799

characteristics can be obtained almost without hysteresis.It is also possible to create asymmetrical characteristicsthat would be important to get a good linearity. Thelinearity can be improved and a stable operation pointcan be achieved by using negative feedback. There is nochange in the sensor parameters up to 803C temperature.Several modi"cations of the GMI sensor were developed.It is possible to detect very low, highly localized magnetic"eld with 40lm resolution. The length of the sensorbased on amorphous wire can be reduced down to 3mmwithout losing sensitivity. There are many prospectiveapplications of this type of sensors, e.g. in medical elec-tronics, in automobiles, in textile industry. The sensorcan be integrated if a thin "lm is applied as a sensingmaterial.

M. Yamaguchi talked about thin xlm sensors. This typeof sensors is mostly associated with thin "lm type of thepreviously discussed GMI sensors. The skin e!ect isutilized in a ferromagnetic body. The impedance of theelement is proportional to the surface current density.With the application of magnetic "eld the permeability ischanged. Very high sensitivity can be achieved by thistype of sensor: the highest possible magnetic "eld resolu-tion is 10~13 T (if thermal #uctuation dominates theresolution). There are many forgoing works with thin"lm type sensors, e.g. Eddy Current Testing (ECT), accel-erometers, magnetic "eld sensors. One of the main e!ortsis to develop thin "lm micromagnetic devices with goodcharacteristics. In the case of these GMI-type sensors thesize can be easily reduced below 1mm by applyingphotolithography technique. By reducing the length, thesensitivity is lost because of the demagnetizing e!ect.Because of this the width of the elements should also bereduced. However, to keep the sensitivity with the minia-turization it is necessary to control the domain con"g-uration. A DC bias is necessary to get high sensitivity.Without using any windings, the bias can be introducedby an integrated thin magnetically hard "lm; SmCo within-plane anisotropy is utilized for this purpose. The driv-ing frequency can be increased up to 300MHz. 1 nTsensitivity has already been demonstrated. These sensorscan be applied in intelligent transportation systems, in"eld compensation in big CRT monitors, in 3D positionsystems, etc. The initial cost can be rather high, but therunning cost is very low, which is one of the main advant-ages of thin "lm sensors. Other advantages are the highsensitivity, small size and the possibility of integration.

H. Hauser reviewed magnetooptical sensors. Mag-netooptical sensors are a relatively new "eld of researchand development. Magnetooptical devices are mainlyused for switching, polarization and amplitude modula-tion of light. The only industrially available magnetoop-tical sensor is magnetooptical current transformermeasuring magnetic "eld of strong currents by the Fara-day rotation in diamagnetic "bers. Our group is develop-ing a new generation of magnetooptical sensors for

measurement of magnetic "eld and position of light.These sensors are based on domain wall motion in softmagnetic garnets and in orthoferrites. The speci"cationsare only preliminary, because only a laboratory proto-type exists. In these sensors the position of domain wallsis detected by means of the Faraday e!ect, using polariz-ed light. A periodic magnetic "eld with a frequency up toseveral MHz can be measured using a simple ruler. Tomake a good sensor, a material with extraordinary prop-erties is needed. One of these materials is the orthoferrite.In these materials the domain wall velocity is very high(up to 20 km/s), Faraday rotation is also very high(29003/cm), and the transparency is also extremely large.The only disadvantage of this material is the high biref-ringence; a complex sample preparation is therefore ne-cessary. The modulation depth of the light transmittedthrough the two-domain structure is about 15%/lT, soat least 1 nT resolution of the sensor can be achieved,depending on the bandwidth. The presently existing100MHz bandwith can be extended up to 1GHz. Theoutput signal of the sensor is usually a time duration, andthe domain walls are excited by an AC "eld. One of theparameters which are dependent on temperature is thesignal/noise ratio. The maximum "eld to be measured isbetween 1 and 10 kA/m, and the size of the plate is10]10mm.

D. Mapps summarized the state of art of magnetoresisi-tive (MR) sensors. This sensor technology has beenknown for quite a long time. MR sensors are widely used,especially in magnetic recording. What has happened inthe last few years is, that materials have been signi"cantlyimproved. This means that MR sensors are being used inmany more applications than just a few years ago. Thesensitivity was improved by reducing the thickness of thesensing material. Even 150 As thick "lms can producea very smooth magnetoresistance vs. applied "eld curve,producing a small noise. It is possible to get 97 dB sig-nal-to-noise ratio which is quite enormous for such a sen-sor. The thickness is very important because in very thin"lms NeeH l walls exist instead of Bloch walls. Due to themuch larger thickness of NeeH l walls, these walls passimperfections with very low change of wall energy, result-ing in very small noise compared to Bloch walls. Thisshows the important role of domain walls in the properoperation of MR devices which are not in a single-domain state. Another improvement which was achievedis to regularize the microstructure of the "lm materialusing new underlayer materials (e.g. platinum), thus im-proving the signal-to-noise ratio. Sophisticated elec-tronic techniques were also used to improve the qualityof the signal. The hysteresis can be removed by applyinghigh-frequency bias "eld. A very sensitive detector can beproduced from a pair of AMR sensors in a switch}biasmode where each sensor is switched alternately to mag-netise in the opposite direction to the other at kHzfrequencies. The resulting di!erential output can be

P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799 797

detected using a phase-sensitive detector. The equipmentfor producing GMR sensors is quite complicated, whichmakes GMR sensors expensive. A drawback is that MRsensors are temperature sensitive and if you do not opti-mise the material they can be noisy. However, they havea multitude of uses in magnetic "eld detection.

J.M. Barandiaran gave an overview about magneto-elastic sensors. Most of these sensors can be separatedinto two categories: static and dynamic. The topic of histalk was the static sensors; dynamic sensors are a subjectof a separate paper. The main characteristic ofmagnetoelastic sensor is the magnetoelastic coupling co-e$cient, which measures the conversion of magnetic toelastic energy or vice versa. In the 1960s there werealready some static sensors based on the stress e!ect onthe hysteresis loop of iron silicon; most of the presentstatic magnetoelastic sensors work on the same basis.The direct application of a static magnetoelastic sensor ismeasurement of stress or strain. It is clear that mag-netoelastic strain sensors are much better than straingauges; their sensitivity is thousand times higher. How-ever, the application of magnetoelastic sensors is morecomplicated than strain gauges because they need ACcurrent supply. They have another interesting and im-portant application possibility, which is the non- contactsensing element. They have given a good performance inrotating shaft measurement of the stresses. Torquemeasurement has been one of the most widespread ap-plications of magnetoelastic sensors. Another interestinguse of these sensors can be in medical applications e.g.sensors of breathing and blood pressure, which werepresented during the conference, but these devices are noton the market yet. The sensitivity and wide dynamicalrange of these sensors are among the best features . Forinstance, the static magnetoelastic sensors have an equiv-alent &gauge factor' of several thousands, as comparedwith resistive semiconductor strain gauges, which havea limit of about 200, and metallic strain gauges that areonly around 2. The temperature dependence is very smallcompared with the other sensors. Typical "gures of tem-perature coe$cient and maximum operating temper-ature are metallic strain gauges (0.05% deg~1 and 803C),semiconductor strain gauges ( 0.1% deg~1 and 1003C),magnetoelastic sensors based on amorphous ribbons( 0.02% deg~1 and 2003C). One of the most importantproblems is that these sensors will never be miniaturizedbecause of the need of coils to drive the sensor and pickup the signal. The dynamical magnetoelastic sensors aremuch better placed for widespread applications at thismoment as they work at resonance, and remote sensing ispossible with distances of about 1 m from the coils to thestripes. They are already present on the market (e.g. forsecurity purposes or for recognizing objects in supermar-kets). Dr. Hasegawa in his talk gave an example ofmagnetoelastic security tags being currently used in theStates. These have better performance than the old tags

based on the second harmonic detection and made up ofsoft magnetic stripes of permalloy or other materials withvery low magnetostriction.

L. Lanotte stressed some applications of dynamicmagnetostriction and in particular recent applicationswhich are based on magnetoelastic wave devices. A newtype of sensor, based on a new type of amorphous alloywas shown, which can be used to measure the strain. Thedevice works without bias magnetic "eld. It is also pos-sible to measure the change in local magnetic "eld. It canbe applied for measuring vibrations without contact. Thesensitive parameter of the sensor is the amplitude ofresonant magnetoelastic waves. It is possible to measurethe deformation of another sample due to application ofmagnetic "eld using magnetostrictive materials. The di-rection is important because the displacement on thevibration can be measured only in the direction which isparallel to the ribbon core. The size is about 30 mm longwith 5 mm diameter. The cost is low. It is only a proto-type, there is no industrial production. However, thetechnical requirements are not di$cult, so industrial ap-plication is possible. In principle it can operate in a largetemperature range.

G. VeH rtesy presented a new type of magnetic "eldsensors (Fluxset) for measuring DC and AC (up to 100kHz frequency) low-level magnetic "elds with high accu-racy. Its principle of operation is close to the pulse-position-type #uxgate magnetometers. The particularadvantage of these magnetometers is an output signalthat can be simply converted into binary code. Themeasurement of a small magnetic "eld is reduced toa high-accuracy time measurement through the displace-ment of the magnetization curve produced by the "eld.The probes are suitable for axial measurement of themagnetic "eld. The transverse sensitivity is negligible.The available maximum resolution of the sensor is below0.1 nT. The linearity is better than 1%. The temperaturestability is extremely good, the measuring head can oper-ate (without measurable change of the parameters) in the!200}#2003C temperature range. The time stability isalso good, as was proved by a long-term measurement(using simultaneously a reference observatory mag-netometer) of the variations of the Earth's magnetic "eld.The length of the 1 mm diameter probe can be variedbetween 5 and 20 mm; a longer probe ensures highersensitivity. The spatial resolution is better than 0.1 mm.One of the main advantages of this sensor is the simpleconstruction and low price. It can be applied in the same"elds like conventional #uxgates. Its parameters make itvery suitable for application in eddy current testing,combining the eddy current method with magnetic "eldmeasurement.

The summaries of the panelists were followed by dis-cussion.

It was inquired whether there is any theoretical limitfor the detectable minimum magnetic xeld in the case of

798 P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799

di!erent types of sensors. Dr. Yamaguchi answered thatin the case of thin "lm-type GMI sensor, if the limitis given by thermal stability and if a Co-based amorph-ous "lm is used, this theoretical limit is about 10~13.T. P. Ripka commented that in case of #uxgate sen-sors, the real detection limit is much larger than thetheoretical value estimated from models for Barkhausennoise. The e!ective "eld noise highly depends on thesensor size and geometry: long station magnetometersmay have 10 pT resolution. But it should be clearly statedthat in most cases (especially when measuring in thepresence of the Earth's "eld) the sensor noise is notcritical. The performance is usually limited by otherfactors like linearity, perming, cross"eld response andtemperature stability.

Another question was raised about the disadvantage ofGMI sensor compared to GMR sensors. Dr. Panina an-swered: GMI is a kind of high-frequency sensor. Thereare problems with impedance mismatching and withre#ected signals, so electronic circuits should be de-veloped to e!ectively operate at high frequency. Sometypes of self-oscillation circuits are being used, but theiroscillation frequency is limited. The problem is that the

oscillation frequency of these devices is not stable (it istoo sensitive to the environment), which will result inunstable sensor parameters. Better results were achievedwith hi-speed CMOS circuits, which generate pulse exci-tation, but their performance is still limited. Anotherdisadvantage of GMI is: Because the origin of this e!ectis a kind of skin e!ect, if we want to reduce the sizefurther, there is a limiting size reduction, and we lose bythis way sensitivity and response speed. If the excitationfrequency is 1GHz, a "eld greater than 100MHz cannotbe detected. For modern computer technology even fas-ter operation is needed.

Dr. Yamaguchi: Another possible disadvantage ofGMI, especially for thin "lm sensor is that this kind ofsensor needs a DC bias to get high sensitivity. In the caseof GMR exchange bias can be utilized, because the active"lm thickness is very small. However, the main disadvan-tage of GMR is its complicated manufacturing process.

The narrow time slot devoted for the panel had ex-pired; most of the topics were only raised, but the fruitfuldiscussions between the participants continued in theinformal part of the conference, which is out of the scopeof the present record.

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