Ž .Sensors and Actuators 81 2000 44–48www.elsevier.nlrlocatersna

Contactless current detection with GMR sensors based on an artificialž /antiferromagnet AAF subsystem

Michael Vieth a,), Wolfgang Clemens a, Hugo van den Berg a, Gunter Rupp a,¨Joachim Wecker a, Matthias Kroeker b

a Siemens, Corporate Technology, ZT MF 1, P.O.Box 3220, D-91050 Erlangen, Germanyb Siemens, Electromechanical Components, D-13629 Berlin, Germany

Abstract

Ž . Ž .The advantages of the newly developed giant magneto-resistive GMR sensor scheme based on artificial antiferromagnetic AAFsubsystem which was already demonstrated in angle detection, also promises a successful application in contactless current sensing.Wheatstone bridge structures were formed by combining sensors with opposite response characteristics. The sensor linearity wasimproved by an additional bias field influencing the slope and the maximum range of the signal curve. In our contactless sensing device,currents up to 40 A were measured reliably. Depending on the measuring range, excess currents up to 10 times of the nominal range donot change the sensitivity and bridge offset of the sensor. q 2000 Elsevier Science S.A. All rights reserved.

Keywords: GMR effect; Spin-valve sensor; Current sensor; Antiferromagnetic coupling; Wheatstone bridge; Excess current

1. Introduction

Ž .Since the giant magneto-resistive GMR effect hadw xbeen detected in magnetic thin film structures 1 it has

attracted large interest, mostly in data storage application.Ž .After anisotropic magnetoresistive AMR sensors have

found access in sensing applications like current detectionand position measurement it is evident that GMR-effectbased sensors will also get greater importance due to their

w xversatility and their ease of operation 2 . Concerning theinterlayer coupling effect in the layer system of GMRsensors, two main principles have to be distinguished:antiferromagnetically coupled and uncoupled systems. Be-cause of their high change in magnetoresistance, coupledlayer systems like the CorCu-multilayer have been exten-sively studied, and GMR effects of more than 70% in thefirst maximum of the oscillatory exchange coupling regime

w xhave been reported 3 . For practical application, largeŽ .saturation fields B of about 1 T for CorCu are disad-s

vantageous, so that sensor concepts based on uncoupledŽ .systems so-called spin-valve structures have been devel-

oped by preference. Among these concepts, a variety ofhard–soft systems with antiferromagnetically coupling

) Corresponding author. E-mail: michael.vieth@erls.siemens.de

FeMn-, IrMn-, or NiO-exchange layers have been pre-w xsented yet 4 . Especially a new sensor scheme based on an

Ž .artificial antiferromagnetic AAF subsystem offers bothunsurpassed rigidity of the hard layers and a large operat-

w xing temperature range 5 .Especially in automotive applications the advanced re-

quirements of power management have increased the de-mand for current sensors. Contactless current sensors re-ported up to now have used the AMR effect which limitsthe available signal height to about 2.5%. In this design

w xBajorek et al. 6 described a sensor bridge for currents upto 500 mA . To prevent the bridge from unexpected strayfields in a perpendicular sensor axis, Ripka developed aperiodical flipping of the bridge magnetisation by meansof an additional coil around the sensor to suppress the

w xdistortion of the sensor characteristic 7 . Recently a mag-netic field sensor concept has been presented, based oncoupled GMR multilayers of the CoNiFerCu type. By useof flux guiding means the effective field in the sensor

w xplane and consequently the sensitivity is increased 8 .

2. Design concept

Our novel concept was first applied in an angular sensorfor the 1808 angle range, and a signal amplitude of about

0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S0924-4247 99 00113-2

( )M. Vieth et al.rSensors and Actuators 81 2000 44–48 45

Fig. 1. Wheatstone bridge set-up consisting of GMR sensors R and constant resistors R , with arrows indicating the direction of the internalGM R L

magnetisations M and M of the AAF, of the field H of a current to be measured and a bias field H ; and bridge signals for normalized current induced1 2 m b

magnetic fields H rH , with bias field at 908 and at 1358 relative to the measuring field.m b

5% has been measured. In a set-up with two sensors,neighboured with 908 inclined sensor axes, the complete3608 angle range can be detected. Its main advantages isthe ability to act as a ‘contactless potentiometer’: in a large

Ž .field range ‘magnetic window’ the angle sensor outputsignal depends only on the direction of an external magnet,and it is independent of the field strength of the rotatingpermanent magnet. A simple electronic compensation canbe used because of the linear temperature dependence ofthe sensor for both ground resistance and signal amplitudew x9 . The design concept of the sensor described in thispaper is based on a GMR spin-valve structure with uncou-pled hard- and soft-magnetic layers. The hard-magneticlayer consists of the novel AAF subsystem, built of aCo Cu Co Cu Co multilayer coupling antiferro-x 1 nm y 1 nm x

magnetically in the first maximum. The choice of thick-nesses t and t of the Co layers, where 2 t y t )0,x y x y

results in a residual net magnetic moment of the AAF. Thedirection of this moment is a reference direction of thesensor. The AAF is in the center of two FerCo detection

Ž .layers, decoupled by relatively thick Cu films )2 nm .For a single GMR sensor, the resistance principally

follows a sinusoidal function, and the effect is dependenton the angle a between the magnetisation of the AAF andthe external field H bym

D RrR A 1ycos a 1Ž . Ž .o

where R is the minimal resistance when the magnetisa-o

tions of the detection layer and the AAF are parallel.Looking at an offset-free Wheatstone bridge, the voltagesignal is determined by

DUrU Asin a 2Ž .o

where U is the voltage supply of the bridge and as0 iso

perpendicular to the residual moment of the AAF in thesensor. To focus on current measuring, it is advantageousto add a bias field H which is superposed with theb

measuring field H . In the insert of Fig. 1 the bridge andm

the different field directions are drawn. So a becomes

asarc tan H rH 3Ž . Ž .m b

and in consequence

DUrU Asin arc tan H rH 4Ž . Ž .Ž .o m b

The plot of this function shows a linear response formeasuring fields H FH r2 and an asymptotic approachm b

Ž .to maximal signal output curve A . For different bias fielddirections this characteristic can be changed, especially inorder to get a larger linear range for unipolar currentsensing for example by rotating the bias field by further

Ž .458 curve B .

3. Sample preparation and mounting

The sensor stack is deposited on thermally oxidised Siwafers. A first Fe buffer layer is made by RF diodesputtering, a second one at the top of the layer stack as softmagnetic detection layer. Between them, the Co and Cu

Fig. 2. Design of the U-shaped current conductor, with the two resistorsR and the biasing magnetic foil in it.GM R

( )M. Vieth et al.rSensors and Actuators 81 2000 44–4846

Ž . Ž .Fig. 3. Influence of the bias field strength on the signal hysteresis left ordinate and on the sensitivity of the bridge right ordinate , and typical bridgey1 Ž .signal for magnetic fields ranging between negative and positive saturation at a bias field of 4 kA m as insert .

films, forming the AAF and the decoupling layers, weresputtered in the DC magnetron mode at an Ar pressure ofabout 10y2 mbar. After deposition and structuring, thesensors are magnetised which sets a fixed direction of thenet magnetic moment of the AAF as a reference directionof the sensor. Rotating or switching magnetic fields smallerthan about 30 kA my1 change the orientation of themagnetisation of the soft magnetic detection layer relativeto the fixed magnetisation of the AAF, and a GMR effectof about 5% is measured for a single sensor element.

As sensing device two GMR sensors R with anti-GMR

parallel magnetisations, each in a conventional SOH-hous-ing, 1 were connected as a half bridge. With two additionalconstant resistors R of identical resistance value, aL

Wheatstone bridge is formed to profit from a temperaturecompensated signal evaluation. A thin hard magnetic foil 2

is set between the GMR sensors to stabilise the sensorsoutput by a bias magnetic field H . This complete sensorb

pack is placed in an U-shaped current conductor whoseŽconnections are separated from the sensor electronics Fig.

.2 . This contactless detection principle allows current mea-surement without breaking the electric circuit under test.

4. Experiments and results

For calibration reasons the sensor bridge without mag-netic foil has been measured in a pair of crossed Helmholtz

1 Detailed data available in Product Information ‘Magnetic Sensors’from Siemens, Semiconductor Group, D-93009 Regensburg, orhttp:rrwww.siemens.dersemiconductorr.

2 From Vacuumschmelze, D-63412 Hanau.

coils to study the dependence of the sensor output on theexciting field H of a current to be measured as well as onm

the magnitude of the bias field H in a wider range. Forb

several bias fields H between 0 and 6 kA my1 the sensorb

characteristics have been measured. Fig. 3 summarises theŽinfluence of the bias field H on the sensitivity defined asb

.the slope of the curve for H s0 and the signal hysteresismŽ .the difference of the curves on the horizontal field axis .A typical field dependent signal, here with a bias field of4.0 kA my1, is shown in the insert. Bias fields between 4and 6 kA my1 are optimal for the given sensor design. Thehysteresis of the signal vanishes, and the sensor character-istic curve is rather linear between y2 and 2 kA my1. A

y1 Ž y1 .y1sensitivity of about 2 mV V kA m is reached.In a separate calibration the field in the U-shaped

current conductor has been measured which gave a relationy1 Ž y1 . y1of 0.12 mT A (0.1 kA m A at the place of the

sensors in the loop. Fitting that to the above deduced linearrange of the sensors, positive or negative currents up to 20A can be linearly measured with the described set-up.

For real application the bias field H was applied by ab

hardmagnetic thin foil which was pasted between the twosensors of the half bridge. Depending on the foil dimen-sions different bias fields act on the place of the sensorswhich influence the behaviour of the sensors in the samemanner as the bias field H of the Helmholtz coils. Forb

practical use a foil piece measuring 5=5 mm was chosenwhich delivers the optimal bias field of about 4 kA my1.This field has been found to be most effective during theinitial calibration.

In the case of unipolar DC current measurements, thesymmetry of the characteristic curve to the origin is notnecessary. For an additional rotation of 458 of the biasfield, the result of the measurement is given in Fig. 4. As

( )M. Vieth et al.rSensors and Actuators 81 2000 44–48 47

Fig. 4. Set-up for the measurement of unipolar currents: bridge signal for magnetic fields ranging between negative and positive saturation, magnetic biasfoil with size 5=5 mm, inclined further 458.

expected from the computation a larger linear range isavailable; hence unipolar currents up to 40 A can bedetected by this simple sensor structure.

Excess currents are crucial for every contactless orcontacting current sensor concept because in the case ofshort-circuit the sensor may be damaged seriously. Theshort-circuit proof was studied in the following manner:after checking the sensor characteristics in its suited rangea first range-exceeding current is applied to the currentline; a repeated check of the sensor characteristics is made,then a new higher current is applied and so on. Theinfluence of more than 20 increasing over-currents on the

sensor performance was studied. Depending on the mea-suring range, excess currents up to 10 times the nominal

Ž .range ;400 A do not destroy the sensitivity and bridgeoffset of the sensor. Over-currents exceeding 400 A beginto change the sensor characteristic: the initial slope of thecharacteristic is diminished as well as the offset of the

Ž .bridge is changed Fig. 5 . Both effects can be understoodby taking into account that field strengths exceeding 30 kAmy1 begin to affect the sensor of the bridge whose AAFwas magnetised in the opposite direction. Its magnetisationis reduced which can be measured as a reduced signalheight as well as a change of the offset of the bridge.

Fig. 5. Dependence of the offset and initial slope of the bridge signal upon the affected excess current.

( )M. Vieth et al.rSensors and Actuators 81 2000 44–4848

Fig. 6. Influence of repeated field exposures of equal field strength on the sensor characteristic, drawn as the slope of the signal between y2 kA my1 andq2 kA my1 ; in the first measurement the slope of 4 mV Vy1 kAy1 my1 was set to 100%.

A detailed study showed an interesting behaviour of asimilar sensor bridge in the field window up to 30 kA my1

where only a small loss of magnetisation was detected:now repeated short circuit currents of equal value areapplied to the bridge sensor, and its characteristic wasmeasured after each current pulse. Fig. 6 shows thatrepeated pulses of 12 kA my1 did not change the initial

Ž .slope at the first measurement normalised to 100% of thesensor characteristic more than 3%. Even at 30 kA my1

pulse field only the first pulses decreased the sensor slopeby 7%; repeated pulses of 30 kA my1 had no significantinfluence to the sensor signal. Therefore, a kind of artifi-cial ageing or sensor conditioning can be done in thiscurrent range. First occurrences of fields larger than 30 kAmy1 strongly affect the initial slope; indeed no furthersevere decrease of the sensor performance was observedafter the fifth pulse of equal strength had been applied tothe sensor bridge.

5. Conclusion

In summary, we have described a new contactless cur-rent sensor set-up, based on the GMR effect, which wasdesigned to measure DC currents up to about 40 A lin-early. Depending on the bias field of an inserted magneticfoil, an optimal sensitivity can be found, free of hysteresis.Excess currents not larger than 10 times of the nominalrange did not affect the sensor behaviour. Currents beyond

this limit affected the sensor signal in a first pulse, later onthe sensor became insensitive to a further exposition.

Acknowledgements

The authors gratefully acknowledge helpful discussionsand technical assistance by G. Gieres, K. Ludwig, H. Maiand K.H. Rojek.

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