A novel highly sensitive magnetic sensor

Sensorsand Actuators

466

A, 37-38(1993)466-410

A novel highly sensitive magnetic sensor U Baqenbruch Unrversriy of Kassel, Sectron of Measurement Technology, Wdhelmshoher ANe 73, D-3500 Kassel (Germany)

A highly sensltlve magnetic sensor urlth a frequency output signal IS introduced The sensor can easily be Integrated m an electrical clrcult It consists of a self-starting tunneldmde oscillator m which the necessary Inductance 1s obtamed by an electncally conductive magnetic thin-film core The resistance and inductance of tlus thm-film core are changed drastically by a small external magnetic field m the range pT to pT Hereby one gets a tuned r f oscillator whose frequency 1s determined by a weak external magnetic field applied to the thin-film core (magnetltally controlled oscillator (MCO)) Thts low-noise magnetic-field-to-frequency converter IS very sunple In construction and shows a sensltlvlty of more than 100 GHz/T It 1s smtable for a magnetic readout system or an integrated magnetic sensor array especially for high-frequency magnetic heads

1. Introduction

Three kinds of highly sensitive magnetic sensors (m the nT range and below) are known from the hterature The first conststs of a superconductmg matenal (SQUID) [ 11, but its construction is complicated and causes high purchase and runmng costs The second consists of a senuconductmg matenal (prnnanly based on the Hall effect) [2-51 and therefore It can easily be integrated and 1s cheap m production, but Its sensitivity 1s hmlted because of its high inherent noise [2] The last consists of a metalhc matenal One type IS based on the amsotroplc magnetoreslstance [6-81, the other IS based on the changes of permeability of a coil-core under the miluence of an external magnetic field [9, lo] (fluxgate magnetometer) Both sensors are complex in their structures (magnetoreslstlve element m a barberpole configu-

+ external apphed magnebc field

B,,

Fig 1 Schematlc diagram of the magnetic sensor &, the whole clrclut resistance, C,, the sum of the external capacitance and &ode capacltancc I,,, the pulsating diode current

0924-4247/93/%600

ratlon for hneaxzatlon and m a bndge connection for mcreasmg sensltlvlty or a coil-magnetometer with many co11 wmdmgs), so they are not very smtable for an integrated circuit The sensor introduced here 1s not very complicated m structure (see Fig 1) so It 1s suitable for a magnetic readout system or an mtegrated magnetic-sensor array The frequency output provides robust mformatlon transrmsslon and snnple dlgltal conversion by countmg the pulses The sensor has been examined from physical and theoretical points of view

2. Detection principle Figure 1 shows a schematic &agram of the magnetic sensor If an external magnetic field is acting on a thin-film core (e g , a small Mumet&@ nbbon or an amorphous metallic nbbon) the r f permeablhty changes m different ways dependmg on the internal magnetic behavlour of the metal nbbon [ 11- 131, which again causes a change m the frequency of the tunneldiode oscillator shown In Fig 1 For given hensions of the strip the sign and the magnitude of this permeability change depend on the excltatlon frequency of the oscillator current (e g , on the pulsating current density m the stnp) and on the magmtude and direction of the external magnetic field relative to the dlrectlon of the internal magnetization m the magnetic strip This IS shown in Fig 2 for the introduced sensor We assume that the strip can be dlvlded into two parts (similar to the Wlegand wire [ 141) wth different magnetic behavlours The mner area has a magnetic easy axis (e a ) parallel to the strip axis The outer margm (border area) has a direction of magnetization (Mb)

@I 1993 -

Elsevier Sequoia All

nghts reserved

Fig 2 Model of the mtemal magnetic behavlour e a , preferred magnetic axw (e g , easy axis), ID, pulsatmg diode current, Hd, demagnetlzatlon field, H,, magnetic field created by the current ID, IV,, M,, magnetlzatlons of the mner and border areas, H,, external dpphed field, t, I, w, tbckness, length and wtdth of the

stnp (for details see text)

which encloses an angle ~1with the easy axis of the mner area (see Fig 2) This strong difference is caused by destruction of the metallic texture by cuttmg the magnetic strip out of a large specimen The above assumption 1s confirmed by theoretical calculations described in Section 4

3. Experimental set-up and results The new sensor is bmlt out of an adjustable and stabilized voltage source (20-200 mv), a tunnel diode and the thin-film strip m a series connection [ 151 (see Fig 1) The necessary capacitance C,, is the sum of the tunnel-diode capacitance (50 pF) and all parasitic external capacitances The resistance depicted m Fig 1 by R, 1s the real part of the whole clrcmt Impedance The oscillator frequency was adjusted to approxnnately 82 MHz The FM output slgnal of the sensor was demodulated by a commercial FM demodulator (Video-PLL Valvo-IC NE 568) The output signal of the demodulation was measured by a lock-m amphfier The lock-m technique was chosen to measure the noise and to reduce the magnetic stray fields because the measurements were camed out m a magnetically unshielded room The noise of the whole measurement system (sensor, demodulator and lock-m) was determmed to 0 6 pV/,/& This corresponds to a muumum detectable frequency change Af = 6 Hz of the magnetic sensor The external magnetic field was generated by a co11 with 1600 windings and gauged by a commercial Gaussmeter (F W Bell, Sacs 9900) In Fig 3 the dependence of the phase-locked-loop output signal on the external magnetic field Be, measured by the lock-m technique is shown (hneanty error < 0 1%) The alternating magnetic field had a frequency of 233 Hz and a bias of 250 pT In this case the magnetic stnp

Fig 3 The output signal of the phase-locked loop (for &tads see text) as a function of the apphed magnetic field B,

f II#l 1s -

m-

Fig 4 The resonance frequencyfof the sensor (Mumeta@ stnp) as a tunctlon of the apphed magnehc field i3, Approxnnatlon of the curve by a striught line at the operatmg pomt of the sensor

was made of Mumetall@ (dnnenslons I= 10 mm, w = 0 05 mm, t = 0 012 mm (see Fig 2)) In Fig 4 one can see the dependence of the oscdlator frequency on the apphed magnetic field B, At the operatmg pomt of the sensor (magnetic bias 250 rir) the curve is approxnnated by a straight line wrth a slope of -66 GHz/T This corresponds to a nummum detectable magnitude field of less than 1 nT wth respect to the noise The curve deplcted m Fig 4 1s not symmetrrcal to the zero of the magnetic field This behavlour 1s caused by the hysteresis of the strrp matenal The reason for the unequal heights of the numma m the curve (Fig 4) 1s unknown at this tnne, but for a whole cycle of magnetic-field strength the curve 1s completely symmetrrcal

to 30 pT) and has a slope at the operating point (magnetic bias of 150 cir> of - 102 GHz/T This corresponds to a magnetic sensltlvlty of less than 100 pT The dependence shows a small hysteresis of about 500 nT This value agrees with the value of the coercive force specified by the producer of the material The remammg behavlour of the Vitrovac strip is the same as that of the Mumetall strip The advantages of the Vltrovac strip are that it 1s more sensltlve and that a much smaller bias magnetic field 1s necessary to adJust the best operating point

4. Theoretical calculation

Fig 5 Dependence of the mductance (a), resistance (b) and resonance frequency (c) of the sensor (MumetaP strip) on the apphed magnetic field B,

In Fig 5 the dependence of the frequency of the sensor osczllator (curve c), the resistance (curve a) and the inductance of the metalhc strip (curve b) on the external magnetic field 1s described The inductance (e g , the nnagmary part of the complex permeablhty) and the resistance (real part) were measured by an nnpedance analyser The asymmetnc behavlour of the curve m Fig 4 can also be seen m curves a and b of Fig 5 In Fig 6 the same measurement as m Fig 5 1s shown, but m this case the matenal of the strip was Vltrovac Z 25@(dlmenslons I= 5 mm, w = 0 05 mm, 1 = 0 025 mm) The frequency dependence 1s also strictly linear (from 0

Fig 6 Dependence of the inductance (a), resistance (b) and resonance frequency (c) of the sensor (Ktrovac@ stnp) on the apphed magnetic field B,

First we consider the theoretical dependence of the frequency w on the complex impedance of series tunnel&ode oscillator circuits [ 15, 161 (for other kinds of oscdlators the calculations are srmllar)

w

&p(l

-Rd

The abbreviations used are resistance R, conductance g, inductance L and capacitance C at the operating point of the tunnel-diode oscillator Developing this expression m a Taylor series, one obtains a new expressron (first-order approxlmatlon m AR/R,, and AL/L,) Aw _Nwo R,, Lo are the resistance and mductance at the operatmg pomt of the tunnel-diode oscdlator without an external apphed field, go is the conductance of the diode at the operating point Here it 1s obvious that the relative frequency change Aw/w, 1s a linear function of the relative resistance (AR/&) and the relative mductance (AL/Lo) changes In usual co11magnetometers the real nnpedance &, 1s comparatively high, so that the relative resistance change (AR/&) IS a small quantity In this way the use of a low-nnpedance amphfier device like the tunnel diode 1s recommended Figures 5 and 6 show that the dependence of the frequency has the greatest slope when the resistance (curve b) and the inductance (curve a) increase In the area of decreasing resistance the slope of the frequency dependence alters (see the bend m Fig 6 curve c at 220 FT (vertical line)) This measured behavlour can be proved by theoretical calculations of the whole oscillator arcmt with a PSPICE@ clrcmt simulator To explam the increase of the resistance and the inductance m Figs 5 and 6, some assumptions are made First, we neglect Bloch-wall movement m the strip because the magnetic excltatlon 1s very high (82 MHz) Secondly, we neglect any changes m the resistance based on the amsotroplc magnetoreslstance

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because the relative changes are of the order of 50% and more Normally the relative resistance change based on the magnetoreslstlve effect 1s of the order of 1% [6] Thirdly, the internal magnetic behavlour in the inner area of the stnp (Fig 2) 1s not destroyed by cutting and it IS exactly as declared by the producer (e g , Vltrovac@ Z 25 has an easy axis parallel to the stnp axls) Fourthly, we assume that the mam magnetic field H, generated by the diode current has a direction vertical to the strip axis Under these assumptions an electrical behavlour as depicted m Figs 5 and 6 cannot be explained rfthe strrp IS homogeneous over the whole width Therefore we assume that the magnetlzatlon m the border area (Rg 2) has a different dlrectlon to that m the inner area Using the magnetization model shown m Fig 2, we tned to fit the inductance and resistance changes (curves b and a m Fig 6) Fzrst step

We compute the direction (angle a) of magnetlzatlon M,, (Fig 2) of the border area of the stnp under the influence of an external magnetic field H,, and a demagnetizing field Hd (Hd perpendicular to the stnp axis) by energy mmumzatlon The energy E(a) 1s gven as [ 121

[tilt Unit]

2

OS’ ’ 2ll-n -Mm

’

lam

’

ml

’0

sm

’

’

’

LCUJ 1swJ ml

BWI A fit of the measureddependence of the resistance(a) and Inductance (b) of the sensor (Vltrovac@stnp) on the applied magnetic field B,, boxes and crosses are measuredvalues, lmes are fitted values Fig 7

tlonal hysteresis power &sslpatlon and is taken to be proportional to the square of the sme of the corresponding angle [ 17] R a V, sm*(@ + V, sin*(y)

E(y) = Kb sin*(y) - H,M, sin(y) - H,,M, cos(y)

In Fig 7 this fit 1s shown, the crosses and the boxes represent the measured resistance and inductance (L ~1) and the straight lines are the fitted values (m units of R, and Lo) of the above magnetization model Takmg into consideration that the model of magnetlzatlon 1s qmte snnple, the overall agreement of the fitted values with the measured ones is very good There are small deviations m the resistance, but It nught be that the skin effect or other damping losses have a small miluence From this calculation we have to assume that the border area takes 30% of the whole area and the angle a for zero external applied field has a value of 90”

With M, = Mb sin(a), the magnetization of the border area 1s m the direction of the easy axls

5. c0lle1u!4olls

E(a) = Kb sin*(a) - H,Mb sm(a) - H,,M, cos(a) Kb 1s the amsotropy constant of the border area Second step

With this eqmhbnum value for the magnetlzatlon direction (angle a), we compute the rotation angle y (see Fig 2) of the above magnetlzatlon by energy mmnnlzatlon under the influence of H, and H, The energy E(y) is given as [ 121

Thzrd step

We compute once more the rotation angle 8 (Ag 2) of the magnetization M, of the mner area by energy mnumlzatlon under the mfluence of H, and H,, The energy E(0) is gven as E(0) = K, sm’(8) - H,M, sm(0) - H,,M, cos(0) K, IS the amsotropy constant of the mner area The permeablhty p IS given by the derlvatlve of the magnetlzatlon Mtotalm the &rectlon perpendicular to the strip axls from the Internal excltmg magnetic field H,, M total- V,M, szn(Q + GM, szn(r),

CCa AM,O,,zlW

V,, V,, are the volumes of the mner and the border area,

respectively

The resistance 1s calculated as the rota-

We have demonstrated that a further reduction of the size of a coil-core magnetometer (fluxgate type) is possible mthout any losses m sensltlvlty The co11can be saved by sending the current (wluch 1s creating the alternating magnetlzatlon) directly through the core (thin-film stnp) In a conventional coil-core magnetometer the magnetic sensltlvlty depends on the mam outer (outside of the cod mre) permeability changes from the co11 The magnetic sensltlvlty of the sensor introduced here depends on the inner (inside of the urlre) permeablhty changes As the inner permeability 1s smaller than the outer, their changes are also, but if one uses the real and Imagmary parts of the permeability for the signal formation, these smaller changes can be compensated In further studies we tned to reduce the

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References 1 T Aoyama and S Mlyake, Hybnd magneto-temperature sensor using contacts on a YBa,CuO,_, superconductor, Sensors and Actuators, A21-A23 (1990) 812-814 2 A Chovet, Ch S Roumemn, G Dlmodopoulos

and N Mathleu, Companson of noise properties of different magnetic-field sensors, Sensors and Actuators, A21 -A23 (1990)

l

time IO msldtv

Fig 8 An oscillogram of the output signal of a commercial magnetic sensor (channel 1) and of the sensor introduced here (channel 2) excited by the same external magnetic field

sue of the stnp, because we found that a reduction m size increases the sensltlvlty A further size reduction cannot be reached mechamcally by cutting a stnp from a foil, but it can be realized by chemical etching or by the molecular beam epltaxy (MBE) technique The MBE technique may be connected with an overall integration of the whole sensor To demonstrate the supenorlty of the introduced magnetic sensor, a direct comparison with commercial magnetometers based on the Hall effect 1s useful Figure 8 (channel 1) shows an osclllogram of the output signal of an a c magnetometer m the rmddle price range (about 5000 $) The external magnetic field B,, has an amplitude of 180 pTwar and a modulation frequency of 233 Hz Channel 2 (Fig 8) shows the oscdlogram of the phase-locked-loop output signal (demodulator) of the sensor introduced here, excited by the same external magnetic field The advantage of the new sensor 1s evident, because m spite of a simple constructlon it shows a clear and strong signal with very low noise compared to the conventional sensors (e g , Hall sensors) to

peaLj

790-794 3 Y Sugyama, H Soga, M Tacano and H P Baltes, Highly

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1977

I would like to thank Professor Dr W -J Becker and Professor Dr K Roll for their helpful dlscusslons

17 R A McCurne and M W Vlccary, Rotational hysteresis m an amsotroplc Mn-AI-C permanent magnet, IEEE Trans Magn, MAG-22 (1986) 1849-1858